Resorcinol Identification: Structure and Chemical Tests

Resorcinol Identification: Key Facts at a Glance

Molecular Formula (C₆H₆O₂) · Molecular Weight (110.11 g/mol) · IUPAC Name (Benzene-1,3-diol) · Common Name (Resorcinol) · Synonym (Resorcin) · Appearance (white crystalline solid; turns pink on exposure to air, light, or iron) · Melting Point (109–111 °C) · Boiling Point (277 °C) · Density (1.28 g/cm³) · Solubility (freely soluble in water and ethanol; insoluble in chloroform) · pKa (9.15) · CAS Number (108-46-3) · Functional Group (two phenolic –OH groups at C1 and C3)
Ignition (Flame) Test · Solubility Test · Litmus Test · Solubility in NaOH · NaHCO₃ Test · FeCl₃ Test (blue-violet) · Bromine Water Test (white precipitate) · Liebermann’s Test (blue → red → blue-green) · Phthalein Dye Test (yellow-green fluorescence) · Azo Dye Test (orange-red) · Seliwanoff’s Test (indirect confirmation) · Confirmatory Tests 1, 2 & 3 · Control Test · Mixed Melting Point (109–111 °C) · Resorcinol Dibenzoate (m.p. 117 °C) · Chemical Structure · Resonance · Chemical Reactions · Glossary · Comparison with Related Compounds · Applications · FAQs · MCQs

Resorcinol physical constants chart showing the benzene-1,3-diol structure with hydroxyl groups at positions 1 and 3, molecular formula C₆H₆O₂, molecular weight 110.11 g/mol, CAS 108-46-3, melting point 109–111 °C, boiling point 277 °C, pKa 9.15 and density 1.28 g/cm³. — Physical constants of resorcinol: meta-dihydroxybenzene structure alongside the thermal, acidity, density and solubility values used in laboratory identification.

What Is Resorcinol?

Resorcinol (pronounced reh-ZOR-sin-ol) is an organic compound belonging to the class of dihydric phenols — phenols that carry two hydroxyl (–OH) groups attached to a benzene ring. Its IUPAC name is benzene-1,3-diol, and it is also widely known by its older name resorcin

In resorcinol, the two –OH groups are positioned at carbons 1 and 3 of the benzene ring, making it the meta isomer of dihydroxybenzene. This places it in a group of three structural isomers: catechol (1,2-positions), resorcinol (1,3-positions), and hydroquinone (1,4-positions).

Resorcinol is classified as a phenol because its –OH groups are directly bonded to the aromatic ring. Like all phenols, it is weakly acidic and shows characteristic reactions with ferric chloride, bromine water, and diazonium salts — all of which form the basis of its qualitative identification.

Physical Constants of Resorcinol (Molecular Formula, Melting Point, Boiling Point & Colour)

Resorcinol, also known as resorcin, 1,3-benzenediol, 1,3-dihydroxybenzene, m-dihydroxybenzene, m-benzenediol, and m-hydroquinone (CAS 108-46-3), is a white crystalline solid — turning pink on prolonged exposure to air, light, or iron — with a molecular formula of C₆H₆O₂, a molecular weight of 110.11 g/mol, and a density of 1.28 g/cm³. It melts at 109–111 °C and boils at 277 °C under atmospheric pressure (178 °C at 16 mmHg under reduced pressure).

Resorcinol is freely soluble in water, ethanol, and ether, but insoluble in chloroform and carbon disulfide. With a pKa of 9.15, it is weakly acidic — consistent with its classification as a dihydric phenol.

Property	Value
IUPAC Name	Benzene-1,3-diol
Common Name	Resorcinol
Synonym	Resorcin · 1,3-Benzenediol · 1,3-Dihydroxybenzene · m-Dihydroxybenzene · m-Benzenediol · m-Hydroquinone
Molecular Formula	C₆H₆O₂
Molecular Weight	110.11 g/mol
CAS Number	108-46-3
Appearance	White crystalline solid (powder and chunks)
Colour	Colourless to white; turns pink on exposure to air, light, or iron
Odour	Faint, characteristic
Melting Point	109–111 °C
Boiling Point	277 °C (atmospheric); 178 °C at 16 mmHg (reduced pressure)
Density	1.28 g/cm³
Solubility in Water	110 g/100 mL at 20 °C (freely soluble)
Solubility in Ethanol	Freely soluble
Solubility in Ether	Freely soluble
Insoluble in	Chloroform; carbon disulfide
pKa	9.15 (weakly acidic)
Vapour Pressure	1 mmHg at 21.1 °C
Flash Point	127 °C (closed cup)

Planning the Identification — Reading Resorcinol’s Structure

Resorcinol (1,3-benzenediol) has a simple but chemically informative structure: two hydroxyl (–OH) groups bonded directly to a benzene ring at the 1 and 3 positions (meta). This single structural reading tells us three things immediately — the compound is a phenol (–OH on aromatic ring), a dihydric phenol (two –OH groups), and carries an activated aromatic ring where positions 2, 4, and 6 are electron-rich and reactive toward electrophiles. The functional groups present in resorcinol directly predict every test in the identification panel below.

What does the structure of resorcinol tell us about its identification?

The structure of resorcinol predicts its identification tests directly:

Phenolic –OH groups (bonded to aromatic ring) → soluble in NaOH; positive FeCl₃ test; positive Liebermann’s test
Two –OH groups (dihydric phenol) → greater polarity than simple phenol due to two hydrogen-bonding sites → freely soluble in water; no effervescence with NaHCO₃.
Meta positions (1,3) → positions 2, 4, and 6 activated → white precipitate with bromine water; yellow-green fluorescence in phthalein dye test.
Aromatic ring → sooty, luminous flame on ignition

Why Are Positions 2, 4, and 6 of Resorcinol More Reactive?

In resorcinol, both –OH groups are electron-donating and direct incoming electrophiles to the ortho and para positions relative to themselves. Since the two –OH groups are at positions 1 and 3, their combined directing effects activate positions 2, 4, and 6 of the ring. The resonance structure of resorcinol shows that electron density is highest at these three positions, making them the preferred sites for electrophilic substitution reactions such as bromination and nitrosation.

This explains why bromine water reacts at all three activated positions to give 2,4,6-tribromoresorcinol, and why Liebermann’s nitrosation occurs specifically at the free para position (C-4).

