Acetaldehyde (Ethanal): Step-by-Step Qualitative Identification Tests

ByDr. Shammas Din
Chart showing acetaldehyde (ethanal, CH3CHO) chemical identification tests, including tests for aromaticity, unsaturation, preliminary tests, carbonyl functional group tests, physical tests, confirmatory tests, and derivatives, with structure and molecular data.
Chemical identification tests for acetaldehyde (ethanal) — functional group, confirmatory tests, and derivatives

Acetaldehyde (Ethanal) Identification — Key Facts at a Glance

  • Molecular Formula (CH₃CHO · C₂H₄O) · Molecular Weight (44.05 g·mol⁻¹) · IUPAC Name (Ethanal) · Common Name (Acetaldehyde) · Other Names (Ethyl aldehyde · Acetic aldehyde) · Abbreviation (MeCHO) · Appearance (colourless, volatile liquid with pungent, fruity odour) · Boiling Point (21°C) · Melting Point (−125°C) · Density (0.785 g/mL at 25°C) · Solubility (fully miscible with water, ethanol) · Flash Point (−38.89°C) · CAS Number (75-07-0) · Acid/Base Nature (neutral) · Polarity (polar) · pKa α-H (17 at 25°C)
  • Physical Appearance Test · Solubility Test · Ignition / Flame Test · Litmus Test · Lassaigne’s Test · Brady’s Test (2,4-DNPH — yellow-orange precipitate) · Tollens’ Test (positive — silver mirror) · Fehling’s Test (positive — brick-red precipitate) · Benedict’s Test (positive — brick-red precipitate) · Schiff’s Test (positive — magenta colour) · Sodium Bisulfite Test (positive — white crystalline adduct) · Iodoform Test (positive — yellow CHI₃ precipitate) · Legal’s Test (positive — red colour) · Molisch’s Test (negative — carbohydrates absent) · Semicarbazone (m.p. 162–163°C) · Oxime (m.p. 47°C) · 2,4-DNPH Hydrazone (m.p. 168°C)
Acetaldehyde physical constants chart showing structure, IUPAC name, molecular formula, condensed formula, molar mass, density, pKa, polarity, melting point, solubility, boiling point, and flash point
Acetaldehyde Physical Constants: Density, Melting Point, Boiling Point & Flash Point

§1 — What Is Acetaldehyde? Introduction and Background

Acetaldehyde (ethanal) is the simplest aliphatic aldehyde — molecular formula CH₃CHO (C₂H₄O), CAS number 75-07-0. The acetaldehyde IUPAC name is ethanal, from the two-carbon chain eth- and the aldehyde suffix -al. It is also known as ethyl aldehyde and acetic aldehyde — abbreviated MeCHO (Me = methyl).
The acetaldehyde pronunciation is a-SÉT-al-duh-hyd; the IUPAC name ethanal is pronounced ETH-uh-nal. Both names — acetaldehyde and ethanal — refer to the same compound. It is the simplest member of the aldehyde family, where the general formula is R–CHO and R = CH₃.
Acetaldehyde occurs naturally in ripe fruit, coffee, fermented beverages, and tobacco smoke, and is formed in the human body as the primary metabolite of ethanol. Industrially, it is produced by Wacker oxidation of ethylene and is a key raw material for acetic acid, acetic anhydride, and ethyl acetate.

§2 — Physical Constants of Acetaldehyde (Ethanal)

The acetaldehyde molecular formula is CH₃CHO — also written as C₂H₄O — a colourless, volatile liquid with an acetaldehyde molecular weight of 44.05 g·mol⁻¹ (CAS 75-07-0). The acetaldehyde boiling point is 21°C (lit.) — critically low, reflecting high volatility — fume cupboard handling is mandatory. The acetaldehyde melting point is −125°C (lit.), confirming it remains liquid under all normal laboratory conditions. The acetaldehyde density is 0.785 g/mL at 25°C — lighter than water. Acetaldehyde solubility in water is complete — fully miscible. The acetaldehyde smell is pungent and fruity, detectable at concentrations as low as 0.0068 ppm. The acetaldehyde pKa of the α-hydrogen is 17 at 25°C. The acetaldehyde flash point is −38.89°C (closed cup) — extremely flammable even at sub-zero temperatures. Acetaldehyde is a polar compound, consistent with its complete miscibility in water and alcohols.
A complete reference table of physical constants and other properties of acetaldehyde for the Qualitative Identification of Acetaldehyde is given below.

Property

Value (Sigma-Aldrich 402788)

IUPAC Name

Ethanal

Common Name

Acetaldehyde; Ethyl aldehyde; Acetic aldehyde

Molecular Formula

CH₃CHO (C₂H₄O)

Condensed Formula

CH₃CHO

Molar Mass

44.05 g·mol⁻¹

CAS Number

75-07-0

EC number

200-836-8

Appearance

Colourless, volatile liquid

Odour

Pungent, fruity

Boiling Point

21°C (lit.)

Melting Point

−125°C (lit.)

Density

0.785 g/mL at 25°C (lit.)

Refractive index

n²⁰/D 1.332 (lit.)

Solubility in Water

Miscible

Flash point

−38.89°C (closed cup)

Vapour pressure

14.63 psi at 20°C

pKa (α-H)

17 (25°C, water)

Polarity

Polar

GHS signal word

DANGER

Carcinogen class

Carc. 1B (H350)

Storage

2–8°C

§3 — Planning the Identification: Decode the Structure First 🔍

Every competent qualitative analysis begins not at the bench, but on paper. Before a single reagent is added, the acetaldehyde structure must be examined carefully — because the structure dictates the entire test panel.

Step 1 — Identify the Functional Groups in Acetaldehyde

  • 🟠 –CHO group (aldehyde functional group) — the carbonyl carbon bonded directly to a hydrogen. This classifies acetaldehyde as an aldehyde, not a ketone.
  • 🟡 CH₃C=O unit (methyl carbonyl) — the methyl group bonded directly to the carbonyl carbon. This identifies acetaldehyde as a methyl carbonyl compound.
  • The acetaldehyde Lewis structure shows 18 valence electrons — C=O double bond with two lone pairs on the oxygen atom.

Step 2 — What Does the Structure Tell Us Before Any Test Is Run?

The acetaldehyde chemical structure contains a –CHO group, confirmed by Tollens’ Test, Fehling’s Test, Schiff’s Test, and Benedict’s Test. The CH₃C=O unit identifies it as a methyl carbonyl compound, confirmed by the Iodoform Test and Legal’s Test. The aliphatic character predicts a clean blue flame in the Ignition Test. The neutral, water-miscible structure predicts a negative Litmus Test and complete miscibility in the Solubility Test. The table below maps each structural clue to its predicted outcome.

Structural Clue

Tests to Confirm It

–CHO group (aldehyde)

Positive Tollens’ · Fehling’s · Schiff’s · Benedict’s

CH₃C=O unit (methyl carbonyl)

Positive Iodoform Test · Legal’s Test

Aliphatic character (no benzene ring)

Clean blue flame — no soot (Ignition Test)

Water miscibility

Solubility Test — fully miscible

Neutral compound (no –COOH, no –OH)

Litmus Test — no colour change

No C=C alkene

Bromine water test excluded

Whole compound (acetaldehyde)

Semicarbazone · Oxime · Molisch’s (negative)

§4 — Acetaldehyde Qualitative Tests: Complete Identification at a Glance

The confirmatory test for acetaldehyde is the Tollens’ Test — the silver mirror that distinguishes it from all ketones — supported by the Iodoform Test and Legal’s Test. The colourless appearance of acetaldehyde immediately eliminates all coloured compounds from consideration — see [Colour of Organic Compounds].
Full procedural details for each test are available on this website: [Solubility Test] · [Litmus Test] · [Brady’s Test] · [Tollens’ Test] · [Fehling’s Test] · [Benedict’s Test] · [Iodoform Test] · [Molisch’s Test] · [Melting Point Determination].

