Why Understanding Solubility Makes Organic Chemistry Easier – Solubility of Organic Compounds in Water

ByMudasra Uncategorized

What Is Solubility?

Solubility is the maximum amount of a substance (solute) that can dissolve in a given amount of a solvent at a specific temperature (and pressure) to form a homogeneous solution.

In simple words, solubility tells us how much of a substance can dissolve in a solvent like water under fixed conditions.

An organic compound is soluble in water if its polar functional group and hydrogen-bonding ability dominate over its hydrocarbon chain; otherwise, it becomes insoluble, but its ionic salts are usually water-soluble.

General Solubility Criteria of organic Compounds in water

  • Short carbon chain → higher solubility
  • Long carbon chain → lower solubility
  • Aromatic rings reduce solubility
  • Hydrogen bonding increases solubility but has limits
  • Ionic salts are usually soluble in water
This diagram is explaining the polarity difference between cyclohexane and glucose molecule.

Effect of Carbon Chain length in Alcohols

As the number of carbon atoms increases, the hydrophobic character of the molecule increases and water solubility decreases.

  • Lower alcohols are soluble.
  • Higher alcohols are insoluble despite having the same –OH group.
  • The same trend applies to acids, amines, and amides.

Key Rule:
Solubility decreases as carbon chain length increases.

This figure explain the solubility of organic compounds (alcohols) in water

Effect of Carbon Chain Length in Carbonyls

As the number of carbon atoms increases, the hydrophobic character of the molecule increases and water solubility decreases.

  • Lower aldehydes are soluble.
  • Higher aldehydes are insoluble despite having the same –CO–group.

Key Rule:
Solubility decreases as carbon chain length increases.

This figure explain the solubility of organic compounds (carbonyls) in water

2. Aromatic Ring Effect

Aromatic rings significantly reduce solubility by increasing hydrophobic character.

  • Benzene is insoluble in water due to its non-polar nature.
  • Phenol is only slightly soluble in water.
  • Benzoic acid is sparingly soluble despite having a –COOH group.

Phenol is partially soluble because the –OH group adds polarity. when number of -OH groups attached to benzene increases, solubility increases.  In this case, the non-polar benzene ring is constant, and adding –OH groups, increases polarity, which increases its solubility in water.

This diagram explain the solubility of aromatic compounds in water that how solubility increases by increasing the -OH group
  • Benzene is insoluble in water, benzoic acid is slightly soluble due to its polar –COOH group, and 1,2-dicarboxylic acid derivatives are more soluble as the additional –COOH group increases polarity and hydrogen bonding.
  • If the polar –OH group remains constant and the non-polar part increases, water solubility decreases. Both phenol and 1-naphthol have one –OH group, but the larger non-polar naphthalene ring in 1-naphthol reduces solubility compared to phenol.
  • The same trend occurs with amino groups: if the –NH₂ group is constant and the non-polar ring increases from benzene to naphthalene, water solubility decreases. This shows that increasing the non-polar part reduces solubility even with different functional groups.

Effect of Polarity and Hydrogen Bonding

Organic compounds that contain polar functional groups such as –OH, –COOH, –NH₂, or –CONH₂ can form hydrogen bonds with water. This interaction promotes solubility.

  • Alcohols, carboxylic acids, amines, and amides are generally soluble only when the hydrocarbon part is small.
  • Hydrogen bonding alone does not guarantee solubility if the non-polar portion dominates.

Example:
Methanol is soluble due to strong hydrogen bonding and a short carbon chain, whereas hexanol is insoluble because the long hydrocarbon chain suppresses polarity.

Ionization and Salt Formation

Non-ionized organic compounds may be poorly soluble. When converted into ionic salts, solubility increases sharply due to strong ion–dipole interactions with water. All compounds of sodium (Na⁺), potassium (K⁺), and ammonium (NH₄⁺) are highly soluble in water due to the strong interaction of these ions with water molecules.

Key Principle:
Salts of organic acids and bases are generally soluble in water.

Benzoic acid is slightly soluble in water due to limited hydrogen bonding of its –COOH group. In contrast, sodium benzoate is highly soluble because the ionic –COO⁻Na⁺ group interacts strongly with water.

This diagram is explaining that the sodium salt of benzoic acid is more polar than the benzoic acid itself. Therefore it is more soluble in water.

Ionization and Salt Formation

Phenol is moderately soluble in water due to its –OH group forming hydrogen bonds, while sodium phenoxide is completely soluble because the ionic –O⁻Na⁺ group interacts strongly with water, enhancing solubility.