Preliminary Tests (Ignition / Flame Test, Solubility Test, Acidity Tests: Litmus, NaOH & NaHCO₃) — Establishing the Phenol Class

Preliminary tests classify an unknown compound on three parameters: aromatic or non-aromatic character (Ignition Test); polarity — polar, non-polar, or intermediate (solubility in water, ethanol, or hexane); and acid-base nature — acidic, basic, or neutral, and if acidic, whether strong or weak (Litmus Test, NaOH Test, NaHCO₃ Test). These three parameters define the compound class before specific chemical tests begin.

i. Ignition (Flame) Test

Procedure: Hold a small amount of resorcinol on a spatula and ignite it in a Bunsen flame.
Observation: Burns with a sooty, luminous flame.
Conclusion: Confirms aromatic character — the benzene ring is present. The high carbon-to-hydrogen ratio of aromatic compounds produces incomplete combustion, resulting in the characteristic sooty flame.

ii. Solubility Test (Water & Ethanol)

Procedure: Add a small amount of resorcinol to water in a test tube. Shake. Repeat with ethanol.
Observation: Resorcinol dissolves freely in both water and ethanol.
Conclusion: Confirms the presence of polar –OH groups capable of hydrogen bonding with the solvent. Resorcinol is more soluble in water than simple phenol due to its two –OH groups, which increase polarity and the number of hydrogen-bonding sites.

iii. Acidity Tests

(a) Litmus Test

Procedure: Place a drop of aqueous resorcinol solution on blue litmus paper.
Observation: Blue litmus turns red.
Conclusion: Confirms weak acidity — consistent with phenolic –OH character. Resorcinol does not turn red litmus blue, confirming it is not basic.

(b) Solubility in NaOH (Sodium Hydroxide Test)

Procedure: Add resorcinol to dilute aqueous NaOH solution.
Observation: Dissolves readily, forming a clear solution of sodium resorcinate.
Conclusion: Confirms phenolic character. The acidic –OH groups react with NaOH to form the water-soluble sodium salt:

C₆H₄(OH)₂ + 2NaOH → C₆H₄(ONa)₂ + 2H₂O

(c) NaHCO₃ Test (Sodium Bicarbonate Test)

Procedure: Add resorcinol to aqueous sodium bicarbonate (NaHCO₃) solution.
Observation: Resorcinol dissolves without producing CO₂ gas.
Conclusion: Rules out carboxylic acid — only acids stronger than carbonic acid (pKa 6.35) react with NaHCO₃ to release CO₂. Since resorcinol has a pKa of 9.15, it is too weak to displace CO₂ from bicarbonate. Combined with NaOH solubility, this confirms phenolic –OH rather than –COOH.

“The ignition test indicates the presence of an aromatic system. Solubility in water and ethanol confirms polar –OH groups. Solubility in sodium hydroxide (NaOH) without effervescence with NaHCO₃ rules out carboxylic acid and establishes phenolic character. The given compound belongs to the phenol class.”

Important Note about the Acidity of Resorcinol: Resorcinol is a weak acid. With a pKa of 9.15, it is more acidic than simple alcohols (pKa ~16) but less acidic than carboxylic acids (pKa ~4–5). Its acidic nature arises from the ability of the phenoxide ion formed after proton loss to stabilise the negative charge by resonance across the aromatic ring. The presence of two –OH groups makes resorcinol slightly more acidic than phenol (pKa 9.99) due to the combined electron-withdrawing inductive effect of the second –OH group.

Chemical Tests for Resorcinol: FeCl₃, Bromine Water, Liebermann’s, Phthalein Dye, Confirmatory Tests and Other Tests

The qualitative identification of resorcinol is based on its two key structural features — the phenolic –OH groups and the activated aromatic ring. The tests below cover class confirmation, group-specific colour reactions, confirmatory tests, and solid derivative preparation. A control test using authentic resorcinol (m.p. 109–111 °C, CAS 108-46-3) must be run alongside all colour-producing tests simultaneously.

i. Ferric Chloride (FeCl₃) Test

Procedure: Add a few drops of neutral ferric chloride solution to an aqueous solution of resorcinol.
Observation: A blue-violet colour develops.
Conclusion: Confirms the presence of a phenolic –OH group. The blue-violet colour results from the formation of an iron(III)–phenolate complex. This colour is characteristic of resorcinol and distinguishes it from catechol (green) and hydroquinone (dark green to black).

Note: Always use a freshly prepared, neutral FeCl₃ solution. An acidic solution suppresses complex formation and may give a false negative. Run a control test with authentic resorcinol alongside the unknown sample.

ii. Bromine Water Test for Resorcinol

Procedure: Add bromine water dropwise to an aqueous solution of resorcinol.
Observation: The bromine water is decolourised immediately and a white precipitate forms.
Conclusion: Confirms an activated aromatic ring. Bromine undergoes electrophilic aromatic substitution at all three activated positions (2, 4, and 6) to give 2,4,6-tribromoresorcinol as a white precipitate. The reaction requires no catalyst — the two –OH groups activate the ring sufficiently for substitution under mild conditions.

C₆H₄(OH)₂ + 3Br₂ → C₆HBr₃(OH)₂ + 3HBr

(Resorcinol + 3 bromine → 2,4,6-tribromoresorcinol + 3 hydrobromic acid)

Note: Positions 1 and 3 carry the –OH groups and are not available for substitution. Only positions 2, 4, and 6 are brominated — hence 3 moles of Br₂ and the tribromo product. Decolourisation alone is not diagnostic; the white precipitate is the key observation.

iii. Liebermann’s Test

Procedure: Dissolve a small amount of resorcinol in concentrated H₂SO₄. Add a crystal of sodium nitrite (NaNO₂). Note the colour. Then pour carefully into water and make alkaline with NaOH.
Observation:
• In H₂SO₄ — blue colour develops
• On dilution with water — turns red
• On addition of NaOH (alkaline) — turns blue-green
Conclusion: Confirms phenolic character. Resorcinol gives a positive result because it has a free para position (position 4, para to the –OH at C-1). Nitrosation occurs at this free para position, forming a nitroso intermediate that tautomerises to a quinone-oxime, producing the characteristic colour changes.

Note: A free para position is the structural requirement for a positive Liebermann’s test — compounds with a blocked para position do not respond. Among the dihydroxyphenols, only resorcinol gives a satisfactory positive result; catechol and hydroquinone do not respond reliably.

iv. Phthalein Dye Test

Procedure: Heat resorcinol with phthalic anhydride in the presence of concentrated H₂SO₄ as catalyst. Cool, dissolve the product in water, and make alkaline with NaOH.
Observation: Yellow-green fluorescence in alkaline solution.
Conclusion: Confirms resorcinol specifically. The reaction of resorcinol with phthalic anhydride under acidic conditions produces fluorescein — a highly fluorescent xanthene dye. This yellow-green fluorescence in alkaline solution is the characteristic result and distinguishes resorcinol from phenol (pink phthalein) and the naphthols.

