Carboxylic acids, acyl derivatives and polyesters

What you’ll learn

• Predict reactions of carboxylic acids and acyl chlorides.
• Distinguish acidic and alkaline ester hydrolysis.
• Identify condensation-polymer linkages.

Understand the idea

Carboxylic acids are weak acids and can form hydrogen bonds, including dimers in suitable non-aqueous environments. Short-chain acids dissolve readily in water, while a longer hydrocarbon chain reduces water solubility. They react with bases and carbonates to form salts, and with alcohols in an acid-catalysed equilibrium to form esters. Reduction of a carboxylic acid gives a primary alcohol.

Acyl chlorides are reactive carboxylic-acid derivatives. Water gives a carboxylic acid, alcohols give esters, and ammonia or primary and secondary amines give amides; hydrogen chloride is produced and may be neutralised by additional base. Acidic ester hydrolysis is reversible and gives an acid plus an alcohol. Alkaline hydrolysis gives a carboxylate salt plus an alcohol and is driven forward by formation of the carboxylate.

Condensation polymerisation joins bifunctional molecules while removing small molecules. A diol and a dicarboxylic acid can form a polyester with repeated ester links; a hydroxycarboxylic acid can also polymerise because each molecule contains both required functional groups. Show the bonds continuing through the repeat-unit brackets and retain the carbonyl oxygen in every ester linkage.

Key formulae

$$\mathrm{RCOOH+R'OH\rightleftharpoons RCOOR'+H_2O}$$

$$\mathrm{RCOCl+H_2O\longrightarrow RCOOH+HCl}$$

$$\mathrm{RCOOR'+OH^-\longrightarrow RCOO^-+R'OH}$$

$$\mathrm{RCOCl+2NH_3\longrightarrow RCONH_2+NH_4Cl}$$

Worked example

Predict the organic products when ethyl propanoate undergoes alkaline hydrolysis, and explain the naming.

1. The propanoate portion contains the carbonyl carbon and comes from propanoic acid.
2. Alkaline hydrolysis produces the propanoate ion rather than the free acid.
3. The ethyl portion attached through oxygen becomes ethanol.

Answer: Propanoate ions and ethanol; the accompanying metal ion determines the carboxylate salt.