Esters

What are they? How are they made? Formed when an alcohol reacts with a carboxylic acid. Very slow reaction, unless! an acid catalyst used (usually sulphuric acid) A condensation reaction The condensation reaction between the hydroxyl group and the carboxylic acid known as esterification. Reverse reaction = ester hydrolysis

Naming Named after alcohol & carboxylic acid from which they are derived. Names of esters end in –oate. CH 3 C CH 2 O O CH 3 ethylethanoate this part from the acid and is named after it this part comes from the alcohol & is named after it let’s name some esters!

Structural formulae Although the previous structural formula are the clearest way of showing esters, they can draw out in a shortened form. ethylethanoate CH 3 C CH 2 O O CH 3 Either… Or… CH 3 C CH 2 O O CH 3 In this version the acid part has been written first Identify the group attached to the C=O – this is from the acid The group attached to the –O- is from the alcohol.

Wait! What’s that smell? Esters have strong, sweet smells. Their bouquet is often floral or fruity. This means they are used in food flavourings & perfumes. Also, very good at dissolving organic compounds so often used as solvents. esterfragrance ethyl methanoateraspberries 3-methylbutyl ethanoatepears ethyl 2-methylbutanoateapples phenylmethyl ethanoatejasmine

Ester hydrolysis Breakdown of an ester by water. Process sped up by catalysis Can use an acid to catalyse (H 2 SO 4 ) Can use an acid to catalyse Alkali catalysts (e.g. sodium hydroxide) can also be used but instead of producing carboxylic acid a carboxylate salt is formed.carboxylate salt Alkaline hydrolysis goes to completion & hence is usually preferred.

This is why it is a condensation reaction because water is produced! R R’C H2OH2O + OH HO O C RO O + ⇋ Definition of a condensation reaction = two molecules reacting to form a larger molecule with the elimination of a small molecule such as water

CH 3 CH CH 2 O O CH 3 CH 2 C CH 3 O O CH CH 3 O O ethylmethanoate methylpropanoate methylmethanoate

R R’C H2OH2O + OH HO O C RO O + ⇌ FORWARD REACTION = condensation reaction, the esterification of an alcohol using acid catalyst under reflux. REVERSE REACTION = ester hydrolysis, same catalyst works for both forward & reverse reactions.

+ OH OH - → CH 3 C CH 2 O O CH 3 ethylethanoate + CH 2 CH 3 C -O-O O ethanol ethanoate ion not a reaction that exists in equilibrium (unlike using acid catalyst)