Carboxylic Acids, Acyl Chlorides, Esters, and Polyesters

Quick Notes

• Carboxylic acids contain the –COOH functional group.

• Hydrogen bonding gives carboxylic acids relatively high boiling points and good solubility in water.
• Can be prepared from:
  • Oxidation of primary alcohols and aldehydes using acidified potassium dichromate (K₂Cr₂O₇/H⁺), heat under reflux
  • Hydrolysis of nitriles, heat under reflux with dilute HCl
• Reactions include:
  • Reduction to alcohols with LiAlH₄
  • With bases to form carboxylate salts
  • With PCl₅ to form acyl chlorides
  • With alcohols to form esters (esterification)
• Acyl chlorides are reactive derivatives of carboxylic acids.
• Esters can be hydrolysed under acidic or alkaline conditions.
  • acidic conditions forms carboxylic acid and alcohol
  • alkaline conditions forms carboxylate salt and alcohol
• Polyesters are formed via condensation polymerisation.

Full Notes

Carboxylic acids contain the –COOH group. This consists of both a carbonyl (C=O) and a hydroxyl (–OH) group on the same carbon atom.

Physical Properties

Boiling Point:
Carboxylic acids have high boiling points due to strong hydrogen bonds between molecules. These require more energy to overcome compared to London Dispersion Forces or dipole-dipole forces for similar sized molecules.

Solubility:
They are soluble in water due to hydrogen bonding with water molecules. Solubility decreases with increasing chain length as the nonpolar hydrocarbon part dominates.

Preparation of Carboxylic Acids

There are two methods of preparing carboxylic acids you need to know.

Oxidation of Alcohols and Aldehydes:

Primary alcohols can be oxidised to carboxylic acids

• Primary alcohols → Aldehydes → Carboxylic acids

Reagent: Acidified potassium dichromate (K₂Cr₂O₇/H⁺)
Conditions: heat under reflux

Example: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

Matt’s Exam Tip

Remember distillation is used to obtain aldehyde and reflux to obtain carboxylic acid - see oxidation of alcohols.

Hydrolysis of Nitriles:

Nitriles can undergo hydrolysis to form a carboxylic acid.
Converts –CN groups to –COOH

Reagent: dilute HCl (or NaOH(aq) followed by H⁺(aq) ).
Conditions: Heat under reflux with

Example: R–CN + 2H₂O + H⁺ → R–COOH + NH₄⁺

Reduction with LiAlH₄

Carboxylic acids can be reduced to primary alcohols using lithium aluminium hydride (LiAlH₄) in dry ether.

LiAlH₄ is a strong reducing agent and must not come into contact with water which is why the reaction is carried out in dry ether.

General equation: R–COOH + 4[H] → R–CH₂OH + H₂O

Matt’s Exam Tip

Remember in organic chemistry that oxidation is the gaining of a bond to a more electronegative element (such as oxygen or nitrogen) and, or the loss of a bond to a less electronegative element (hydrogen). Reduction is the opposite of this - the losing of a bond to a more electronegative element (such as oxygen or nitrogen) and the gaining of a bond to hydrogen.

Reactions of Carboxylic Acids

Carboxylic acids are weak acids and can donate H⁺ ions in reactions, producing a carboxylate ion (COO⁻) that forms a salt. Because they are weak acids they only partially dissociate in solution.

There are several reactions of carboxylic acids you need to know.

Redox Reaction with Reactive Metals

Example: 2CH₃COOH + Zn → (CH₃COO)₂Zn + H₂↑

Explanation: Metal displaces hydrogen from the –COOH group, producing hydrogen gas and a carboxylate salt.

Neutralisation with Alkalis

Example: CH₃COOH + NaOH → CH₃COONa + H₂O

Explanation: OH⁻ accepts the H⁺ from the –COOH group, forming a salt and water.

Reaction with Carbonates

Example: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂↑ + H₂O

Explanation: Typical acid–carbonate reaction producing carbon dioxide, water, and a salt.

With PCl₅:

Converts –COOH to acyl chloride (R–COCl)

Example:CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

With Alcohols (Esterification)

In presence of concentrated H₂SO₄, produces esters

Reversible, so carried out under reflux and water removed

Example:CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂O

Acyl Chlorides and Esters

Acyl chlorides (R–COCl) are more reactive than carboxylic acids.

There are some key reactions of acyl chlorides you need to know.

Hydrolysis with Water

RCOCl + H₂O → RCOOH + HCl
This is a vigorous reaction; steamy fumes of HCl are released.

Reaction with Alcohols

RCOCl + R'OH → RCOOR' + HCl
Forms an ester without needing a catalyst (faster and more direct than using carboxylic acids).

Reaction with Ammonia (NH₃)

Reaction with Primary Amines

RCOCl + R'NH₂ → RCONHR' + HCl
(Can also react with Secondary amines to form tertiary amide
RCOCl + R'₂NH → RCONR'₂ + HCl)
Forms substituted amides. HCl is neutralised by excess amine.

Hydrolysis of Esters

Esters can be broken apart in hydrolysis reactions. The products obtained depend on the conditions used.

Acid Hydrolysis:

Acid hydrolysis of an ester is reversible and forms a carboxylic acid and alcohol.

• Reagent: Dilute H₂SO₄ or HCl
• Reaction:
  Ester + Water ⇌ Carboxylic Acid + Alcohol

Example: CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

Alkaline Hydrolysis:

Alkaline hydrolysis of an ester goes to completion (rather than reversible) and forms a carboxylate salt and alcohol.

• Reagent: NaOH
• Reaction:
  Ester + NaOH → Carboxylate Salt + Alcohol

Example: CH₃COOCH₂CH₃ + NaOH → CH₃COO⁻Na⁺ + CH₃CH₂OH

Polyesters

Polyesters are made when monomers containing alcohol (–OH) and carboxylic acid (–COOH) groups react.

They are examples of condensation polymers and are formed in condensation polymerisation.

Examples you need to know are given below.

Diol + Dicarboxylic Acid or Dioyl Chloride

A diol (–OH group at both ends) reacts with a dicarboxylic acid (or dioyl chloride).

Each ester bond forms with loss of H₂O (or HCl if using acyl chlorides).

Hydroxycarboxylic Acid

A hydroxycarboxylic acid has both an –OH and –COOH group meaning it can polymerise with itself, forming a polyester.

The –OH of one molecule reacts with –COOH of another, forming an ester bond.

Summary

• Carboxylic acids contain the COOH group and form hydrogen bonds.
• Can be prepared by oxidising primary alcohols or by hydrolysing nitriles.
• Can be reduced to primary alcohols using LiAlH₄ in dry ether.
• Acids react with metals, alkalis and carbonates to form salts.
• PCl₅ converts acids to acyl chlorides which are highly reactive.
• Esterification makes esters with acid catalysts under reflux.
• Acyl chlorides form esters and amides and hydrolyse to acids.
• Esters hydrolyse to acids and alcohols in acid and to salts and alcohols in alkali.
• Polyesters form by condensation of diols with diacids or by self-condensation of hydroxyacids.