#

Test

What It Establishes

What It Rules Out

Preliminary Tests

1

Physical Appearance

Colourless, volatile liquid; pungent, fruity odour

Coloured compounds — absent

2

Solubility Test

Polar compound; fully miscible with water

Non-polar compounds — absent

3

Ignition / Flame Test

Clean, non-sooty blue flame — aliphatic character

Aromatic compounds — absent

4

Litmus Test

Neutral compound

Carboxylic acids — absent · Phenols — absent · Amines — absent

5

Lassaigne’s Test

No nitrogen, sulfur, or halogens present

Amines — absent · Amides — absent · Halogenated compounds — absent

Functional Group Tests

6

Brady’s Test (2,4-DNPH Test)

Carbonyl group (C=O) confirmed

Non-carbonyl compounds — absent

7

Tollens’ Test

Positive — silver mirror — aldehyde confirmed

Ketones — absent

8

Fehling’s Test

Positive — brick-red precipitate — aliphatic aldehyde confirmed

Ketones — absent · Aromatic aldehydes — absent

9

Benedict’s Test

Positive — brick-red precipitate — corroborating Fehling’s

Ketones — absent

10

Schiff’s Test

Positive — magenta colour — aldehyde confirmed

Ketones — absent

11

Sodium Bisulfite Test

Positive — aldehyde or methyl ketone confirmed

Non-reactive carbonyls — absent

Confirmatory Tests

12

Iodoform Test

Yellow precipitate (CHI₃) — CH₃CHO unit confirmed

Non-methyl carbonyls — absent

13

Legal’s Test (Sodium Nitroprusside)

Red colour — methyl carbonyl unit confirmed

Non-methyl carbonyls — absent

14

Molisch’s Test

Negative — carbohydrate class excluded

Carbohydrates — absent

15

Semicarbazone Derivative

m.p. 162–163°C — confirms acetaldehyde specifically

Other aldehydes — absent

16

Oxime Derivative

m.p. 47°C — independent m.p. confirmation

Other aldehydes — absent

§5 — Materials and Reagents

Safety note: Acetaldehyde is stored at 2–8°C, flash point −38.89°C — handle in a fume cupboard only; keep away from all ignition sources.
Full details of each reagent are available on this website: [Solubility Test] · [Litmus Test] · [Brady’s Test] · [Tollens’ Test] · [Fehling’s Test] · [Benedict’s Test] · [Iodoform Test].

#

Reagent

Composition / Preparation

Purpose in Identification

1

Acetaldehyde sample (unknown)

Stored at 2–8°C; handle in fume cupboard only

Test compound

2

Distilled water

—

Solubility test; reagent preparation

3

Brady’s Reagent (2,4-DNPH)

2,4-Dinitrophenylhydrazine dissolved in methanol and dilute H₂SO₄

Detects carbonyl group (C=O) — Brady’s Test

4

Tollens’ Reagent

AgNO₃ in water + NaOH (Ag₂O ppt) + dilute NH₃ dropwise until clear [Ag(NH₃)₂]OH — prepare freshly; never store

Distinguishes aldehyde from ketone — positive for acetaldehyde

5

Fehling’s Solution

Fehling’s A: CuSO₄ in water · Fehling’s B: sodium potassium tartrate + NaOH — mix equal volumes before use

Detects aliphatic aldehydes — positive for acetaldehyde

6

Benedict’s Reagent

CuSO₄ + sodium carbonate + sodium citrate in distilled water — stable single solution

Corroborates Fehling’s — positive for acetaldehyde

7

Schiff’s Reagent

Basic fuchsin (1 g) + sodium metabisulfite (Na₂S₂O₅, 1.9 g) + 0.15N HCl (100 mL) — shake, decolourise with activated charcoal, filter to colourless; prepare freshly; store in dark

Detects aldehydes — positive for acetaldehyde

8

Saturated Sodium Bisulfite

Saturated aqueous NaHSO₃ — prepare freshly

Detects aldehydes and methyl ketones

9

Iodine Solution (Lugol’s)

I₂ dissolved in aqueous KI

Used with NaOH in Iodoform Test

10

Sodium Hydroxide (NaOH)

10% aqueous solution

Required for Iodoform Test and Tollens’ preparation

11

Sodium Nitroprusside

Na₂[Fe(CN)₅NO] in distilled water

Legal’s Test — methyl carbonyl detection

12

Molisch’s Reagent

α-Naphthol (5–10%) dissolved in 95% ethanol — prepare freshly, protect from light

Molisch’s Test — carbohydrate exclusion

12a

Conc. Sulphuric Acid (H₂SO₄)

Added separately during Molisch’s Test — layered carefully down tube wall; never mixed with Molisch’s reagent

Dehydrates carbohydrates to furfural — produces purple ring at interface

13

Semicarbazide hydrochloride

sodium acetate in water

Semicarbazone derivative — m.p. 162–163°C

14

Hydroxylamine hydrochloride

sodium acetate in water

Oxime derivative — m.p. 47°C

15

Authentic acetaldehyde sample

Known compound — same source

Control test — comparison with authentic sample

16

Blue and red litmus paper

—

Litmus Test — acid/base character

§6 — Preliminary Tests for Acetaldehyde Identification

In qualitative analysis, a single preliminary observation can eliminate multiple compound classes at once. A neutral litmus result rules out all acidic and basic compounds together.
A clean, non-sooty flame rules out all aromatic compounds in one step. This is the power of preliminary testing — systematic elimination before a single chemical reagent is used.ll

6.i — Physical Appearance

  • Observation: Examine the compound visually and by odour under safe laboratory conditions — do not inhale directly — colourless, volatile liquid with a pungent, fruity odour — liquid state consistent with low boiling point (21°C).
  • Conclusion: Coloured compounds — absent · Solid compounds — absent · Aldehyde-class fruity odour — indicated

6.ii — Solubility Test

Reagents: Distilled water · Ethanol · Dilute NaOH solution

  • Water: Add 2–3 drops of acetaldehyde to 2 mL of water → completely miscible in all proportions.
    Conclusion: Polar functional group — confirmed · Aromatic hydrocarbons — absent · Non-polar compounds — absent
  • Ethanol: Add 2–3 drops to 2 mL of ethanol → completely miscible.
    Conclusion: Low-molecular-weight aliphatic compound — confirmed
  • NaOH: Add 2–3 drops to dilute NaOH solution → gelatinous/resinous precipitate forms.
    Conclusion: Carboxylic acids — absent · Phenols — absent · Aldehyde character with α-hydrogen — indicated

6.iii — Ignition / Flame Test

Reagent: Bunsen burner (Caution: flash point −38.89°C — fume cupboard mandatory; keep away from all other reagents)

  • Observation: Place a few drops on a clean evaporating dish, ignite carefully with a taper → burns readily with a clean, non-sooty blue flame.
  • Conclusion: Aromatic compounds — absent · Phenols — absent · Aliphatic character — confirmed

6.iv — Litmus Test

  • Red litmus: Dissolve a few drops in distilled water, test with red litmus paper → no colour change.
    Conclusion: Carboxylic acids — absent · Phenols — absent · Sulfonic acids — absent
  • Blue litmus: Test with blue litmus paper → no colour change.
    Conclusion: Amines — absent · Anilines — absent
  • Overall Conclusion: No change to either red or blue litmus paper → compound is neutral → aldehydes, ketones, or carbohydrates — indicated

6.v — Lassaigne’s Test (Elemental Detection)

Reagents: Sodium metal · Glucose · Distilled water · FeSO₄ solution · FeCl₃ solution · Sodium nitroprusside solution · AgNO₃ solution · Dilute HNO₃

Preparation of Lassaigne’s Extract: Place sodium metal in the ignition tube, add a small quantity of acetaldehyde carefully, then fill the remaining space with glucose — glucose contains no nitrogen, sulfur, or halogens and does not interfere with any detection test; it reduces vapour escape and makes the fusion safer. Heat the ignition tube until red hot, cool cautiously, add distilled water carefully, boil, and filter to obtain the Lassaigne’s extract.