2 C₆H₄(OH)₂ + C₈H₄O₃ → C₂₀H₁₂O₅ + 2H₂O

(2 × Resorcinol + phthalic anhydride → fluorescein + 2 water)

Note: The yellow-green fluorescence is visible even at very high dilution — this makes the phthalein dye test one of the most sensitive tests for resorcinol.

v. Azo Dye Test

Procedure:
1. Prepare benzene diazonium chloride (aniline + dilute HCl + NaNO₂ added dropwise with stirring, 0–5 °C maintained with crushed ice, 3 minutes). Test with KI-starch paper — a blue-violet colour confirms diazotization is complete.
2. Add alkaline resorcinol solution to the cold benzene diazonium chloride solution.
Observation: An orange-red azo dye forms immediately.
Conclusion: Confirms an activated aromatic ring. The diazonium ion couples at the free para position (C-4) of resorcinol to give a coloured azo dye — an electrophilic aromatic substitution under mild alkaline conditions at 0–5 °C.

C₆H₅N₂⁺Cl⁻ + C₆H₄(OH)₂ → C₆H₅–N=N–C₆H₃(OH)₂ + HCl

Note: Keep temperature at 0–5 °C — diazonium salt decomposes above this range.

vi. Confirmatory Tests for Resorcinol

Confirmatory Test 1

Procedure: Dissolve 0.1 g resorcinol in 1 ml water. Add 1 ml strong NaOH solution and 0.1 ml (approx. 2 drops) chloroform. Heat and allow to cool.
Observation: An intense deep-red colour develops. On addition of a slight excess of dilute HCl, the colour turns pale yellow.

Confirmatory Test 2

Procedure: Mix resorcinol with potassium hydrogen phthalate and heat. Cool, then add dilute NaOH.
Observation: Intense green fluorescence develops in alkaline solution.

Confirmatory Test 3

Procedure: Dissolve resorcinol in ethanol. Add NaOH and CH₂Cl₂ (dichloromethane).
Observation: Green fluorescence develops.

Control Test

Run all three confirmatory tests simultaneously with the unknown sample and authentic resorcinol (m.p. 109–111 °C, CAS 108-46-3). All observations must match across both samples to confirm identity.

vii. Seliwanoff’s Test

Reagent: Dissolve 50 mg resorcinol in 33 ml concentrated HCl and make up to 100 ml with distilled water.
Procedure: Add 1 ml test sample to 2 ml Seliwanoff’s reagent. Heat in boiling water bath for 1–2 minutes.
Observation: Cherry-red colour develops.
Conclusion: Confirms resorcinol. Do not over-heat — prolonged heating gives false positive with aldoses.

Note: In Seliwanoff’s test, resorcinol serves as the analytical reagent, not the analyte. A solution of the unknown compound is prepared in dilute HCl and tested against fructose (a ketose). A cherry-red colour confirms that the unknown compound functions as Seliwanoff’s reagent — thereby confirming the presence of resorcinol indirectly.

viii. Control Test

What is a Control Test?

A control test is a parallel experiment run simultaneously with the unknown sample using a known authentic standard of the compound under investigation. Its purpose is to provide a direct visual comparison of results under identical conditions — same reagents, same temperature, same timing, same analyst. Without a control test, a colour observation alone cannot be considered conclusive.

Procedure: Run the following tests simultaneously with the unknown sample and authentic resorcinol (m.p. 109–111 °C, CAS 108-46-3):
• FeCl₃ test
• Liebermann’s test
• Phthalein dye test
• Azo dye test
• Confirmatory Tests 1, 2, and 3
Confirmation: All colours and fluorescence results must match the authentic standard simultaneously. Identity of resorcinol is confirmed only when all results match across every test.

ix. Mixed Melting Point

Procedure: Mix a small amount of the unknown sample with an equal quantity of authentic resorcinol (m.p. 109–111 °C, CAS 108-46-3). Determine the melting point of the mixture using a melting point apparatus.
Observation: Melting point of the mixture — 109–111 °C.
Conclusion: No depression in melting point confirms identity of resorcinol.

x. — Solid Derivative — Resorcinol Dibenzoate (m.p. 117 °C)

Procedure: Add benzoyl chloride dropwise to a solution of resorcinol in pyridine with cooling. Allow to stand, then pour into cold water. Filter the white precipitate, wash with cold water, and recrystallise from ethanol.
Observation: White crystalline solid — m.p. 117 °C.
Conclusion: Formation of resorcinol dibenzoate confirms resorcinol identity. Both –OH groups are benzoylated. The observed melting point is compared with the reported literature value (117 °C) — agreement confirms identity.

Chemical Reactions of Resorcinol

The chemical reactions of resorcinol can be classified into five reaction types: acid-base reaction, coordination complex formation, electrophilic aromatic substitution (EAS), condensation reaction, and esterification. Each reaction type reflects a specific structural feature of resorcinol — the acidic –OH groups, the activated aromatic ring, or the free para position.

Understanding these reaction types explains the chemistry behind the qualitative identification of resorcinol and connects laboratory observations to organic chemistry theory.

1. Acid-Base Reaction — NaOH Test

Resorcinol acts as a weak Brønsted acid, donating protons from both phenolic –OH groups to sodium hydroxide. The reaction produces the water-soluble sodium resorcinate salt. This acid-base behaviour confirms the phenolic nature of resorcinol and explains its solubility in NaOH solution.

C₆H₄(OH)₂ + 2NaOH → C₆H₄(ONa)₂ + 2H₂O

Reaction type: Acid-base (proton transfer)

2. Coordination Complex Formation — FeCl₃ Test

Resorcinol acts as a ligand, donating electron pairs from the oxygen atoms of its phenolic –OH groups to Fe³⁺ (Lewis acid) to form a coloured iron(III)–phenolate coordination complex. The blue-violet colour is the observable result of this complex formation. This is not a simple colour reaction — it is coordination chemistry, where the phenolic oxygen acts as an electron-pair donor to the metal centre.

Reaction type: Coordination chemistry (Lewis acid-base / complex formation)

3. Electrophilic Aromatic Substitution (EAS) — Bromination

Bromine (Br⁺) acts as the electrophile, attacking positions 2, 4, and 6 of the activated ring — no catalyst required. All three positions are substituted simultaneously to give 2,4,6-tribromoresorcinol.

