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18. Carboxylic acids and derivatives

Syllabus
9701–2028–2029
Section
18
Level
AS

18.1 Carboxylic acids

Syllabus
9701–2028–2029
Topic
18.1
Level
AS

Prepare carboxylic acids by complete oxidation or hydrolysis

Starting material Reagent and conditions Immediate product / final step
primary alcohol or aldehyde acidified K₂Cr₂O₇ or acidified KMnO₄, reflux carboxylic acid
nitrile dilute acid, heat carboxylic acid + NH₄⁺
nitrile dilute alkali, heat carboxylate + NH₃; acidify to carboxylic acid
ester dilute acid, heat carboxylic acid + alcohol
ester dilute alkali, heat carboxylate + alcohol; acidify to carboxylic acid

Reflux keeps a primary alcohol, the aldehyde intermediate and the oxidant together so oxidation can reach the acid. Distillation would instead remove the aldehyde and stop the sequence early.

RCN+2 HX2O+HX+→RCOOH+NHX4X+\ce{RCN + 2H2O + H+ -> RCOOH + NH4+}

RCN+HX2O+OHX−→RCOX2X−+NHX3\ce{RCN + H2O + OH- -> RCO2- + NH3}

The nitrile carbon becomes the carboxyl carbon, so hydrolysing a nitrile preserves the total carbon count. After alkaline hydrolysis, acidification protonates RCO₂⁻; it is required when the requested product is RCOOH.

Carboxylic acids form salts, esters and primary alcohols

Reagent Reaction type Products / condition
reactive metal redox carboxylate salt + H₂
alkali neutralisation carboxylate salt + H₂O
carbonate acid–base carboxylate salt + H₂O + CO₂
alcohol esterification / condensation ester + H₂O; concentrated H₂SO₄ catalyst
LiAlH₄ reduction primary alcohol

2 RCOOH+2 Na→2 RCOONa+HX2\ce{2RCOOH + 2Na -> 2RCOONa + H2}

RCOOH+NaOH→RCOONa+HX2O\ce{RCOOH + NaOH -> RCOONa + H2O}

2 RCOOH+NaX2COX3→2 RCOONa+HX2O+COX2\ce{2RCOOH + Na2CO3 -> 2RCOONa + H2O + CO2}

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

RCOOH+4 [H]→LiAlHX4RCHX2OH+HX2O\ce{RCOOH + 4[H] ->[LiAlH4] RCH2OH + H2O}

Hydrogen is evolved with a reactive metal, whereas carbon dioxide is the diagnostic gas with a carbonate. Concentrated sulfuric acid catalyses esterification; it is not consumed in the overall equation. LiAlH₄ reduces the carboxyl carbon to a primary-alcohol carbon.

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.