  • Detection of Nitrogen: Lassaigne’s extract + FeSO₄ → boil → cool → add FeCl₃ → acidify with dilute H₂SO₄ → no Prussian blue → nitrogen absent → amines and amides absent.
  • Detection of Sulfur: Alkaline Lassaigne’s extract + sodium nitroprusside solution → no violet/purple colour → sulfur absent → sulfonic acids and thio-compounds absent.
  • Detection of Halogens: Lassaigne’s extract + dilute HNO₃ + AgNO₃ solution → no precipitate → halogens absent → halogenated compounds absent.

Preliminary Test Results — Identifying the Compound Class of Acetaldehyde

Group

Compound Class

Characteristic Tests (exact)

Our Observation

Verdict

A

Carboxylic Acids

(i) Litmus test (ii) Sodium bicarbonate test (iii) Ferric chloride test (iv) Esterification test (v) Sodium hydroxide solubility test (vi) Flame test (vii) Sublimation test (viii) Melting point / mixed melting point (ix) Solid derivative — amide (x) Solid derivative — anilide

Litmus neutral · No CO₂ with NaHCO₃ · No clean dissolution in NaOH

Ruled out

B

Phenols

(i) Litmus test (ii) Ferric chloride test (iii) Bromine water test (iv) Liebermann’s nitroso test (v) Phthalein dye test (vi) Azo dye test (vii) Millon’s test (viii) Sodium hydroxide solubility test (ix) Sodium bicarbonate test (negative) (x) Melting point / mixed melting point (xi) Solid derivative — tribromophenol

Litmus neutral · Clean blue flame · Gelatinous ppt with NaOH

Ruled out

D

Amines

(i) Litmus test (ii) Carbylamine test (iii) Nitrous acid test (iv) Hinsberg test (v) Azo dye test (vi) Acetylation test (vii) Benzoylation test (Schotten–Baumann) (viii) Lassaigne’s test (ix) Solid derivative — acetyl / benzoyl

Litmus neutral · Nitrogen absent (Lassaigne’s)

Ruled out

E

Aromatic Hydrocarbons

(i) Baeyer’s test (negative) (ii) Bromine water test (substitution) (iii) Nitration test (iv) Sulfonation test (v) Friedel–Crafts test (vi) Combustion / flame test (sooty flame) (vii) Lassaigne’s test (viii) Melting point / mixed melting point (ix) Solid derivative — picrate (x) Solid derivative — charge transfer complex

Clean non-sooty blue flame · No heteroatoms (Lassaigne’s)

Ruled out

C

Aldehydes · Ketones · Carbohydrates

(i) Brady’s test (ii) Tollens’ test (iii) Fehling’s test (iv) Benedict’s test (v) Schiff’s test (vi) Sodium bisulfite addition test (vii) Iodoform test (viii) Molisch’s test (ix) Seliwanoff’s test (x) Barfoed’s test (xi) Osazone formation test (xii) 2,4-DNPH derivative melting point

Neutral · Polar · Aliphatic · No heteroatoms · Gelatinous ppt with NaOH

Indicated ✅
  • Group A — Carboxylic Acids — Ruled Out: Litmus test is neutral — no colour change on red or blue litmus · no CO₂ effervescence with sodium bicarbonate → carboxylic acids eliminated.
  • Group B — Phenols — Ruled Out: Ignition test produces a clean, non-sooty blue flame — phenols burn with a sooty flame · NaOH produces a gelatinous precipitate — phenols dissolve cleanly forming sodium phenoxide → phenols eliminated.
  • Group D — Amines — Ruled Out: Litmus test is neutral — amines turn blue litmus deeper blue · Lassaigne’s test confirms nitrogen absent — no Prussian blue → amines and amides eliminated.
  • Group E — Aromatic Hydrocarbons — Ruled Out: Ignition test produces a clean, non-sooty blue flame — aromatic hydrocarbons burn with a characteristically sooty flame · Lassaigne’s test confirms no heteroatoms → aromatic hydrocarbons eliminated.
  • Group C — Aldehydes, Ketones, and Carbohydrates — Indicated ✅: Neutral litmus · fully miscible with water · clean blue flame · no heteroatoms in Lassaigne’s test · gelatinous precipitate with NaOH — all observations consistent with Group C. Functional group tests of Group C now apply.

§7 — Functional Group Tests: Carbonyl Detection in Acetaldehyde

The preliminary tests confirmed acetaldehyde is neutral, aliphatic, polar, and contains only carbon, hydrogen, and oxygen. The functional group tests below determine which specific compound it is.

Step 1 — Confirm carbonyl group: Brady’s Test (2,4-DNPH Test) → reacts with both aldehydes and ketones → immediate orange/yellow crystalline precipitate → carbonyl group (C=O) confirmed · carbohydrates eliminated.
Step 2 — Aldehyde or ketone? Tollens’ Test → silver mirror · Fehling’s Test → brick-red precipitate · Benedict’s Test → brick-red precipitate · Schiff’s Test → magenta colour → all four positive → aldehyde confirmed · ketones eliminated.
Step 3 — Which aldehyde? Sodium Bisulfite Test → white crystalline adduct · Iodoform Test → yellow precipitate (CHI₃) → CH₃CHO unit confirmed · Legal’s Test → red colour → methyl carbonyl unit confirmed → acetaldehyde specifically indicated.

§

Test

Result for Acetaldehyde

§8

Brady’s Test (2,4-DNPH Test)

Positive — carbonyl confirmed

§9

Tollens’ Test

Positive — silver mirror — aldehyde confirmed

§10

Fehling’s

Positive — brick-red precipitate

§11

Benedict’s Test

Positive — brick-red precipitate

§12

Schiff’s Test

Positive — magenta colour — aldehyde confirmed

§13

Sodium Bisulfite Test

Positive — CH₃CHO adduct confirmed

§14

Iodoform Test

Positive — yellow CHI₃ precipitate — CH₃CHO confirmed

§15

Legal’s Test

Positive — red colour — methyl carbonyl confirmed

§16

Molisch’s Test

Negative — carbohydrates excluded

§8 — Test 1: Brady’s Test (2,4-DNPH Test)

Brady’s Test (2,4-DNPH Test) detects the carbonyl group (C=O) in aldehydes and ketones. A positive result confirms a carbonyl compound is present — it does not yet distinguish aldehyde from ketone. For acetaldehyde, the expected result is a yellow crystalline precipitate.

Full details of Brady’s Test — including its mechanism, derivative melting points, and applications — are available at [Brady’s Test — chemistrysh.com].

Test

Reagent

Procedure

Positive Result

Inference

Brady’s Test (2,4-DNPH Test)

2,4-Dinitrophenylhydrazine (0.5 g) dissolved in concentrated H₂SO₄ (10 mL), then added to ethanol (15 mL) and distilled water (5 mL) — Brady’s reagent. Prepare freshly or use within 10 days.

Add 3–4 drops of acetaldehyde to 2 mL of Brady’s reagent in a clean test tube. Shake well and allow to stand for 2–3 minutes at room temperature.

Yellow crystalline precipitate — aliphatic aldehyde or ketone · Orange precipitate — α,β-unsaturated carbonyls · Red precipitate — aromatic aldehydes and ketones

Carbonyl group (C=O) confirmed. Acetaldehyde is an aldehyde or ketone. Alcohols, carboxylic acids, and esters give no precipitate.

§9 — Test 2: Tollens’ Test (Silver Mirror Test)

Tollens’ Test uses [Ag(NH₃)₂]OH — the diamminesilver(I) complex — to distinguish aldehydes from ketones. Acetaldehyde gives a positive Tollens’ test — confirming an aldehyde, not a ketone.

Full details of Tollens’ Test are available at [Tollens’ Test — chemistrysh.com].

Test

Reagent

Procedure

Positive Result

Inference

Tollens’ Test (Silver Mirror Test)

Freshly prepared Tollens’ reagent — AgNO₃ + dilute NaOH → Ag₂O precipitate → add dilute NH₃ dropwise → clear [Ag(NH₃)₂]OH solution. Never store.

