Summary — What This Chapter Covers
- Carboxylic acids come from oxidising primary alcohols or aldehydes (reflux), or hydrolysing nitriles or esters.
- Acid reactions: metals → salt + H2; alkalis → salt + H2O; carbonates → salt + H2O + CO2.
- Esterification (concentrated H2SO4) and reduction by LiAlH4 to a primary alcohol.
- Ester hydrolysis: reversible with dilute acid; complete with dilute alkali.
1. Carboxylic Acids
18.1
Making carboxylic acids
- Oxidation of a primary alcohol or an aldehyde: acidified K2Cr2O7 or KMnO4, heat under reflux. CH3CHO + [O] → CH3COOH
- Hydrolysis of a nitrile: heat under reflux with dilute acid, or with dilute alkali followed by acidification.
CH3CH2CN + 2H2O + HCl → CH3CH2COOH + NH4Cl
CH3CH2CN + NaOH + H2O → CH3CH2COONa + NH3, then add acid
- Hydrolysis of an ester: dilute acid or dilute alkali and heat, followed by acidification if alkali was used.
Reactions
| Reaction | Equation (ethanoic acid) | Observation |
|---|
| reactive metal (redox) | 2CH3COOH + Mg → (CH3COO)2Mg + H2 | effervescence, metal dissolves |
| alkali (neutralisation) | CH3COOH + NaOH → CH3COONa + H2O | no visible change |
| carbonate (acid–base) | 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2 | effervescence; gas turns limewater milky |
| esterification | CH3COOH + CH3OH ⇌ CH3COOCH3 + H2O (concentrated H2SO4, heat) | sweet, fruity smell |
| reduction | CH3COOH + 4[H] → CH3CH2OH + H2O (LiAlH4 in dry ether) | primary alcohol formed |
Effervescence with a carbonate is a quick way to tell a carboxylic acid from an alcohol: both react with sodium, but alcohols are far too weakly acidic to release CO2.
Worked Example
0.300 g of ethanoic acid (Mr 60.0) is added to excess Na2CO3(aq). Calculate the volume of CO2 produced at room temperature and pressure (24.0 dm3 mol−1).
Practice Question
Give a two-step route from 1-bromopropane to butanoic acid.
2. Esters
18.2
Making esters
An ester forms in a condensation reaction between an alcohol and a carboxylic acid, heated with concentrated H2SO4 as catalyst. The reaction is reversible. The ester is named alcohol part first: CH3CH2COOCH3 is methyl propanoate.
Esterification
RCOOH + R′OH ⇌ RCOOR′ + H2O
Hydrolysis of esters
| Dilute acid (e.g. H2SO4), heat under reflux | Dilute alkali (e.g. NaOH), heat under reflux |
|---|
| Equation | CH3COOC2H5 + H2O ⇌ CH3COOH + C2H5OH | CH3COOC2H5 + NaOH → CH3COONa + C2H5OH |
| Products |
carboxylic acid + alcohol |
| Extent |
3
−
+
3