19.3Esters

19.3    Esters

Introduction to esters

• Esters are derivatives of carboxylic In an ester, the hydrogen from the -COOH group of carboxylic acid is replaced by an alkyl group. The alkyl group came from the alcohol/phenol. Some common esters and their naming:
• Note that the name of an ester is ‘alcohol + carboxylic acid’.

• Physical properties of esters:
  1. Esters have lower melting and boiling points than carboxylic acids. This is because they are not capable of forming intermolecular hydrogen bonding.
  2. Esters are insoluble in water and often identified by their strong fruity smell.

• Unlike carboxylic acids, esters are neutral. This is because they cannot donate or accept a proton

Preparation of esters

• Esters can be prepared from the reaction between alcohol and carboxylic acid(AS).

• Between alcohol and carboxylic acid:
  1. To prepare an ester from carboxylic acid and alcohol, both them are heated under reflux in the presence of concentrated sulfuric acid as catalyst. Esters can be detected from a sweet-smelling odour. The general  equation:
  2. For example, to make methyl butanoate, methanol and butanoic acid are used.
     CH3OH + CH3CH2CH2COOH ⇌ CH3CH2CH2COOCH3 + H2O
  3. This reaction cannot be used to produce esters when the -OH group is attached to a benzene ring, also known as phenol. In other words, carboxylic acid cannot react with phenol to produce ester.

Hydrolysis of  esters

• Acidic hydrolysis of esters:
  i) When ester is heated under reflux with a dilute acid such as dilute hydrochloric acid, the corresponding carboxylic acid and alcohol will be formed.
  ii) Using methyl propanoate as an example, propanoic acid and methanol is formed.
  iii) This is just the reverse process of esterification
• Alkaline hydrolysis of esters:

1. When ester is heated under reflux with a base such as sodium hydroxide solution, the corresponding carboxylate salt and alcohol will be formed.
2. Using methyl propanoate as example, sodium propanoate and methanol is formed.

• The sodium propanoate can be converted to propanoic acid by adding dilute acid.

Uses of esters

• As solvents:

1. Small esters such as methyl ethanoate, ethyl ethanoate and butyl ethanoate are very widely used as solvents

• As perfumes and flavourings:

1. The smell and taste of fruits such as oranges, apples, pears, raspberries, strawberries, and so on, are due to naturally occurring esters. Many foods will have those same esters added to them to simulate the smell and taste of real fruit.
2. Flower and other smells are also produced by naturally occurring esters
3. Perfumes will have the same esters Benzyl ethanoate, for example, is used to produce jasmine or gardenia fragrances.