Add 2–3 drops of acetaldehyde to 2 mL of Tollens’ reagent. Warm in water bath at 60°C for 5 minutes. Observe inner walls of test tube.

Bright silver mirror on inner walls of test tube — positive result for acetaldehyde. (No mirror = ketone — negative result)

Aldehyde confirmed. Ag⁺ reduced to Ag metal by the aldehyde. Ketones give no mirror.
  • Silver mirror formed → aldehyde confirmed (acetaldehyde — positive result)
  • No silver mirror → ketone confirmed (negative result)
  • Special case: alpha-hydroxy ketones may give a weak positive — acetaldehyde gives a strong positive

§10 — Test 3: Fehling’s Test and Benedict’s Test

Fehling’s Test and Benedict’s Test detect reducing aldehydes using Cu²⁺ ions — producing a brick-red precipitate of Cu₂O on positive result. Acetaldehyde gives a positive result in both — confirming an aliphatic aldehyde. Aromatic aldehydes (e.g. benzaldehyde) give a negative Fehling’s test.

Full details are available at [Fehling’s Test — chemistrysh.com] and [Benedict’s Test — chemistrysh.com].

Test

Reagent

Procedure

Positive Result

Inference

Fehling’s Test

Fehling’s A (CuSO₄ solution) + Fehling’s B (sodium potassium tartrate + NaOH) — mix equal volumes immediately before use

Add 1 mL of acetaldehyde to 2 mL of freshly mixed Fehling’s solution. Heat in boiling water bath for 5 minutes. Observe.

Brick-red Cu₂O precipitate — positive result for acetaldehyde

Aliphatic aldehyde confirmed. Ketones and aromatic aldehydes give no precipitate.

Benedict’s Test

Benedict’s reagent — CuSO₄ + sodium carbonate + sodium citrate in water — stable single solution

Add 1 mL of acetaldehyde to 2 mL of Benedict’s reagent. Heat in boiling water bath for 5 minutes. Observe.

Brick-red precipitate — positive result for acetaldehyde

Corroborates Fehling’s result. Reducing aldehyde confirmed.

§11 — Test 4: Schiff’s Test

Schiff’s reagent — a decolourised solution of fuchsin dye — is restored to its pink/magenta colour by aldehydes. Ketones do not respond under normal conditions. Acetaldehyde gives a positive Schiff’s test — confirming an aldehyde, not a ketone.

Full details of Schiff’s Test are available at [Schiff’s Test — chemistrysh.com].

Test

Reagent

Procedure

Positive Result

Inference

Schiff’s Test

Schiff’s reagent — basic fuchsin (1 g) + sodium metabisulfite (Na₂S₂O₅, 1.9 g) + 0.15N HCl (100 mL) — decolourise with activated charcoal, filter to colourless; prepare freshly; store in dark.

Add 2–3 drops of acetaldehyde to 2 mL of Schiff’s reagent in a clean test tube. Allow to stand at room temperature for 5 minutes. Do not heat. Observe colour change.

Immediate pink/magenta colour — positive result for acetaldehyde

Aldehyde confirmed. Ketones give no colour change under normal conditions.

§12 — Test 5: Sodium Bisulfite Test

The sodium bisulfite test detects aldehydes and methyl ketones by nucleophilic addition of HSO₃⁻ to the carbonyl carbon — forming a white crystalline bisulfite adduct. Acetaldehyde gives a positive result — confirming an aldehyde or methyl ketone is present.

Test

Reagent

Procedure

Positive Result

Inference

Sodium Bisulfite Test

Saturated aqueous NaHSO₃ solution — prepare freshly before use

Add 1 mL of acetaldehyde to 2 mL of saturated sodium bisulfite solution in a clean test tube. Shake well and allow to stand for 15–20 minutes. Observe.

White crystalline precipitate (CH₃CH(OH)SO₃Na) forms — positive result for acetaldehyde

Aldehyde or methyl ketone confirmed. Sterically hindered ketones give no precipitate.

§13 — Test 6: Iodoform Test

The Iodoform Test is a key confirmatory test for the CH₃CHO unit. Acetaldehyde reacts with iodine (I₂) and sodium hydroxide (NaOH) to produce iodoform (CHI₃) — a yellow crystalline precipitate with a characteristic antiseptic odour. A positive iodoform test confirms the CH₃CHO group is present.

Full details of the Iodoform Test are available at [Iodoform Test — chemistrysh.com].

Test

Reagent

Procedure

Positive Result

Inference

Iodoform Test

Iodine solution (I₂ in aqueous KI) + sodium hydroxide solution (10% NaOH)

Add 1 mL of acetaldehyde to 2 mL of NaOH solution in a clean test tube. Add iodine solution dropwise with shaking until a faint permanent yellow colour persists. Warm gently if no precipitate forms. Observe.

Yellow crystalline precipitate of iodoform (CHI₃) with characteristic antiseptic odour — positive result for acetaldehyde

CH₃CHO unit confirmed. CH₃CHO + 3I₂ + 4NaOH → CHI₃↓ + HCOONa + 3NaI + 3H₂O

§14 — Test 7: Legal’s Test (Sodium Nitroprusside Test)

Legal’s Test is specific to compounds containing the CH₃C=O unit — methyl ketones and acetaldehyde. Acetaldehyde gives a positive Legal’s Test — producing a red colour in alkaline medium. This is a special case: most aldehydes give a negative Legal’s Test, but acetaldehyde gives a positive result because it contains the CH₃CHO unit, which behaves analogously to a methyl ketone in this reaction.

Test

Reagent

Procedure

Positive Result

Inference

Legal’s Test (Sodium Nitroprusside Test)

Freshly prepared sodium nitroprusside solution — Na₂[Fe(CN)₅NO] dissolved in distilled water + sodium hydroxide solution (10% NaOH). Prepare freshly before use.

1. Dissolve sodium nitroprusside in 2 mL distilled water in a clean test tube. 2. Add 1 mL of acetaldehyde. Shake gently. 3. Add 10% NaOH solution dropwise. 4. Observe colour change immediately.

Red colour — positive result for acetaldehyde. Colour may shift to yellow on prolonged standing. Note: colour variation is normal — exact shade depends on concentration of sodium nitroprusside and alkalinity of the medium.

CH₃CHO unit confirmed. Acetaldehyde gives a positive Legal’s Test — a special case among aldehydes. Most aldehydes give no colour — negative result.

§15 — Test 8: Molisch’s Test

Molisch’s Test is a general test for carbohydrates. Acetaldehyde gives a negative Molisch’s Test — confirming it is not a carbohydrate. This negative result eliminates glucose, fructose, and all other reducing sugars from consideration.

Test

Reagent

Procedure

Positive Result

Inference

Molisch’s Test

Molisch’s reagent — α-naphthol (5–10%) in 95% ethanol (prepare freshly, protect from light) · Conc. H₂SO₄ added separately during test

Add 2–3 drops of acetaldehyde to 2 mL of distilled water. Add 2 drops of Molisch’s reagent. Shake gently. Tilt the test tube and carefully layer 1 mL of conc. H₂SO₄ down the tube wall. Do not mix. Observe the interface.

No purple ring at interface — negative result for acetaldehyde

Carbohydrate class absent — acetaldehyde is not a sugar. Positive result (purple ring) would indicate carbohydrate.

§16 — Solid Derivatives of Acetaldehyde and Their Melting Points

Solid derivatives provide the final confirmatory test for acetaldehyde through melting point determination. The melting point of acetaldehyde derivatives — particularly the semicarbazone (162–163°C) and oxime (47°C) — acts as a fingerprint that distinguishes acetaldehyde from other aldehydes sharing positive test results.