C₆H₄(OH)₂ + 3Br₂ → C₆HBr₃(OH)₂ + 3HBr

Reaction type: Electrophilic aromatic substitution — halogenation

4. Electrophilic Aromatic Substitution (EAS) — Nitrosation

The nitrosonium ion (NO⁺), generated from NaNO₂ and concentrated H₂SO₄, acts as the electrophile. It attacks the free para position (C-4) of resorcinol — the most electron-rich available position. The initial nitroso product tautomerises to a quinone-oxime, which undergoes further colour changes with water and NaOH to give the characteristic blue → red → blue-green sequence observed in Liebermann’s test.

Reaction type: Electrophilic aromatic substitution — nitrosation

5. Electrophilic Aromatic Substitution (EAS) — Diazonium Coupling

The benzene diazonium ion (C₆H₅N₂⁺) acts as a weak electrophile. It couples at the free para position (C-4) of resorcinol under mild alkaline conditions. Alkaline conditions are essential — they convert resorcinol to its phenoxide ion form, which is a stronger electron donor, making the ring more reactive toward the weakly electrophilic diazonium ion. The product is a coloured azo dye containing the characteristic –N=N– chromophore.

C₆H₅N₂⁺Cl⁻ + C₆H₄(OH)₂ → C₆H₅–N=N–C₆H₃(OH)₂ + HCl

Reaction type: Electrophilic aromatic substitution — diazonium coupling

6. Condensation Reaction — Fluorescein Synthesis

Resorcinol reacts with phthalic anhydride in the presence of concentrated H₂SO₄ as acid catalyst. Two molecules of resorcinol condense with one molecule of phthalic anhydride with elimination of water to form fluorescein — a highly fluorescent xanthene dye. The yellow-green fluorescence observed in alkaline solution is the direct result of the extended conjugated system of the fluorescein molecule.

2 C₆H₄(OH)₂ + C₈H₄O₃ → C₂₀H₁₂O₅ + 2H₂O

Reaction type: Condensation (acid-catalysed)

7. Esterification — Solid Derivative

Benzoyl chloride (acylating agent) reacts with both –OH groups of resorcinol in pyridine — pyridine acts as base, neutralising the HCl produced. The product is resorcinol dibenzoate (m.p. 117 °C), a sharp-melting solid derivative used for identity confirmation.

C₆H₄(OH)₂ + 2C₆H₅COCl → C₆H₄(OOCC₆H₅)₂ + 2HCl

Reaction type: Esterification (acylation)

Results and Discussion: Qualitative Identification of Resorcinol

Summary of Test Results

The qualitative analysis of resorcinol yielded positive results across all identification tests. The FeCl₃ test produced a blue-violet colour, bromine water gave a white precipitate of 2,4,6-tribromoresorcinol, and the phthalein dye test gave an intense yellow-green fluorescence — the three most diagnostic observations for resorcinol. The mixed melting point confirmed 109–111 °C with no depression, and the solid derivative resorcinol dibenzoate melted at 117 °C, in agreement with reported literature values.

Test	Observation	Conclusion
Ignition (Flame) Test	Sooty, luminous flame	Aromatic compound — benzene ring present
Solubility in Water	Freely soluble	Polar –OH groups present
Solubility in Ethanol	Freely soluble	Polar –OH groups present
Litmus Test	Blue litmus turns red	Weakly acidic
NaOH Test	Dissolves — clear solution	Phenolic –OH confirmed
NaHCO₃ Test	No effervescence	Carboxylic acid ruled out
FeCl₃ Test	Blue-violet colour	Phenolic –OH — iron(III) complex
Bromine Water Test	Decolourisation + white precipitate	Activated aromatic ring confirmed
Liebermann’s Test	Blue → red → blue-green	Free para position — phenol class
Phthalein Dye Test	Yellow-green fluorescence	Fluorescein formed — resorcinol specific
Azo Dye Test	Orange-red azo dye	Coupling at C-4 confirmed
Confirmatory Test 1	Intense deep-red → pale yellow with HCl	Resorcinol confirmed
Confirmatory Test 2	Intense green fluorescence	Resorcinol confirmed
Confirmatory Test 3	Green fluorescence	Resorcinol confirmed
Seliwanoff’s Test	Cherry-red colour	Resorcinol confirmed indirectly — functions as Seliwanoff’s reagent
Mixed Melting Point	109–111 °C — no depression	Identity confirmed
Solid Derivative	White solid — m.p. 117 °C	Resorcinol dibenzoate — identity confirmed

Discussion

All physical and chemical observations are consistent with the identity of the compound as resorcinol (benzene-1,3-diol, CAS 108-46-3). The compound burned with a sooty, luminous flame and dissolved freely in water and ethanol, indicating an aromatic compound with polar –OH groups. Blue litmus turned red, confirming weak acidity. Dissolution in dilute NaOH gave a clear solution of sodium resorcinate, while no effervescence was observed with NaHCO₃ — establishing phenolic –OH and ruling out carboxylic acid. The FeCl₃ test gave a blue-violet colour, confirming iron(III)–phenolate complex formation and placing the compound in the phenol class.

Within the phenol class, bromine water gave immediate decolourisation and a white precipitate of 2,4,6-tribromoresorcinol — confirming an activated aromatic ring with three free positions at C-2, C-4, and C-6. Liebermann’s test gave the characteristic blue → red → blue-green sequence, confirming a free para position at C-4, consistent with 1,3-dihydroxy substitution. The phthalein dye test gave intense yellow-green fluorescence — fluorescein formation — specific to resorcinol and distinguishing it from phenol (pink) and the naphthols. The azo dye test gave an orange-red azo dye, confirming coupling at C-4. All three confirmatory tests matched the authentic standard simultaneously, identifying the compound as a dihydric phenol with meta –OH substitution — resorcinol.

The mixed melting point showed no depression at 109–111 °C, and the solid derivative resorcinol dibenzoate melted at 117 °C — in agreement with reported literature values. It is concluded that the given compound is resorcinol (benzene-1,3-diol, CAS 108-46-3) — a dihydric phenol with hydroxyl groups at positions 1 and 3 of the benzene ring.

Conclusion: Identification of Resorcinol Confirmed

The melting point (109–111 °C) and free solubility in water and ethanol establish the physical profile. Solubility in NaOH without effervescence with NaHCO₃ places the compound in the phenol class. The FeCl₃ test (blue-violet), bromine water test (white precipitate of 2,4,6-tribromoresorcinol), and Liebermann’s test (positive — free para position at C-4) confirm a dihydric phenol with 1,3-substitution.

