1. Carboxylic Acids: Structure & Acidity
Carboxylic acids contain the carboxyl functional group (-COOH). They are weak acids that dissociate partially in aqueous solution:
RCOOH(aq) <=> RCOO-(aq) + H+(aq)
Carboxylic acids are much stronger acids than alcohols because the resulting carboxylate ion (RCOO-) is stabilised by delocalisation of negative charge across both oxygen atoms.
Characteristic Reaction with Carbonates
Carboxylic acids react with sodium carbonate or sodium hydrogencarbonate with vigorous effervescence (bubbles of CO2 gas):
2CH3COOH(aq) + Na2CO3(aq) -> 2CH3COONa(aq) + CO2(g) + H2O(l)
This rapid effervescence provides the standard chemical test to distinguish carboxylic acids from phenols and alcohols (which are not acidic enough to liberate CO2 from carbonates).
2. Esterification & Ester Hydrolysis
Esterification (Fischer Esterification)
Carboxylic acids react reversibly with alcohols in the presence of a concentrated sulfuric acid (H2SO4) catalyst under reflux:
RCOOH + R'OH <=> RCOOR' + H2O
Esters are named with the alkyl group from the alcohol first, followed by the alkanoate stem from the carboxylic acid (e.g. ethanoic acid + methanol -> methyl ethanoate).
Ester Hydrolysis: Acid vs Base
| Hydrolysis Type | Reagents & Conditions | Chemical Equation | Yield & Reversibility |
|---|---|---|---|
| Acid Hydrolysis | Dilute HCl or H2SO4; reflux | RCOOR' + H2O <=> RCOOH + R'OH | Reversible equilibrium; poor yield (equilibrium mixture of reactants and products). |
| Base Hydrolysis (Saponification) | Aqueous NaOH; reflux | RCOOR' + OH- -> RCOO- + R'OH | Irreversible reaction; complete conversion, high yield. (Carboxylate salt is acidified to recover acid). |
3. Vegetable Oils, Fats, Soaps & Biodiesel
Animal fats and vegetable oils are triesters of glycerol (propane-1,2,3-triol) with long-chain fatty acids (triglycerides).
- Saponification (Soap Making): Alkaline hydrolysis of triglycerides with hot aqueous NaOH produces glycerol (propane-1,2,3-triol) and sodium salts of long-chain fatty acids, which act as soap.
- Biodiesel Production: Triglycerides react with methanol in the presence of a sodium hydroxide / potassium hydroxide catalyst in a transesterification process to yield glycerol and a mixture of methyl esters (biodiesel).
4. Acyl Chlorides & Acid Anhydrides
Acyl chlorides (RCOCl) and acid anhydrides ((RCO)2O) are reactive acid derivatives that undergo nucleophilic addition-elimination reactions at room temperature without requiring an acid catalyst:
| Nucleophile | Reaction with Ethanoyl Chloride (CH3COCl) | Reaction with Ethanoic Anhydride ((CH3CO)2O) | Organic Product Class |
|---|---|---|---|
| Water (H2O) | CH3COCl + H2O -> CH3COOH + HCl(g) | (CH3CO)2O + H2O -> 2CH3COOH | Carboxylic acid |
| Alcohol (CH3OH) | CH3COCl + CH3OH -> CH3COOCH3 + HCl(g) | (CH3CO)2O + CH3OH -> CH3COOCH3 + CH3COOH | Ester |
| Ammonia (NH3) | CH3COCl + 2NH3 -> CH3CONH2 + NH4Cl | (CH3CO)2O + 2NH3 -> CH3CONH2 + CH3COONH4 | Primary amide |
| Primary Amine (CH3NH2) | CH3COCl + 2CH3NH2 -> CH3CONHCH3 + CH3NH3+Cl- | (CH3CO)2O + 2CH3NH2 -> CH3CONHCH3 + CH3COONH3CH3 | N-substituted amide |
5. Nucleophilic Addition-Elimination Mechanism Diagram
6. Industrial Synthesis of Aspirin
Aspirin (acetylsalicylic acid) is synthesised by the esterification of the phenolic -OH group on 2-hydroxybenzoic acid (salicylic acid) using ethanoic anhydride:
2-hydroxybenzoic acid + ethanoic anhydride -> aspirin + ethanoic acid
Why Ethanoic Anhydride is Preferred Industrially over Ethanoyl Chloride
- Safety (No toxic HCl gas): Ethanoyl chloride produces choking, corrosive, highly toxic fumes of hydrogen chloride gas (HCl). Ethanoic anhydride produces harmless, recyclable ethanoic acid.
- Controlled exotherm: Ethanoic anhydride reacts less violently than ethanoyl chloride, making thermal runaway easier to manage on an industrial tonnage scale.
- Cost & storage: Ethanoic anhydride is cheaper and does not hydrolyse as readily with atmospheric moisture during storage.
7. Worked Synthesis & Mechanism
(a) Ethylamine (CH3CH2NH2)
(b) Water
Solution:
- (a) With ethylamine: Forms an N-substituted amide and an alkylammonium salt:
CH3CH2COCl + 2CH3CH2NH2 -> CH3CH2CONHCH2CH3 + CH3CH2NH3+Cl-
Organic product: N-ethylpropanamide. - (b) With water: Rapid hydrolysis releasing steamy fumes of HCl:
CH3CH2COCl + H2O -> CH3CH2COOH + HCl(g)
Organic product: propanoic acid.
Equation:
C3H5(OOCC17H35)3 + 3CH3OH -> C3H5(OH)3 + 3C17H35COOCH3
Reactants: 1 mole of triglyceride + 3 moles of methanol.
Products: 1 mole of glycerol (propane-1,2,3-triol) + 3 moles of methyl stearate (biodiesel).