Aniline is slightly soluble in water because the –NH₂ group forms limited hydrogen bonds. In contrast, anilinium chloride is readily soluble due to the ionic –NH₃⁺Cl⁻ group interacting strongly with water.

Nature of the Functional Group

Different functional groups influence solubility differently.

  • Amides are more soluble than alcohols and amines due to stronger hydrogen bonding.
  • Carboxylic acids are more soluble than corresponding phenols.
  • Hydrocarbons lack polarity and are insoluble.

“Like Dissolves Like”

Water dissolves polar substances, and hexane dissolve none polar compounds.

Chemistry Terms Related to Solubility, Polarity, and Intermolecular Forces

Terms

Explanation

Examples

Polar

Having Partial +ve an –ve charges

Water, Ethanol

Hydrophilic

Having affinity towards water

Sugars (glucose), Alcohols

Hydrophobic

Having no affinity towards water

Alkanes , Oils and fats

Soluble

Substances which dissolve in water

Sugars

Sparingly soluble

Only a very small amount dissolves

Benzophenone

Insoluble

Does not dissolve in water

Benzene, Hexane

Miscible

Two liquids that mix completely

Alcohol + Water

Immiscible

Two liquids that don’t mix with each other

Hexane + Water

Partially soluble

More than sparingly soluble

Ether

Like dissolve like

Polar solute dissolves in polar solvent

Non polar solutes dissolves in non-polar solvents

Sugar in Water (polar)

Grease (Hydrocarbon) in Hexane (non-polar)

Hydrogen Bonding

Specific type of interaction between hydrogen atom and electronegative atom (like F, O and N)

Water and Ammonia

London Dispersion Forces

Very weak intermolecular forces between non-polar molecules

Hexane

Benzene

Multiple Choice Questions

MCQ 1

1. Which factor primarily decreases the solubility of alcohols in water as the series increases?

A. Increase in hydrogen bonding

B. Increase in polarity

C. Increase in alkyl chain length

D. Presence of hydroxyl group

MCQ 2

2. Which alcohol is most soluble in water?

A. Pentanol

B. Butanol

C. Propanol

D. Methanol

MCQ 3

3. Why is phenol only slightly soluble in water despite having an –OH group?

A. Absence of hydrogen bonding

B. Presence of aromatic ring

C. Low molecular weight

D. High polarity

MCQ 4

4. Which statement correctly explains the effect of carbon chain length on solubility?

A. Longer chains increase solubility in water

B. Chain length has no effect on solubility

C. Longer chains increase hydrophobic character

D. Short chains decrease hydrogen bonding

MCQ 5

5. Which compound becomes highly soluble in water after conversion to its salt?

A. Phenol

B. Ethanol

C. Benzoic acid

D. Hexane

MCQ 6

6. Why are ionic salts of organic acids and bases generally soluble in water?

A. Due to covalent bonding

B. Due to ion–dipole interactions

C. Due to aromaticity

D. Due to weak polarity

FAQ’s

Lower alcohols are soluble due to hydrogen bonding with water, but solubility decreases as the non-polar alkyl chain length increases..

An increase in carbon atoms enhances hydrophobic character, which weakens interaction with water and lowers solubility.

The aromatic ring in phenol increases non-polar character, reducing the effect of hydrogen bonding with water.

Aromatic rings increase hydrophobicity and generally reduce water solubility.

Salt formation produces ions that interact strongly with water through ion–dipole forces, increasing solubility.

Hydrogen bonding increases solubility, but it is effective only when the hydrocarbon part of the molecule is small.

As non-polar character increases, compounds dissolve better in non-polar or weakly polar organic solvents.

References

  1. Vogel, A. I., Furniss, B. S., Hannaford, A. J., Smith, P. W. G., & Tatchell, A. R. (1989). Vogel’s Textbook of Practical Organic Chemistry (5th ed.). Longman / Wiley.
  2. Pavia, D. L., Lampman, G. M., Kriz, G. S., & Engel, R. G. (2016). A Small-Scale Approach to Organic Laboratory Techniques (3rd ed.). Cengage Learning.
  3. Atkins, P., de Paula, J., & Keeler, J. (2018). Atkins’ Physical Chemistry (11th ed.). Oxford University Press.
  4. Clayden, J., Greeves, N., Warren, S., & Wothers, P. (2012). Organic Chemistry (2nd ed.). Oxford University Press.
  5. Solomons, T. W. G., & Fryhle, C. B. (2011). Organic Chemistry (11th ed.). Wiley.
  6. Mann, F. G., & Saunders, B. C. (1990). Practical Organic Chemistry (3rd ed.). Pearson Education.

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