The phthalein dye test (yellow-green fluorescence — fluorescein) is the single most specific test for resorcinol within the phenol class. Three confirmatory tests, mixed melting point (no depression at 109–111 °C), and the solid derivative resorcinol dibenzoate (m.p. 117 °C) provide final confirmation. The given compound is resorcinol (benzene-1,3-diol, CAS 108-46-3).

Glossary of Key Terms Used in the Article on Identification of Resorcinol

Acylating agent

A chemical reagent that introduces an acyl group (RCO–) into a molecule by reacting with a hydroxyl or amino group.
Example: In the preparation of resorcinol dibenzoate, benzoyl chloride acts as the acylating agent, reacting with both –OH groups of resorcinol in the presence of pyridine.

Acidic strength

Acidic strength is the ability of an acid to donate a proton (H⁺) in solution, measured by its pKa value — the lower the pKa, the stronger the acid.
Example: Resorcinol (pKa 9.15) is more acidic than ethanol (pKa 15.9) but less acidic than acetic acid (pKa 4.76) — placing it between alcohols and carboxylic acids on the acid strength scale.

Anthelmintic

A pharmaceutical agent that expels or destroys parasitic worms (helminths) from the body.
Example: Hexylresorcinol, a derivative of resorcinol, possesses anthelmintic properties in addition to its antiseptic activity.

Antiseptic

A substance that inhibits the growth of microorganisms on living tissue, used topically to prevent infection.
Example: Resorcinol is used as an antiseptic in topical pharmaceutical preparations for skin infections and dermatological conditions.

Benzoylation

A reaction in which a benzoyl group (C₆H₅CO–) is introduced into a molecule by reaction with benzoyl chloride in the presence of a base such as pyridine.
Example: Resorcinol undergoes benzoylation at both –OH groups to form resorcinol dibenzoate (m.p. 117 °C), used as a solid derivative for identity confirmation.

Brønsted acid

A substance that donates a proton (H⁺) to another species in a chemical reaction, as defined by the Brønsted–Lowry theory of acids and bases.
Example: Resorcinol acts as a Brønsted acid when it donates protons from both its phenolic –OH groups to sodium hydroxide, forming sodium resorcinate.

Chromophore

The part of a molecule responsible for its colour, typically a system of conjugated double bonds or specific functional groups that absorb visible light.
Example: The –N=N– group in the azo dye formed from resorcinol and benzene diazonium chloride is the chromophore responsible for its characteristic orange-red colour.

Conjugated system

A molecular system in which alternating single and double bonds allow delocalisation of π electrons across multiple atoms, resulting in enhanced stability and often visible colour or fluorescence
Example: The extended conjugated system of the fluorescein molecule — formed from resorcinol and phthalic anhydride — is responsible for its intense yellow-green fluorescence in alkaline solution.

Depigmentation

The reduction or removal of pigmentation from the skin, achieved through chemical agents that interfere with melanin production or distribution.
Example: Resorcinol is used as a mild depigmentation agent in skin-lightening formulations, working through keratolysis rather than direct inhibition of tyrosinase.

Diazotization

The reaction of a primary aromatic amine with sodium nitrite (NaNO₂) and dilute hydrochloric acid at 0–5 °C to form a diazonium salt.
Example: In the azo dye test for resorcinol, aniline undergoes diazotization with NaNO₂ and dilute HCl at 0–5 °C to form benzene diazonium chloride, which then couples with resorcinol.

Dihydroxybenzene isomers

The three structural isomers of dihydroxybenzene — catechol (1,2-positions), resorcinol (1,3-positions), and hydroquinone (1,4-positions) — sharing the molecular formula C₆H₆O₂ but differing in the position of their –OH groups.
Example: The dihydroxybenzene isomers are distinguished by their FeCl₃ colours: catechol gives green, resorcinol gives blue-violet, and hydroquinone gives dark green to black.

Electrophile

An electron-deficient species that accepts an electron pair from a nucleophile or attacks electron-rich sites such as activated aromatic rings.
Example: In the bromination of resorcinol, Br⁺ acts as the electrophile, attacking the electron-rich positions 2, 4, and 6 of the activated aromatic ring.

Fluorescence

The emission of light by a substance that has absorbed light or other electromagnetic radiation, typically at a longer wavelength than the absorbed radiation.
Example: Resorcinol produces an intense yellow-green fluorescence when it reacts with phthalic anhydride under acidic conditions to form fluorescein — visible even at very high dilution.

Inductive effect

The transmission of electronic effects through sigma bonds in a molecule, causing a shift in electron density that affects the acidity, basicity, or reactivity of nearby functional groups.
Example: The electron-withdrawing inductive effect of the second –OH group in resorcinol makes it slightly more acidic than simple phenol (pKa 9.15 vs 9.99).

Keratolytic agent

A substance that softens, loosens, and facilitates removal of the outer keratin layer of skin, used in dermatological treatments for conditions involving thickened or scaly skin.
Example: Resorcinol cream acts as a keratolytic agent in the treatment of acne, seborrheic dermatitis, and psoriasis by dissolving the excess keratin layer.

KI-starch paper

Potassium iodide-starch paper used as an indicator to confirm the presence of an oxidising agent; turns blue-violet in the presence of free iodine generated by oxidation of iodide ions.
Example: In the azo dye test for resorcinol, KI-starch paper confirms that diazotization is complete — a blue-violet colour confirms the presence of excess nitrous acid.

Lewis acid

A substance that accepts an electron pair from a Lewis base to form a coordinate bond, as defined by the Lewis theory of acids and bases.
Example: In the FeCl₃ test for resorcinol, Fe³⁺ acts as a Lewis acid, accepting electron pairs from the phenolic oxygen atoms of resorcinol to form a blue-violet iron(III)–phenolate coordination complex.

Nitrosonium ion

The electrophilic species NO⁺, generated from sodium nitrite (NaNO₂) and concentrated sulfuric acid (H₂SO₄), responsible for nitrosation of activated aromatic rings.
Example: In Liebermann’s test for resorcinol, the nitrosonium ion (NO⁺) attacks the free para position (C-4) of resorcinol to initiate the nitrosation reaction.