Three standard derivatives are prepared:

#

Derivative

Reagent

Appearance

Melting Point

CAS

1

2,4-Dinitrophenylhydrazone (2,4-DNPH hydrazone)

Brady’s reagent — 2,4-DNPH in methanol/H₂SO₄

Yellow-orange crystalline solid

168°C

1019-57-4

2

Semicarbazone

Semicarbazide hydrochloride + sodium acetate in water

White crystalline solid

162–163°C

—

3

Oxime (Acetaldoxime)

Hydroxylamine hydrochloride + sodium acetate in water

White crystalline solid

47°C

—

How to Prepare Acetaldehyde Derivatives in the Laboratory

This procedure is a standard organic chemistry practical carried out in any laboratory experiment involving carbonyl compound identification.

  1. Dissolve acetaldehyde in the minimum volume of ethanol or water as appropriate. (Caution: flash point −38.89°C — work in fume cupboard only)
  2. Add the reagent solution.
  3. Allow to stand or warm gently until crystallisation occurs.
  4. Filter and wash with cold solvent.
  5. Recrystallise to obtain a pure sample.
  6. Determine the melting point of the acetaldehyde derivative and compare with the tabulated values above.

Note: The 2,4-DNPH hydrazone melting point is sensitive to traces of acid in the crystals. Wash with dilute sodium bicarbonate solution before recrystallisation to obtain a sharp, reproducible value. The semicarbazone derivative (m.p. 162–163°C) is the preferred confirmatory derivative — it gives a sharp, reproducible melting point and is stable at room temperature.

§17 — Chemical Reactions Involved in the Identification of Acetaldehyde

The identification tests performed on acetaldehyde involve distinct chemical reactions. For each test, the chemical reaction, type of reaction, and observation are given below.

17.i — Brady’s Test (2,4-DNPH Test)

Chemical Reaction:

CH₃CHO + (O₂N)₂C₆H₃NHNH₂ → CH₃CH=NNH·C₆H₃(NO₂)₂ + H₂O

Type of Reaction: Condensation reaction (nucleophilic addition-elimination) — the 2,4-DNPH adds across the C=O double bond, followed by elimination of water to form the hydrazone.

Observation: Yellow-orange crystalline precipitate — acetaldehyde 2,4-dinitrophenylhydrazone (m.p. 168°C).

17.ii — Tollens’ Test

Chemical Reaction:

CH₃CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag↓ + 4NH₃ + H₂O

Type of Reaction: Oxidation-reduction — acetaldehyde is oxidised to acetate; silver ion (Ag⁺) is reduced to silver metal (Ag↓), depositing as a mirror on the inner walls of the test tube.
Observation: Bright silver mirror on inner walls of test tube — positive result.

17.iii — Fehling’s Test and Benedict’s Test

Chemical Reaction:

CH₃CHO + 2Cu²⁺ + 5OH⁻ → CH₃COO⁻ + Cu₂O↓ + 3H₂O

Type of Reaction: Oxidation-reduction — acetaldehyde is oxidised to acetate; copper(II) ions (Cu²⁺, blue) are reduced to copper(I) oxide (Cu₂O, brick-red precipitate).
Observation: Brick-red Cu₂O precipitate — positive result for both Fehling’s and Benedict’s tests.

17.iv — Schiff’s Test

Chemical Reaction:

CH₃CHO + Schiff’s reagent → magenta colour restored

Type of Reaction: The aldehyde group reacts with the decolourised fuchsin-bisulfite complex, regenerating the chromophore and restoring the magenta colour.
Observation: Immediate pink/magenta colour — positive result.

17.v — Sodium Bisulfite Test

Chemical Reaction:

CH₃CHO + NaHSO₃ → CH₃CH(OH)SO₃Na

Type of Reaction: Nucleophilic addition — the bisulfite ion (HSO₃⁻) acts as a nucleophile and adds to the carbonyl carbon to form the crystalline bisulfite adduct.
Observation: White crystalline precipitate of sodium acetaldehyde bisulfite adduct — positive result.

17.vi — Iodoform Test

Chemical Reaction:

CH₃CHO + 3I₂ + 4NaOH → CHI₃↓ + HCOONa + 3NaI + 3H₂O

Type of Reaction: Halogenation followed by nucleophilic cleavage — hydroxide ion acts as a base to allow successive iodination of the methyl group, then cleaves the C–C bond to release iodoform (CHI₃) and sodium formate (HCOONa).
Observation: Yellow crystalline precipitate of iodoform (CHI₃) with characteristic antiseptic odour — positive result.

17.vii — Legal’s Test (Sodium Nitroprusside Test)

Chemical Reaction:

CH₃CHO + Na₂[Fe(CN)₅NO] + NaOH → red/pink complex

Type of Reaction: Coordination complex formation — the methyl carbonyl unit of acetaldehyde reacts with sodium nitroprusside in alkaline conditions to form a red-coloured iron coordination complex.
Observation: Red colour in alkaline medium — positive result. Colour may shift to yellow on prolonged standing.

17.viii — Semicarbazone Formation

Chemical Reaction:

CH₃CHO + NH₂NHCONH₂ → CH₃CH=NNHCONH₂ + H₂O (m.p. 162–163°C)

Type of Reaction: Condensation reaction — nucleophilic addition of semicarbazide to the carbonyl group, followed by elimination of water.
Observation: White crystalline solid — semicarbazone derivative, m.p. 162–163°C.

17.ix — Oxime Formation

Chemical Reaction:

CH₃CHO + NH₂OH → CH₃CH=NOH + H₂O (m.p. 47°C)

Type of Reaction: Condensation reaction — hydroxylamine adds to the carbonyl group, followed by elimination of water to form acetaldoxime.
Observation: White crystalline solid — oxime derivative (acetaldoxime), m.p. 47°C.

§18 — Acetaldehyde Test Results: Qualitative Identification of Acetaldehyde

The qualitative identification of acetaldehyde — the acetaldehyde test sequence — was carried out systematically through preliminary tests (Physical Appearance · Solubility Test · Ignition Test · Litmus Test · Lassaigne’s Test), functional group tests (Brady’s Test · Tollens’ Test · Fehling’s Test · Benedict’s Test · Schiff’s Test · Sodium Bisulfite Test), and confirmatory tests (Iodoform Test · Legal’s Test · Molisch’s Test · Semicarbazone Derivative · Oxime Derivative) following standard organic qualitative analysis protocol. The results of all tests are summarised in the table below.

Qualitative Test for Acetaldehyde — Summary of All Test Results

#

Test

Observation

Conclusion

1

Physical Appearance

Colourless, volatile liquid; pungent, fruity odour

Coloured compounds — absent

2

Solubility Test

Completely miscible with water and ethanol; gelatinous precipitate with NaOH

Polar compound — aromatic hydrocarbons absent · aldehyde character with α-hydrogen indicated

3

Ignition / Flame Test

Clean, non-sooty blue flame

Aromatic compounds and phenols — absent

4

Litmus Test

Neutral — no colour change

Carboxylic acids, phenols, amines — absent

5

Lassaigne’s Test

N, S, halogens all absent

Amines, amides, halogenated compounds — absent

6

Brady’s Test (2,4-DNPH Test)

Yellow-orange crystalline precipitate

Carbonyl group (C=O) — confirmed

7

Tollens’ Test

Bright silver mirror — positive

Aldehyde confirmed · ketones — absent

8

Fehling’s Test

Brick-red Cu₂O precipitate — positive

Aliphatic aldehyde confirmed · aromatic aldehydes — absent

9

Benedict’s Test

Brick-red precipitate — positive

Reducing aldehyde — confirmed

10

Schiff’s Test

Immediate pink/magenta colour — positive

Aldehyde confirmed · ketones — absent

11

Sodium Bisulfite Test

White crystalline precipitate — positive

Aldehyde or methyl ketone — confirmed

12

Iodoform Test

Yellow precipitate (CHI₃), antiseptic odour — positive

CH₃CHO unit — confirmed

13

Legal’s Test (Sodium Nitroprusside)

Red colour — positive

Methyl carbonyl unit — confirmed

14

Molisch’s Test

No purple ring — negative

Carbohydrates — absent

15

Semicarbazone Derivative

White crystalline solid — m.p. 162–163°C

Matches reported literature value — confirmed

16

Oxime Derivative

White crystalline solid — m.p. 47°C

Independent m.p. confirmation

Confirming Acetaldehyde as an Aliphatic Aldehyde — Test Result Interpretation

The compound does not turn blue litmus red — carboxylic acids, phenols, and sulfonic acids eliminated. It does not turn red litmus blue — amines and anilines eliminated. The ignition test produces a clean, non-sooty blue flame — aromatic hydrocarbons, benzene, toluene, phenol, and cresol eliminated. Lassaigne’s test shows no nitrogen, sulfur, or halogens — amines, amides, and halogenated compounds eliminated. Brady’s Test produces a yellow-orange precipitate — carbonyl group (C=O) confirmed, alcohols, carboxylic acids, and esters eliminated. Tollens’, Fehling’s, Benedict’s, and Schiff’s tests all give positive results — aldehyde confirmed, ketones eliminated. The Iodoform Test and Legal’s Test are both positive — CH₃CHO unit confirmed, non-methyl carbonyls eliminated. The semicarbazone derivative melting point is 162–163°C, matching the reported literature value — the compound is therefore identified as acetaldehyde (ethanal).

