Esters (OCR A Level Chemistry)

• Esters are a carboxylic acid derivative which contains the ester group, -COO-
• An ester is named after the parent carboxylic acid from which it is derived
• The nomenclature of esters follows the pattern:
  • Remove the -oic acid suffix from the parent carboxylic acid and replace with -oate
  • The alkyl chain attached to the oxygen atom of the -COO- group is then added as the first word in the name
    • This part of the name comes from the alcohol, e.g. propanol becomes propyl

• Ester names are confusing because the name is written backwards from the way the structure is drawn

Structure of methyl ethanoate

Esters Examples Table

Esterification

• Esters are characterised by their sweet and fruity smells
• They are prepared from the condensation reaction between a carboxylic acid and alcohol with concentrated H₂SO₄ as catalyst
  • This is also called esterification

• A condensation reaction involves the elimination of a small molecule not always water
  • Esterification is one example of a condensation reaction as water is eliminated from the acid and alcohol reacting together
• The reaction is reversible
• The reaction is quite slow and needs heating under reflux, (often for several hours or days).
• Low yields(50% ish) tend to be achieved by this route

Esters are formed from the condensation reaction between carboxylic acids and alcohols

• Esterification can also take place by reacting acid anhydrides with alcohols at room temperature

 

• Acid anhydrides are also derivatives of carboxylic acids formed by substitution of the -OH group by an alkanoate (e.g. ethanoate, propanoate)
  • Acid anhydrides are named by identifying the parent hydrocarbon chain and adding the suffix -oic anhydride
  • They can also be named by removing the -oic acid from the carboxylic acid and adding -oic anyhydride

 

• The acid anhydrides are more reactive than carboxylic acids
• The reaction is not reversible and a higher yield is achieved.

• Esters can be hydrolysed to reform the carboxylic acid and alcohol by either dilute acid or dilute alkali and heat
• When an ester is heated under reflux with dilute acid (eg. sulfuric acid) an equilibrium mixture is established as the reaction is reversible

Ester hydrolysis by dilute acid is a reversible reaction forming carboxylic acid and alcohol

• However, heating the ester under reflux with dilute alkali (eg. sodium hydroxide) is an irreversible reaction as the ester is fully hydrolysed
• This results in the formation of a sodium carboxylate salt which needs further acidification to turn into a carboxylic acid
  • The sodium carboxylate (-COO^-) ion needs to get protonated by an acid (such as HCl) to form the carboxylic acid (-COOH)

Ester hydrolysis by dilute alkali is an irreversible reaction forming a sodium carboxylate salt and alcohol

Table showing differences in hydrolysis of esters