What is an ester and how is it formed?

• A. An ester is RCOOR' formed by reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst (Fischer esterification)
• B. An ester is RCOO-R' formed by the dehydration of an alcohol
• C. An ester is RCOOR' formed by reaction of an alcohol with carbonate
• D. An ester is ROOR' formed by reaction of a carboxylic acid with water

Answer: (A)

Esters have the structure RCOOR', a carbonyl carbon bonded to an alkoxy group. They are most commonly formed by Fischer esterification, where a carboxylic acid reacts with an alcohol in the presence of an acid catalyst. The acid protonates the carbonyl oxygen, making the carbonyl carbon more electrophilic so the alcohol can attack. After a series of proton transfers and the loss of water, the ester is formed. This route directly produces the RCOOR' linkage, which is exactly what defines an ester. Dehydration of an alcohol typically yields alkenes, not esters. Reacting an alcohol with a carbonate would give a carbonate ester, not the simple ester described by RCOOR'. And a carboxylic acid reacting with water does not form an ester.