Esters

What is the functional group of an ester?

–COO–

How are esters formed from carboxylic acids?

A carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester and water.

What is the general equation for esterification?

Carboxylic acid + alcohol ⇌ ester + water.

What catalyst is used in esterification?

A strong acid catalyst such as H₂SO₄.

What conditions are used for esterification?

Heat under reflux.

Is esterification reversible?

Yes.

Why is esterification a relatively slow reaction?

It is a slow reaction and requires heating under reflux.

Why is the yield from esterification relatively low?

The reaction is reversible, so an equilibrium is established and the reaction does not go completely to the products.

How are esters named?

The –yl part comes from the alcohol and the –anoate part comes from the carboxylic acid.

What ester is formed from ethanoic acid and ethanol?

Ethyl ethanoate.

Write the equation for ethanoic acid reacting with ethanol.

CH₃CO₂H + CH₃CH₂OH ⇌ CH₃CO₂CH₂CH₃ + H₂O

What does the –yl part of an ester name tell you?

The alcohol from which the ester was formed.

What does the –anoate part of an ester name tell you?

The carboxylic acid from which the ester was formed.

What are esters used for?

Perfumes, flavourings, solvents and plasticisers.

Why are esters used in perfumes and flavourings?

They often have pleasant/sweet smells.

What properties are needed for an ester to be suitable for use in perfumes?

It should be non-toxic, soluble in a suitable solvent such as ethanol, volatile and not react with water.

Why are esters useful as solvents?

They can dissolve polar organic substances.

What is ethyl ethanoate used as a solvent for?

Glues and printing inks.

Why do esters have lower boiling points than the carboxylic acids they came from?

Esters cannot form hydrogen bonds with each other because there is no hydrogen bonded to a highly electronegative atom.

Why can esters not form hydrogen bonds with each other?

They have no hydrogen bonded directly to a highly electronegative atom.

Are esters soluble in water?

They are almost insoluble in water.

What are plasticisers used for?

They are added to polymers to increase flexibility.

How do plasticisers increase the flexibility of polymers?

They allow polymer chains to move more easily over one another.

What is ester hydrolysis?

The reaction in which an ester is split into its components using water under acidic or alkaline conditions.

What are the two ways esters can be hydrolysed?

By heating with dilute acid or by heating with sodium hydroxide.

What are the conditions for acid hydrolysis of an ester?

Dilute acid such as HCl and heat under reflux.

What products are formed during acid hydrolysis of an ester?

A carboxylic acid and an alcohol.

Is acid hydrolysis reversible?

Yes.

Why is acid hydrolysis the reverse of ester formation?

The carboxylic acid and alcohol produced can react to reform the ester and water.

What are the conditions for alkaline hydrolysis of an ester?

Aqueous NaOH and heat under reflux.

What products are formed during alkaline hydrolysis of an ester?

A carboxylate salt and an alcohol.

Is alkaline hydrolysis reversible?

No, it effectively goes to completion.

Why does alkaline hydrolysis go to completion?

The carboxylate ion formed is resistant to attack by weak nucleophiles such as alcohols.

Write the equation for alkaline hydrolysis of methyl propanoate.

CH₃CH₂COOCH₃ + NaOH → CH₃CH₂COO⁻Na⁺ + CH₃OH

How can a carboxylate salt be converted back into the carboxylic acid?

Add a strong acid such as HCl.

Write the equation for sodium propanoate reacting with HCl.

CH₃CH₂COO⁻Na⁺ + HCl → CH₃CH₂COOH + NaCl

What happens when ethyl benzoate is hydrolysed using NaOH?

It forms sodium benzoate and ethanol.

What conditions are used to hydrolyse ethyl benzoate with NaOH?

Heat under reflux for about 30 minutes.

What happens after hydrolysis of ethyl benzoate?

The mixture is cooled and HCl is added, producing a precipitate of benzoic acid.

Why does benzoic acid precipitate when HCl is added to sodium benzoate?

The ionic sodium benzoate is converted into non-ionic benzoic acid, which is insoluble in water.