Oxidative hair dye

A hair colouring formulation that uses oxidative coupling reactions between a primary intermediate and a coupler to develop colour within the hair shaft.
Example: Resorcinol is widely used as a coupler in oxidative hair dye formulations, reacting with primary intermediates under alkaline oxidative conditions to produce brown and auburn shades.

pKa

The negative logarithm of the acid dissociation constant (Ka), used to express the strength of an acid — the lower the pKa, the stronger the acid.
Example: Resorcinol has a pKa of 9.15, placing it between carboxylic acids (pKa ~4–5) and alcohols (pKa ~16) on the acid strength scale.

Phenoxide ion

The negatively charged ion (ArO⁻) formed when a phenol loses a proton; stabilised by resonance delocalisation of the negative charge across the aromatic ring.
Example: Under alkaline conditions in the azo dye test, resorcinol forms its phenoxide ion, which is a stronger electron donor than neutral resorcinol, making the ring more reactive toward the diazonium ion.

Pyridine

A basic heterocyclic organic compound (C₅H₅N) used as a solvent and base in organic synthesis, particularly in acylation reactions where it neutralises the acid produced.
Example: In the preparation of resorcinol dibenzoate, pyridine acts as both solvent and base, neutralising the HCl produced during benzoylation of resorcinol’s two –OH groups.

Qualitative analysis

A branch of analytical chemistry concerned with identifying the chemical composition and functional groups of an unknown compound through systematic physical and chemical tests.
Example: The qualitative analysis of resorcinol involves a panel of tests including the FeCl₃ test, Liebermann’s test, and phthalein dye test, each confirming a specific structural feature of the compound.

Quinone-oxime

A tautomeric form of a nitroso compound in which a C=N–OH group is present; formed as an intermediate in Liebermann’s nitrosation reaction with phenols carrying a free para position.
Example: In Liebermann’s test for resorcinol, the initial nitroso product at C-4 tautomerises to a quinone-oxime, which undergoes further colour changes with water and NaOH to give the blue → red → blue-green sequence.

Recrystallisation

A purification technique in which a solid is dissolved in a hot solvent and allowed to crystallise slowly on cooling, producing a purer product with a sharper melting point.
Example: After preparation of resorcinol dibenzoate, the crude product is recrystallised from ethanol to give a pure white crystalline solid with a sharp melting point of 117 °C.

Resonance

The delocalisation of electrons across a molecule through a system of alternating single and double bonds, represented by two or more contributing structures that together describe the actual electron distribution.
Example: The resonance structures of resorcinol show that electron density is highest at positions 2, 4, and 6, explaining why these positions are preferentially attacked by electrophiles in bromination and nitrosation.

Tautomerism

The interconversion of two structural isomers (tautomers) that differ in the position of a proton and a double bond, with both forms existing in equilibrium.
Example: In Liebermann’s test, the nitroso intermediate formed at C-4 of resorcinol undergoes tautomerism to the quinone-oxime form, producing the characteristic colour changes observed in the test.

Tyrosinase

A copper-containing enzyme that catalyses the oxidation of tyrosine to melanin in the skin pigmentation pathway; a key target for skin-lightening agents.
Example: Unlike kojic acid, which inhibits tyrosinase directly, resorcinol achieves skin-lightening through keratolysis and mild depigmentation rather than direct tyrosinase inhibition.

UV absorber

A substance that absorbs ultraviolet radiation and dissipates it as heat, preventing UV-induced damage to skin or materials.
Example: Resorcinol is used as a UV absorber in sunscreen formulations, providing protection against UV radiation in outdoor and sports applications.

Xanthene dye

A class of synthetic fluorescent dyes based on the xanthene ring system, characterised by intense fluorescence and high molar absorptivity.
Example: Fluorescein — formed from resorcinol and phthalic anhydride — is a xanthene dye that produces intense yellow-green fluorescence in alkaline solution, making the phthalein dye test one of the most sensitive tests for resorcinol.

Resorcinol vs Catechol, Hydroquinone, Phenol, α-Naphthol and β-Naphthol: Chemical Test Comparison

Comparison of Physical Constants and Names — Catechol, Resorcinol, Hydroquinone and Related Phenols

Resorcinol (benzene-1,3-diol, 1,3-dihydroxybenzene) is one of three benzenediol isomers — catechol (1,2-positions), resorcinol (1,3-positions), and hydroquinone or quinol (1,4-positions) — that differ only in the relative positions of their two –OH groups.

Despite sharing the same molecular formula (C₆H₆O₂) and molecular weight (110.11 g/mol), the three isomers differ significantly in melting point and chemical behaviour. The acidic strength order of catechol, resorcinol, and quinol reflects their structural differences and is an important differentiating parameter in qualitative analysis.

Compound	IUPAC Name	Common Name	Alternative Names	Formula	MW	M.P. (°C)	CAS No.
Phenol	Phenol	Phenol	Carbolic acid, hydroxybenzene	C₆H₆O	94.11	40–42	108-95-2
Catechol	Benzene-1,2-diol	Catechol	1,2-dihydroxybenzene, pyrocatechol	C₆H₆O₂	110.11	104–105	120-80-9
Resorcinol	Benzene-1,3-diol	Resorcinol	1,3-dihydroxybenzene, resorcin, m-dihydroxybenzene	C₆H₆O	110.11	109–111	108-46-3
Hydroquinone	Benzene-1,4-diol	Hydroquinone	1,4-dihydroxybenzene, quinol, p-dihydroxybenzene	C₆H₆O₂	110.11	172–175	123-31-9
α-Naphthol	Naphthalen-1-ol	α-Naphthol	1-naphthol, 1-hydroxynaphthalene	C₁₀H₈O	144.17	94–96	90-15-3
β-Naphthol	Naphthalen-2-ol	β-Naphthol	2-naphthol, 2-hydroxynaphthalene	C₁₀H₈O	144.17	121–123	135-19-3

Comparison of Chemical Tests — Resorcinol vs Catechol, Hydroquinone, Phenol, α-Naphthol, and β-Naphthol

The chemical tests below distinguish resorcinol from five closely related phenolic compounds. Resorcinol and catechol are both dihydric phenols, but their FeCl₃ colours, Liebermann’s test results, and phthalein dye responses are markedly different — making these three tests the most reliable for differentiation. Resorcinol vs hydroquinone is best distinguished by the phthalein dye test (yellow-green fluorescence vs deep purple) and the Liebermann’s test (positive vs negative). The FeCl₃ colour, Liebermann’s test result, and phthalein dye fluorescence together provide unambiguous identification of resorcinol within this group.