Identification Flowchart

  • Carbonyl present? Brady’s Test positive → Yes
  • Aldehyde or ketone? Tollens’, Fehling’s, Benedict’s, Schiff’s all positive → Aldehyde confirmed
  • Methyl ketone? Iodoform Test positive + Legal’s Test positive → CH₃CHO unit confirmed
  • Which aldehyde specifically? Semicarbazone m.p. 162–163°C matches reported literature value → Acetaldehyde confirmed

§19 — Conclusion

The unknown compound is not aromatic — clean blue flame confirmed. It is not acidic — blue litmus unchanged. It is not basic — red litmus unchanged. It is not a carbohydrate — Molisch’s Test negative. It is not a ketone — Tollens’, Fehling’s, Benedict’s, and Schiff’s tests all positive, ruling out all ketones. It is not a non-methyl aldehyde — Iodoform Test and Legal’s Test both positive, confirming the CH₃CHO unit.
The compound gives a positive Tollens’ Test (silver mirror), a positive Iodoform Test (yellow CHI₃ precipitate), and a positive Legal’s Test (red colour) — all three confirmatory tests consistent with acetaldehyde and acetaldehyde alone.
The semicarbazone derivative melting point (162–163°C) matches the reported literature value for acetaldehyde semicarbazone.
The unknown compound is therefore conclusively identified as acetaldehyde (ethanal) — molecular formula CH₃CHO (C₂H₄O) · molar mass 44.05 g·mol⁻¹ · boiling point 21°C · melting point −125°C · density 0.785 g/mL at 25°C · CAS 75-07-0.

§20 — Acetaldehyde Chemistry — Key Terms and Definitions for Qualitative Identification

α-Hydrogen

Any carbon atom directly bonded to a functional group is called the α-carbon. A hydrogen atom bonded to the α-carbon is called the α-hydrogen.

  • In acetaldehyde: The CH₃ carbon is directly bonded to the –CHO group → CH₃ is the α-carbon → the three H atoms on CH₃ are the α-hydrogens
  • pKa of acetaldehyde α-H: 17 — more acidic than acetone (pKa = 20) because the aldehyde carbonyl is more electron-withdrawing than the ketone carbonyl

Nucleophilic Addition

A reaction in which a nucleophile — an electron-rich species — attacks the electrophilic carbonyl carbon and adds across the C=O bond.

  • Nucleophile: The attacking species (e.g. HSO₃⁻ in Bisulfite Test · OH⁻ in Iodoform Test)
  • Electrophile: The carbonyl carbon of acetaldehyde (C=O)
  • Product: A new C–Nu bond forms at the carbonyl carbon

Bisulfite Adduct

A white crystalline addition product formed when sodium bisulfite (NaHSO₃) reacts with an aldehyde or methyl ketone by nucleophilic addition.

  • Nucleophile: HSO₃⁻ (bisulfite ion)
  • Electrophile: Carbonyl carbon of acetaldehyde
  • Product: CH₃CH(OH)SO₃Na — white crystalline solid
  • Reversibility: Dissolves on addition of dilute acid or dilute alkali

Condensation Reaction

A reaction in which two molecules combine to form a larger product with simultaneous elimination of a small molecule — usually water.

  • In Brady’s Test: Acetaldehyde + 2,4-DNPH → yellow-orange hydrazone + H₂O (m.p. 168°C)
  • In semicarbazone formation: Acetaldehyde + semicarbazide → semicarbazone + H₂O (m.p. 162–163°C)
  • In oxime formation: Acetaldehyde + hydroxylamine → acetaldoxime + H₂O (m.p. 47°C)

Oxidising Agent

A substance that accepts electrons from another substance is called an oxidising agent. The oxidising agent is always reduced in the reaction.

  • In Brady’s Test: Acetaldehyde + 2,4-DNPH → yellow-orange hydrazone + H₂O (m.p. 168°C)
  • In semicarbazone formation: Acetaldehyde + semicarbazide → semicarbazone + H₂O (m.p. 162–163°C)
  • In oxime formation: Acetaldehyde + hydroxylamine → acetaldoxime + H₂O (m.p. 47°C)

Reducing Agent

A substance that provides electrons to another substance is called a reducing agent. The reducing agent is always oxidised in the reaction.

  • Tollens’ Test: Acetaldehyde provides electrons to Ag⁺ → acetaldehyde is oxidised to acetate and acts as the reducing agent · Ag⁺ accepts electrons → Ag⁺ is reduced to Ag↓ and acts as the oxidising agent
  • Fehling’s Test: Acetaldehyde provides electrons to Cu²⁺ → acetaldehyde is oxidised to acetate and acts as the reducing agent · Cu²⁺ accepts electrons → Cu²⁺ is reduced to Cu⁺ and acts as the oxidising agent
  • Benedict’s Test: Acetaldehyde provides electrons to Cu²⁺ → acetaldehyde is oxidised to acetate and acts as the reducing agent · Cu²⁺ accepts electrons → Cu²⁺ is reduced to Cu⁺ and acts as the oxidising agent
  • Note: Acetone does not provide electrons → gives negative Tollens’, Fehling’s, and Benedict’s → acetone does not act as a reducing agent

Oxidation-Reduction (Redox)

A reaction involving simultaneous transfer of electrons — one species loses electrons (oxidation) and another gains electrons (reduction).

  • Oxidation: Loss of electrons — acetaldehyde loses electrons → oxidised to acetate
  • Reduction: Gain of electrons — Ag⁺ gains electrons → reduced to Ag↓ (Tollens’) · Cu²⁺ gains electrons → reduced to Cu⁺ (Fehling’s/Benedict’s)
  • Memory aid: OIL RIG — Oxidation Is Loss · Reduction Is Gain

Example — Benedict’s Test (balanced equation):
CH₃CHO + 2Cu²⁺ + 5OH⁻ → CH₃COO⁻ + Cu₂O↓ + 3H₂O

  • Oxidised: CH₃CHO (acetaldehyde) → CH₃COO⁻ (acetate) — acetaldehyde is the reducing agent
  • Reduced: Cu²⁺ → Cu⁺ (Cu₂O, brick-red precipitate) — Cu²⁺ is the oxidising agent

Base

A substance that accepts a proton (H⁺) from another substance is called a base.

  • In Iodoform Test: NaOH acts as a base — abstracts the α-hydrogen from CH₃CHO to form an enolate, enabling successive iodination of the methyl group (intermediate: CI₃CHO) → final cleavage releases CHI₃↓
  • Note: In the Iodoform Test, OH⁻ acts as both a base (abstracts α-H) and a nucleophile (cleaves C–C bond) — two different roles in the same reaction

Nucleophile

An electron-rich species that donates an electron pair to an electrophilic centre is called a nucleophile.

