Chapter 18 · Organic Chemistry
18.15Carboxylic Acids and Esters
10 min · two checks
Predict
What functional group makes vinegar acidic, and what related group makes many fruit smells?
The idea
- Recognize carboxylic acids and esters.
- Describe how an ester relates to an acid and an alcohol.
A carboxylic acid has the group –COOH: a carbonyl and a hydroxyl on the same carbon. The names end in -oic acid: methanoic acid (formic), ethanoic acid (acetic). They are weak acids. The O–H is far more acidic than the O–H of an alcohol because the conjugate base is resonance-stabilized. Small carboxylic acids mix with water and smell sharp. Longer ones — fatty acids — become oily.
An ester is what you get, formally, when a carboxylic acid and an alcohol lose water: the –OH of the acid and the H of the alcohol leave, and the pieces join as R–COO–R′. Names sound like ethyl ethanoate (ethyl acetate). Fats are triesters of glycerol. Esters often smell pleasant; acids usually do not. Hydrolysis of an ester in acid or base returns the parent pieces, which is what happens when soap is made from fat and lye — saponification frees the fatty-acid salts.
Keep these
- Carboxylic acids: –COOH, weak acids, -oic acid names.
- Esters: RCOOR′, often fragrant, formed from acid plus alcohol.
- Fats are esters. Saponification hydrolyzes them.