CourseCourseQ BankQuestion BankDocsDocuments

18.1 Carboxylic acids

Syllabus
9701–2028–2029
Topic
18.1
Level
AS

Learning objectives

18.1.1Preparation of carboxylic acids• Recall preparation of carboxylic acids- (a) oxidation of primary alcohols and aldehydes with acidified K 2Cr2O7 or acidified KMnO4 and refluxing- (b) hydrolysis of nitriles with dil. acid or dil. alkali followed by acidification- (c) hydrolysis of esters with dil. acid or dil. alkali and heat followed by acidification18.1.2Describe• Describe:- (a) the redox reaction with reactive metals to produce a salt and H2(g)- (b) the neutralisation reaction with alkalis to produce a salt and H2O(l )- (c) the acid–base reaction with carbonates to produce a salt and H2O(l) and CO2(g)- (d) esterification with alcohols with conc. H2SO4 as catalyst- (e) reduction by LiAl H4 to form a primary alcohol

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.