  • In Iodoform Test: OH⁻ acts as a nucleophile — attacks the carbonyl carbon of the triiodinated intermediate (CI₃CHO), cleaving the C–C bond and releasing CHI₃↓
  • In Bisulfite Test: HSO₃⁻ acts as a nucleophile — attacks the carbonyl carbon of acetaldehyde to form the bisulfite adduct (CH₃CH(OH)SO₃Na)
  • Note: OH⁻ acts as both a base (abstracts α-H) and a nucleophile (cleaves C–C bond) in the Iodoform Test — two different roles in the same reaction

Oxidation of Acetaldehyde

Acetaldehyde is readily oxidised to acetic acid (ethanoic acid) — or acetate in alkaline medium.

Reaction

Oxidising Agent

Reduced to

Product from Acetaldehyde

Tollens’ Test

Ag⁺ (silver ion)

Ag↓ (silver mirror)

CH₃COO⁻ (acetate)

Fehling’s Test

Cu²⁺ (copper II)

Cu⁺ (Cu₂O↓)

CH₃COO⁻ (acetate)

Benedict’s Test

Cu²⁺ (copper II)

Cu⁺ (Cu₂O↓)

CH₃COO⁻ (acetate)

Industrial

O₂ (oxygen)

H₂O

CH₃COOH (acetic acid)

In every case, acetaldehyde is the reducing agent — it donates electrons and is converted to acetate/acetic acid.

§21 — Real-Case Problems: Distinguishing Acetaldehyde from Similar Compounds

The Tollens’ Test, Iodoform Test, and Brady’s Test are powerful tools — but several compounds share identical results in one or more of these tests. Correct identification of acetaldehyde requires knowing which single test resolves each ambiguity. Three real-case diagnostic problems are presented below:

  • (a) Acetaldehyde vs Acetone —both neutral · both Brady’s Test positive · both Iodoform Test positive · both Sodium Bisulfite Test positive
  • (b) Acetaldehyde vs Benzaldehyde—both neutral · both Brady’s Test positive · both Tollens’ Test positive
  • (c) Acetone vs Ethanol and Isopropanol (all neutral · all water-miscible · all Iodoform Test positive)

Problem (a): Acetone vs Acetaldehyde

Acetaldehyde and acetone share identical observations in the Litmus Test, Brady’s Test, Iodoform Test, and Sodium Bisulfite Test. What is the chemical test to distinguish between acetaldehyde and acetone?

  • Litmus Test → neutral for both
  • Brady’s Test (2,4-DNPH) → positive for both — yellow-orange precipitate
  • Iodoform Test → positive for both — yellow CHI₃ precipitate
  • Sodium Bisulfite Test → positive for both — white crystalline precipitate

Although acetaldehyde and acetone respond identically to all four tests above, none of these tests can serve as the basis for differentiation.

Tollens’ Test resolves the ambiguity — acetaldehyde reduces Tollens’ reagent to give a silver mirror (positive), while acetone gives no silver mirror (negative).

Test

Acetone

Acetaldehyde

Conclusion

Tollens’ Test

No silver mirror — negative

Silver mirror formed — positive

Acetaldehyde is an aldehyde — acetone is a ketone

Problem (b): Acetone vs Acetophenone

Acetaldehyde and benzaldehyde share identical observations in Brady’s Test and Tollens’ Test. How can these two compounds be differentiated?

  • Brady’s Test (2,4-DNPH) → positive for both
  • Tollens’ Test → positive for both — silver mirror

Fehling’s Test resolves the ambiguity — acetaldehyde (aliphatic aldehyde) gives a brick-red Cu₂O precipitate (positive), while benzaldehyde (aromatic aldehyde) gives no precipitate (negative).

Test

Acetaldehyde

Benzaldehyde

Conclusion

Fehling’s Test

Brick-red precipitate — positive

No precipitate — negative

Acetaldehyde is aliphatic · benzaldehyde is aromatic

Ignition / Flame Test

Clean blue flame — no soot

Sooty flame

Aliphatic vs aromatic character confirmed

Iodoform Test

Yellow CHI₃ precipitate — positive

No precipitate — negative

CH₃CHO unit confirmed in acetaldehyde only

Problem (c): Acetaldehyde vs Formaldehyde

Acetaldehyde and formaldehyde share identical observations in Brady’s Test, Tollens’ Test, Fehling’s Test, and Benedict’s Test. How can these two compounds be differentiated?

  • Brady’s Test → positive for both
  • Tollens’ Test → positive for both — silver mirror
  • Fehling’s Test → positive for both — brick-red precipitate
  • Benedict’s Test → positive for both — brick-red precipitate

Iodoform Test resolves the ambiguity — acetaldehyde gives a yellow CHI₃ precipitate (positive) because it contains the CH₃CHO unit, while formaldehyde gives no precipitate (negative) because it has no methyl group.

Test

Acetaldehyde

Formaldehyde

Conclusion

Iodoform Test

Yellow CHI₃ precipitate — positive

No precipitate — negative

CH₃CHO unit present in acetaldehyde — absent in formaldehyde

Schiff’s Test

Magenta colour — positive

Magenta colour — positive

No differentiation — both positive

Legal’s Test

Red colour — positive

No colour — negative

Methyl carbonyl unit in acetaldehyde only

§24 — Practice Exercise

Test your understanding of the qualitative identification of acetaldehyde with the two exercises below. Part A tests recall — Part B tests reasoning.

Part A — Complete the Summary Table

The table below is partially completed. Fill in the missing observations and conclusions based on what you have learned in this article.

#

Test

Observation

Conclusion

1

Physical Appearance

Coloured compounds — absent

2

Litmus Test

Neutral — no colour change

3

Brady’s Test (2,4-DNPH)

Carbonyl group (C=O) — confirmed

4

Tollens’ Test

Aldehyde confirmed · ketones absent

5

Fehling’s Test

Brick-red precipitate — positive

6

Schiff’s Test

Aldehyde confirmed · ketones absent

7

Iodoform Test

Yellow precipitate, antiseptic odour

8

Legal’s Test

Methyl ketone — confirmed

9

Molisch’s Test

No purple ring — negative

10

Semicarbazone derivative

m.p. 162–163°C

Part B — Identify the Unknown

  • Litmus Test → neutral
  • Ignition Test → clean, non-sooty blue flame
  • Lassaigne’s Test → nitrogen, sulfur, and halogens all absent
  • Brady’s Test (2,4-DNPH) → yellow-orange crystalline precipitate
  • Tollens’ Test → bright silver mirror — positive
  • Fehling’s Test → brick-red precipitate — positive
  • Iodoform Test → yellow CHI₃ precipitate — positive
  • Semicarbazone derivative → m.p. 162–163°C

Question: What is the unknown compound? Which single test distinguishes it from acetone? Which single test distinguishes it from benzaldehyde? Which single test distinguishes it from formaldehyde?

Answer: The unknown compound is acetaldehyde (ethanal). Tollens’ Test distinguishes it from acetone — silver mirror positive for acetaldehyde, negative for acetone. Fehling’s Test distinguishes it from benzaldehyde — positive for acetaldehyde, negative for benzaldehyde. Iodoform Test distinguishes it from formaldehyde — positive for acetaldehyde, negative for formaldehyde.

§25 — Acetaldehyde Questions and Answers: Viva Questions on Qualitative Identification of Acetaldehyde

The IUPAC name of acetaldehyde is ethanal — derived from the two-carbon chain eth- and the aldehyde suffix -al. Acetaldehyde is the retained common name; ethanal is the preferred systematic name (CAS 75-07-0).

The acetaldehyde molecular formula is CH₃CHO — also written as C₂H₄O. The acetaldehyde molecular weight is 44.05 g·mol⁻¹.

No. The Cannizzaro reaction requires an aldehyde with no α-hydrogen — such as formaldehyde or benzaldehyde. Acetaldehyde has three α-hydrogens on the CH₃ group and undergoes aldol condensation instead.

Yes. Acetaldehyde gives a positive iodoform test — a yellow crystalline precipitate of iodoform (CHI₃) with a characteristic antiseptic odour. This confirms the presence of the CH₃CHO unit, which is essential for the iodoform reaction.

