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18.2 Esters

Syllabus
9701–2028–2029
Topic
18.2
Level
AS

Condense an alcohol and carboxylic acid to make an ester

An alcohol and a carboxylic acid form an ester and water in a reversible condensation reaction. Concentrated H₂SO₄ is the catalyst and is not consumed in the overall equation.

RCOOH+RX′OH⇌conc ⋅  HX2SOX4RCOORX′+HX2O\ce{RCOOH + R'OH <=>[conc.\ H2SO4] RCOOR' + H2O}

The alcohol loses H from O–H and supplies the group attached to the single-bonded ester oxygen, R′. The acid loses –OH and supplies the acyl part RCO–; together those atoms account for the eliminated water.

CHX3COOH+CHX3CHX2OH⇌CHX3COOCHX2CHX3+HX2O\ce{CH3COOH + CH3CH2OH <=> CH3COOCH2CH3 + H2O}

Name the alcohol-derived group first as an alkyl name, then the acid-derived part as an alkanoate: ethanol + ethanoic acid gives ethyl ethanoate. Do not reverse the two name parts or include the catalyst in the product.

Ester hydrolysis gives different products in acid and alkali

Dilute acid hydrolysis of an ester is reversible and gives a carboxylic acid plus an alcohol. Dilute alkali hydrolysis gives a carboxylate salt plus an alcohol and is effectively driven to completion.

Heat supplies energy and the reagent determines whether the carboxyl group remains protonated. Acidification after alkaline hydrolysis converts the carboxylate into the free acid.

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH in acid; with NaOH, products are CH₃COONa + CH₃CH₂OH, followed by acidification if ethanoic acid is required.

Do not write identical products for acid and alkaline hydrolysis, and do not forget reversibility in the acid route.