2 Esters

Esterification Carboxylic acids react with alcohols, in the presence of a strong sulphuric acid catalyst, to form esters and water. C C C O O C H H H H H H H H Esters have two parts to their names, eg methyl propanoate. The bit ending in –yl comes from the alcohol that has formed it and is next to the single bonded oxygen. The bit ending in –anoate comes from the carboxylic acid and includes the C in the C=O bond. Carboxylic Acid + Alcohol Ester + water H+ The reaction is reversible. The reaction is quite slow and needs heating under reflux, (often for several hours or days). Low yields (50% ish) are achieved. An acid catalyst (H2SO4 ) is needed. H C + + H2O 3 C O OH C C O O C C H H H H H H H H H+ C O H H H C H H H CH3CO2H + CH3CH2OH CH3CO2CH2CH3 + H2O Ethanoic acid Ethanol Ethyl Ethanoate Oxidation of methanoic acid Carboxylic acids cannot be oxidised by using oxidising agents but methanoic acid is an exception as its structure has effectively an aldehyde group C O O H H C O O H + [O]
H O It forms carbonic acid (H2 CO3 ) which can decompose to give CO2 N Goalby chemrevise.org Change in functional group: acid anhydride
ester Reagent: alcohol Conditions: room temp. (RCO)2O + CH3CH2OH
RCO2CH2CH3 +RCO2H CH3 C O O CH3 C O C C O O C C H H H H H H H + CH3CH2OH
H + CH3CO2H Esterification using acid anhydrides The acid anhydrides are more reactive than carboxylic acids. The reaction is not reversible and a higher yield is achieved. Hydrolysis of esters Esters can be hydrolysed and split up by either heating with acid or with sodium hydroxide. i) with acid reagents: dilute acid (HCl) conditions: heat under reflux This reaction is the reverse reaction of ester formation. When an ester is hydrolysed a carboxylic acid and an alcohol are formed. CH3CH2CO2CH2CH3 + H2O CH3CH2CO2H + CH3CH2OH ethyl propanoate H+ This reaction is reversible and does not give a good yield of the products. ii) with sodium hydroxide reagents: dilute sodium hydroxide conditions: heat under reflux CH3CH2CO2CH3 + NaOH
CH3CH2CO2 – Na+ + CH3OH methyl propanoate sodium propanoate methanol The carboxylic acid salt product is the anion of the carboxylic acid. The anion is resistant to attack by weak nucleophiles such as alcohols, so the reaction is not reversible. This reaction goes to completion.