Tollens’ Test and Fehling’s Test both distinguish acetaldehyde from acetone — acetaldehyde gives a silver mirror (Tollens’) and a brick-red precipitate (Fehling’s), while acetone gives negative results in both. Both give positive Brady’s Test and positive Iodoform Test — so neither can be used for this distinction.

Acetaldehyde is an aldehyde — not a ketone. It contains the –CHO functional group in which the carbonyl carbon is bonded directly to a hydrogen atom. Ketones have two carbon substituents on the carbonyl carbon with no aldehydic hydrogen.

The acetaldehyde smell is pungent and fruity — often described as similar to green apples or overripe fruit. It is detectable at concentrations as low as 0.0068 ppm, making it one of the most odour-active aldehydes.

Acetaldehyde is a liquid at room temperature — boiling point 21°C. However, it is extremely volatile — vapours form rapidly in air, and it behaves practically as a gas under warm laboratory conditions.

The confirmatory test for acetaldehyde is the Tollens’ Test (silver mirror) — confirming aldehyde character — supported by the Iodoform Test (yellow CHI₃ precipitate) and Legal’s Test (red colour), both confirming the CH₃CHO unit specifically.

Acetaldehyde gives a positive Tollens’ test because it contains the –CHO group — the aldehydic hydrogen makes it a reducing agent that reduces Ag⁺ to Ag↓ (silver mirror). Acetone is a ketone with no aldehydic hydrogen — it cannot reduce Tollens’ reagent and gives a negative result.

§26 — Multiple Choice Questions on the Qualitative Identification of Acetaldehyde

MCQ 1

1. The acetaldehyde IUPAC name is:

A. Ethanone

B. Methanal

C. Ethanal ✓

D. Propanal

  • Ethanal is the IUPAC preferred name — derived from eth- and -al (CAS 75-07-0).

MCQ 2

2. The acetaldehyde molecular formula is:

A. CH₃COCH₃ 

B. CH₃CHO ✓

C. HCHO

D. C₂H₅OH

  • Acetaldehyde is CH₃CHO (C₂H₄O) — molar mass 44.05 g·mol⁻¹. It is the simplest aliphatic aldehyde.

MCQ 3

3.  The confirmatory test for acetaldehyde that distinguishes it from all ketones is:

A. Brady’s Test

B. Sodium Bisulfite Test

C. Tollens’ Test ✓

D. Litmus Test

  • Tollens’ Test gives a bright silver mirror with acetaldehyde (positive) but no mirror with ketones (negative).

MCQ 4

4. Acetaldehyde gives a positive iodoform test because it contains:

A. A benzene ring

B. A hydroxyl group (–OH)

C. The CH₃CHO unit ✓

D. A carboxylic acid group (–COOH)

  • Only compounds containing the CH₃C=O unit give a yellow CHI₃ precipitate with iodine and NaOH.

MCQ 5

5. The acetaldehyde boiling point is 21°C. This low value indicates:

A. Acetaldehyde is a solid at room temperature

B. Acetaldehyde forms strong hydrogen bonds between its own molecules

C. Acetaldehyde is extremely volatile — fume cupboard handling is mandatory ✓

D. Acetaldehyde is ionic in nature

  • The boiling point of 21°C means acetaldehyde vaporises at or near room temperature.

MCQ 6

6. Which test distinguishes acetaldehyde from benzaldehyde?

A. Tollens’ Test — both give positive results

B. Fehling’s Test — acetaldehyde positive, benzaldehyde negative ✓

C. Brady’s Test — both give positive results

D. Litmus Test — both are neutral

  • Fehling’s Test is positive for aliphatic aldehydes (acetaldehyde) and negative for aromatic aldehydes (benzaldehyde).

MCQ 7

7. Which test distinguishes acetaldehyde from formaldehyde?

A. Tollens’ Test — both give positive results

B. Fehling’s Test — both give positive results

C. Iodoform Test — acetaldehyde positive, formaldehyde negative ✓

D. Schiff’s Test — both give positive results

  • Acetaldehyde contains the CH₃CHO unit → positive iodoform. Formaldehyde (HCHO) has no methyl group → negative iodoform.

MCQ 8

8. In Tollens’ Test, acetaldehyde acts as:

A. An oxidising agent

B. A reducing agent ✓

C. A nucleophile

D. A catalyst

  • Acetaldehyde donates electrons to Ag⁺ → oxidised to acetate and acts as the reducing agent · Ag⁺ is reduced to Ag↓ (silver mirror).

MCQ 9

9. Can acetaldehyde give the Cannizzaro reaction?

A. No — acetaldehyde has α-hydrogens and undergoes aldol condensation instead ✓

B. Yes — all aldehydes give the Cannizzaro reaction

C. IYes — but only in dilute NaOH

D. No — because acetaldehyde is a liquid

  • The Cannizzaro reaction requires aldehydes with no α-hydrogens. Acetaldehyde has three α-hydrogens and undergoes aldol condensation preferentially.

MCQ 10

10. The acetaldehyde smell is best described as:

A. Sweet, floral odour similar to roses

B. Sharp, vinegar-like odour

C. Pungent, fruity odour detectable at 0.0068 ppm ✓

D. Odourless

  • Acetaldehyde has a characteristic pungent, fruity odour — detectable at very low concentrations.

MCQ 11

11. What colour precipitate does acetaldehyde produce in Fehling’s Test?

A. White precipitate

B. Yellow precipitate

C. Blue precipitate

D. Brick-red precipitate ✓

  • Fehling’s Test produces a brick-red precipitate of Cu₂O — acetaldehyde reduces Cu²⁺ (blue) to Cu⁺ (brick-red Cu₂O). Ketones give no precipitate.

MCQ 12

12. The acetaldehyde pKa of the α-hydrogen is:

A. 10 — same as phenol

B. 20 — same as acetone

C. 17 — more acidic than acetone ✓

D. 25 — same as terminal alkynes

  • The α-hydrogen pKa of acetaldehyde is 17 — more acidic than acetone (pKa = 20).

MCQ 13

13. In Legal’s Test, acetaldehyde gives a positive result because it contains:

A. A benzene ring

B. A carboxylic acid group

C. The methyl carbonyl unit (CH₃C=O) ✓

D. A hydroxyl group

  • Legal’s Test (sodium nitroprusside + NaOH) is specific to the CH₃C=O unit — positive for acetaldehyde and methyl ketones, negative for most other aldehydes.

MCQ 14

14. Acetaldehyde is classified as which GHS hazard?

A. WARNING — irritant only

B. CAUTION — low flammability

C. DANGER — extremely flammable, suspected carcinogen (Carc. 1B) ✓

D. No GHS classification required

  • Acetaldehyde carries GHS signal word DANGER — flash point −38.89°C and Carc. 1B (H350) carcinogen classification.

§27 — Further Readings and References

[1] Carey, F. A., & Sundberg, R. J. (2007). Advanced organic chemistry, Part A: Structure and mechanisms (5th ed., p. 607). Springer.
[2] chemistrysh.com. (2024). Iodoform test: Procedure, mechanism, and applications. https://chemistrysh.com/iodoform-test/
[3] Shriner, R. L., Hermann, C. K. F., Morrill, T. C., Curtin, D. Y., & Fuson, R. C. (2004). The systematic identification of organic compounds (8th ed.). Wiley.
[4] Sigma-Aldrich. (n.d.). Acetaldehyde, ACS reagent, ≥99.5% (Product No. 402788). MilliporeSigma. https://www.sigmaaldrich.com/catalog/product/sigma/402788
[5] Vogel, A. I., Tatchell, A. R., Furnis, B. S., Hannaford, A. J., & Smith, P. W. G. (1989). Vogel’s textbook of practical organic chemistry (5th ed.). Longman.

Suggestions for Further Readings

THE ODOROUS CONSTITUENTS OF APPLES. EMANATION OF ACETALDEHYDE FROM THE RIPE FRUIT.

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