Compound	FeCl₃ Colour	Bromine Water	Liebermann’s Test	Phthalein Dye Test	Azo Dye Test
Phenol	Violet	White ppt (tribromophenol)	Positive	Red colour	Orange-red dye
Catechol	Green	White ppt	Negative (blocked para)	Blue — alizarin	Orange-red dye
Resorcinol	Blue-violet	White ppt (tribromoresorcinol)	Positive (free para at C-4)	Yellow-green fluorescence (fluorescein)	Orange-red dye
Hydroquinone	Dark green to black	White ppt	Negative (blocked para)	Deep purple	Orange-red dye
α-Naphthol	Violet	White ppt	Positive	Green	Orange-red dye
β-Naphthol	Purple	White ppt	Positive	Faint green with slight fluorescence	Scarlet-red dye

Applications of Resorcinol in Medicine, Industry and Chemistry

Resorcinol has a wide range of applications in chemistry, medicine, and industry.

Acne and skin treatment — used in resorcinol cream for acne, seborrheic dermatitis, and psoriasis as a keratolytic agent
Adhesive — resorcinol adhesive used for rubber-to-metal bonding and structural applications
Analytical reagent — used in Seliwanoff’s test for ketose detection in carbohydrate chemistry
Antiseptic — used in topical preparations for skin infections
Dyestuff — precursor in fluorescein, eosin, and other xanthene dye synthesis
Flame retardant — stabiliser in plastics and polymer formulations
Formaldehyde resin — resorcinol-formaldehyde resin used in tyre cord bonding and wood lamination
Hair dye — key component in oxidative hair dye formulations
Hexylresorcinol — derivative with antiseptic and anthelmintic properties
Sunscreen — used as UV absorber in sunscreen formulations

Check Your Learning: Identification of Resorcinol by Chemical Tests

The table below is partially completed. Without consulting test results discussed above, fill in the missing observations and conclusions from memory. When complete, check your answers against the above results.

Test	Observation	Conclusion
Ignition (Flame) Test	Sooty, luminous flame
Solubility in Water		Polar –OH groups present
Solubility in Ethanol	Freely soluble
Litmus Test	Blue litmus turns red	Weakly acidic
NaOH Test	Dissolves — clear solution
NaHCO₃ Test
FeCl₃ Test		Phenolic –OH — iron(III) complex
Bromine Water Test	Decolourisation + white precipitate
Liebermann’s Test		Free para position — phenol class
Phthalein Dye Test
Azo Dye Test	Orange-red azo dye
Confirmatory Test 1		Resorcinol confirmed
Confirmatory Test 2	Intense green fluorescence
Confirmatory Test 3	Green fluorescence
Seliwanoff’s Test	Cherry-red colour
Mixed Melting Point		Identity confirmed
Solid Derivative	White solid — m.p. 117 °C

FAQs

Resorcinol — also called resorcin — is a dihydric phenol with two hydroxyl (–OH) groups on a benzene ring at the 1 and 3 positions. Its meaning in chemistry refers to its role as a meta-substituted dihydroxybenzene used widely in medicine and industry.

The IUPAC name of resorcinol is benzene-1,3-diol (also written 1,3-benzenediol). Its structure shows a benzene ring with –OH groups at C-1 and C-3 (meta positions), giving the molecular formula C₆H₆O₂ and making it the meta isomer of the three dihydroxybenzene isomers.

Resorcinol (pronounced reh-ZOR-sin-ol) is a phenol — not a steroid. It belongs to the dihydric phenol class, carrying two –OH groups directly bonded to an aromatic ring. Steroids have a completely different four-ring carbon skeleton and bear no structural resemblance to resorcinol.

Resorcinol gives a blue-violet colour with neutral ferric chloride (FeCl₃) solution. This differentiates it from phenol (violet), catechol (green), and hydroquinone (dark green to black). The blue-violet colour results from iron(III)–phenolate coordination complex formation.

When bromine water is added to resorcinol (C₆H₆O₂), immediate decolourisation occurs and a white precipitate of 2,4,6-tribromoresorcinol forms. This electrophilic aromatic substitution requires no catalyst — unlike the bromine water test for simple phenol, resorcinol reacts at all three activated positions simultaneously.

Resorcinol is freely soluble in water (110 g/100 mL at 20 °C) due to its two polar –OH groups forming hydrogen bonds with water molecules. It also dissolves readily in dilute NaOH, forming sodium resorcinate — confirming its phenolic –OH character.

Resorcinol is weakly acidic with a pKa of 9.15 — more acidic than simple alcohols (pKa ~16) but less acidic than carboxylic acids (pKa ~4–5). Its acidic character is confirmed by the litmus test (blue litmus turns red) and by dissolution in NaOH without reaction with NaHCO₃.

The uses and benefits of resorcinol span multiple fields: it functions as a keratolytic agent in acne creams, an antiseptic in topical preparations, a UV absorber in sunscreens, a component in oxidative hair dyes, a precursor in fluorescein dye synthesis, and an adhesive resin in tyre cord bonding.

Resorcinol is not a retinol. Retinol is a form of vitamin A; resorcinol is a dihydric phenol. In skin-lightening, resorcinol is milder than hydroquinone with fewer side effects. Unlike kojic acid, which inhibits tyrosinase directly, resorcinol works through keratolysis and mild depigmentation.

Resorcinol is not classified as a confirmed human carcinogen. Its known side effects include skin irritation and potential thyroid disruption at high concentrations. Under normal topical use conditions, it is not considered carcinogenic by major regulatory agencies.

Resorcinol melts at 109–111 °C. It is a white crystalline solid that gradually turns pink on prolonged exposure to air, light, or contact with iron. Its colour change on standing is a useful preliminary observation in identification work.

The molecular formula of resorcinol is C₆H₆O₂ and its molecular weight is 110.11 g/mol. It shares the same molecular formula and weight as catechol and hydroquinone — all three are isomers of dihydroxybenzene — making chemical tests essential for their differentiation.

Resorcinol, catechol, and hydroquinone (quinol) are the three isomers of dihydroxybenzene, differing only in the position of their –OH groups: 1,2 (catechol), 1,3 (resorcinol), 1,4 (hydroquinone). They are distinguished by FeCl₃ colour (green / blue-violet / dark green to black), Liebermann’s test (negative / positive / negative), and phthalein dye test (blue-alizarin / yellow-green / deep purple).

Lactose gives a positive Benedict’s test result. The solution will change from blue to green, yellow, orange, or brick red depending on the concentration of lactose. This is because lactose is a reducing sugar with a When resorcinol reacts with phthalic anhydride in the presence of concentrated H₂SO₄ and the product is treated with dilute NaOH, an intense yellow-green fluorescence is produced — confirming fluorescein formation. This result is unique to resorcinol among the dihydric phenols and distinguishes it clearly from phenol and the naphthols.anomeric carbon on the glucose unit.

