Structure of Carboxylic Acids

Carboxylic acids contain a carboxyl group (–COOH), consisting of a carbonyl group (C=O) and a hydroxyl group (–OH). The general formula is:

R–COOH where R is an alkyl or aryl group.

1. Bonding and Geometry

• Carbon atom is sp² hybridized → trigonal planar geometry.
• O=C–OH angle ≈ 120°.
• C–O bonds equalized due to resonance.

2. Resonance

Carboxyl group shows resonance, delocalizing charge on both oxygen atoms, stabilizing the molecule.

   O⁻            O
    \\          //
     C   ⇌    C
    /          \
  O–H          O⁻

3. Hydrogen Bonding

Carboxylic acids associate via strong intermolecular hydrogen bonding, forming dimers.

R–COOH ⋯⋯ OOC–R (hydrogen-bonded dimer)

4. Acidity and Electron Distribution

The carbonyl oxygen withdraws electrons → hydroxyl hydrogen becomes acidic.

R–COOH ⇌ R–COO⁻ + H⁺

Carboxylate ion is resonance-stabilized → higher acidity compared to alcohols and phenols.

5. 3D Representation

–COOH group is planar (sp² hybridized), allowing conjugation and resonance.

Conclusion: Resonance, hydrogen bonding, and planar geometry explain the acidity and unique properties of carboxylic acids.

MCQs on Structure of Carboxylic Acids

1. Hybridization of carboxyl carbon is:
   a) sp
   b) sp²
   c) sp³
   d) none
   Answer: b) sp²
2. The resonance in –COOH group:
   a) Increases acidity
   b) Decreases acidity
   c) No effect
   d) Makes it neutral
   Answer: a) Increases acidity
3. Carboxylic acids often exist as:
   a) Monomers
   b) Dimers
   c) Polymers
   d) None
   Answer: b) Dimers

Fill in the Blanks

• The general formula of carboxylic acids is R–COOH.
• The O=C–OH bond angle is approximately 120°.
• Carboxylic acids form hydrogen-bonded dimers in non-polar solvents.

Quick Quiz

Q: Why are carboxylic acids more acidic than alcohols?

A: Because the conjugate base (carboxylate ion) is stabilized by resonance, unlike the alkoxide ion in alcohols.

FAQs on Structure of Carboxylic Acids

Q1: What is the functional group of carboxylic acids?
A: The carboxyl group (–COOH).

Q2: Why do carboxylic acids have higher boiling points?
A: Due to intermolecular hydrogen bonding and dimer formation.

Q3: What makes carboxylic acids acidic?
A: The resonance stabilization of the carboxylate ion after losing H⁺.