Acyl Chloride and Ester Functional Groups

Acyl chlorides and esters are members of the carboxylic acid derivative family in organic chemistry.
Acyl chlorides often have the functional group -COCl. This group is highly reactive due to the presence of a good leaving group, the chloride ion.
They are usually prepared through the reaction of a carboxylic acid with thionyl chloride (SOCl2). The by-products of this reaction are sulphur dioxide (SO2) and hydrogen chloride (HCl).
Due to their high reactivity, acyl chlorides can be used to make a variety of other organic compounds such as amides, esters, and carboxylic acids.
Esters have the functional group -COO-. They are commonly formed through a condensation reaction, also known as esterification, between a carboxylic acid and an alcohol.
As an example, upon reacting methanol with ethanoic acid, the ester formed is methyl ethanoate and a molecule of water, by the process of esterification.
The reverse reaction, the hydrolysis of esters, can occur under acidic or basic conditions. This results in either a carboxylic acid or a carboxylate salt, respectively, along with an alcohol.
Esters are often used in artificial flavourings and fragrances due to their sweet smells. They also find extensive use in making polymers and in the biosynthesis of lipids.
In terms of nomenclature, the first part of an ester’s name derives from the alcohol used and the second part from the acid. For example, the ester of ethanol and methanoic acid is called ethyl methanoate.
Both acyl chlorides and esters can undergo nucleophilic acyl substitution. The carbonyl carbon in both molecules is electrophilic, allowing it to be attacked by nucleophiles.
Further oxidation of acyl chloride is not possible as it is already in the highest oxidation state. In contrast, esters can be hydrolysed and oxidised to carboxylic acids under the right conditions.

Get well-acquainted with the different reactions of acyl chlorides and esters, being able to identify them in organic compound structures and understanding their behaviour and reactivity will yield better understanding of Organic Chemistry II concepts.