Multiple Choice Questions

MCQ 1

1. What is the molecular formula of resorcinol and its molecular weight?

A. C₆H₅OH — 94.11 g/mol

B. C₆H₆O₂ — 110.11 g/mol

C. C₆H₄(OH)₂ — 126.11 g/mol

D. C₁₀H₈O — 144.17 g/mol

MCQ 2

2. Which of the following is the correct IUPAC name and structure of resorcinol?

A. Benzene-1,2-diol — ortho dihydroxybenzene

B. Benzene-1,4-diol — para dihydroxybenzene

C. Benzene-1,3-diol — meta dihydroxybenzene

D. Naphthalen-2-ol — beta naphthol

MCQ 3

3. What is the melting point of resorcinol in Celsius, and what is its boiling point under atmospheric pressure?

A. M.P. 40–42 °C / B.P. 182 °C

B. M.P. 104–105 °C / B.P. 245 °C

C. M.P. 109–111 °C / B.P. 277 °C

D. M.P. 121–123 °C / B.P. 295 °C

MCQ 4

4. What colour does resorcinol produce in the ferric chloride test, and what type of reaction does this represent?

A. Green colour — reduction reaction

B. Violet colour — acid-base reaction

C. Dark green to black — precipitation reaction

D. Blue-violet colour — coordination complex formation

MCQ 5

5. When resorcinol reacts with excess bromine water, the product formed is:

A. 2,4-Dibromoresorcinol — decolourisation only

B. 2,4,6-Tribromoresorcinol — decolourisation and white precipitate

C. 2,6-Dibromophenol — yellow precipitate

D. 2,4,6-Tribromophenol — white precipitate

MCQ 6

6. The pKa of resorcinol is 9.15. Which statement correctly describes its acidic or basic nature?

A. Resorcinol is strongly acidic — reacts vigorously with NaHCO₃

B. Resorcinol is weakly basic — dissolves in dilute HCl

C. Resorcinol is weakly acidic — dissolves in NaOH but not NaHCO₃

D. Resorcinol is neutral — shows no reaction with NaOH or NaHCO₃

MCQ 7

7. Which of the following correctly describes the solubility of resorcinol?

A. Freely soluble in water, insoluble in ethanol, soluble in chloroform

B. Insoluble in water, freely soluble in ethanol, soluble in ether

C. Freely soluble in water and ethanol, insoluble in chloroform and carbon disulfide

D. Slightly soluble in water, freely soluble in chloroform

MCQ 8

8. In Liebermann’s test for resorcinol, the correct sequence of colour changes is:

A. Red → blue → green in alkaline medium

B. Blue (in H₂SO₄) → red (on dilution) → blue-green (with NaOH)

C. Green → violet → colourless

D. Yellow → orange → red on addition of NaOH

MCQ 9

9. When resorcinol reacts with phthalic anhydride in concentrated H₂SO₄ followed by dilute NaOH, the observation is:

A. Pink fluorescence — phenolphthalein formation

B. No colour change — resorcinol is unreactive

C. Yellow-green fluorescence — fluorescein formation

D. Deep purple colour — alizarin formation

MCQ 10

10. In the azo dye test for resorcinol, benzene diazonium chloride is prepared at 0–5 °C and coupled with alkaline resorcinol. The product and its colour are:

A. A yellow sulphonated dye — coupling at ortho position only

B. A blue indigo dye — reduction of diazonium salt

C. An orange-red azo dye — coupling at the free para position (C-4)

D. A white precipitate — no coupling reaction occurs

MCQ 11

11. The resonance structure of resorcinol shows that electron density is highest at which positions, making them most reactive toward electrophilic substitution?

A. Positions 1, 3, and 5

B. Positions 2, 4, and 6

C. Positions 1, 2, and 4

D. Positions 3, 5, and 6

MCQ 12

12. Which combination correctly distinguishes resorcinol from catechol and hydroquinone using FeCl₃ colour and Liebermann’s test?

A. Resorcinol: green / negative — Catechol: blue-violet / positive — Hydroquinone: dark green / negative

B. Resorcinol: blue-violet / positive — Catechol: green / negative — Hydroquinone: dark green to black / negative

C. Resorcinol: violet / positive — Catechol: blue-violet / positive — Hydroquinone: green / negative

D. Resorcinol: dark green / negative — Catechol: violet / positive — Hydroquinone: blue-violet / positive

MCQ 13

13. Which of the following correctly lists the pharmaceutical uses and benefits of resorcinol?

A. Antifungal, antihistamine, and diuretic agent

B. Keratolytic agent, antiseptic, and component in hair dye formulations

C. Analgesic, anti-inflammatory, and antipyretic agent

D. Beta-blocker, antihypertensive, and vasodilator

MCQ 14

14. Which statement correctly describes the toxicology and side effects of resorcinol?

A. Resorcinol is non-toxic and classified as GRAS for all uses

B. Resorcinol is a confirmed human carcinogen classified by IARC Group 1

C. Resorcinol may cause skin irritation and thyroid disruption at high concentrations but is not classified as a confirmed human carcinogen

D. Resorcinol is highly flammable with NFPA flammability rating of 4

MCQ 15

15. In Seliwanoff’s test, resorcinol acts as the analytical reagent dissolved in dilute HCl. What colour confirms a positive result and what does it indicate?

A. Blue colour — presence of an aldose sugar

B.Yellow colour — presence of a disaccharide

C. Cherry-red colour — presence of a ketose sugar such as fructose

D. Green colour — presence of a reducing sugar

References

British Pharmacopoeia Commission. (2024). Resorcinol monograph. In British Pharmacopoeia 2024 (Vol. 1, pp. 1823–1824). The Stationery Office.
Chughtai, M. I. Systematic organic analysis. Majeed Book Depot, Lahore.
Mohrig, J. R., Hammond, C. N., & Schatz, P. F. (2014). Techniques in organic chemistry (4th ed.). W. H. Freeman.
National Institute of Standards and Technology. (2024). NIST WebBook: Resorcinol (CAS 108-46-3). National Institute of Standards and Technology. https://webbook.nist.gov
National Center for Biotechnology Information. (2024). PubChem compound summary: Resorcinol (CID 5054). National Library of Medicine. https://pubchem.ncbi.nlm.nih.gov/compound/5054
Derivatives of resorcinol