Carboxylic Acids

Carboxylic acids are prepared by oxidising primary alcohols, aldehydes and alkylbenzenes, by hydrolysing nitriles, amides, acyl chlorides, anhydrides and esters, and by treating Grignard reagents with carbon dioxide.

This lesson works through the main laboratory methods for preparing carboxylic acids, with balanced equations and examples. Oxidation of a primary alcohol or an aldehyde keeps the number of carbon atoms, while vigorous oxidation of an alkylbenzene cuts any side chain down to a single –COOH group. The nitrile route and the Grignard route each add one carbon atom to an alkyl halide. Hydrolysis of acid derivatives returns the parent acid, and every method that ends in alkaline conditions gives a salt that must be acidified to release the free acid.

Methods of Preparation of Carboxylic Acids

Detailed Summary of Carboxylic Acid Preparation

Most methods for making carboxylic acids fall into three families: oxidation (of primary alcohols, aldehydes and alkylbenzenes), hydrolysis (of nitriles, amides, acyl chlorides, anhydrides and esters) and carbon–carbon bond formation (Grignard reagents with carbon dioxide). A useful habit is to count carbon atoms: decide whether the product has the same number of carbons as the starting material, one more, or fewer, and that points you to the right method.

From Primary Alcohols and Aldehydes

• Primary alcohols are oxidised to carboxylic acids by potassium permanganate (in neutral, acidic or alkaline solution), by potassium dichromate in dilute sulphuric acid, or by chromium trioxide in sulphuric acid (Jones reagent).
• RCH2OH + 2[O] → RCOOH + H2O. For example, CH3CH2OH + 2[O] → CH3COOH + H2O. With acidified potassium dichromate, the solution turns from orange to green as Cr2O7²⁻ is reduced to Cr³⁺.
• The aldehyde is an intermediate: RCHO + [O] → RCOOH. Aldehydes are oxidised so easily that even mild oxidising agents such as Tollens' reagent and Fehling's solution convert them into the acid (as its salt).
• With alkaline KMnO4 the product is the potassium salt, RCOOK; adding dilute acid releases the free carboxylic acid.
• The number of carbon atoms does not change: ethanol gives ethanoic acid and propan-1-ol gives propanoic acid.

From Alkylbenzenes

• Vigorous oxidation with hot alkaline KMnO4 (followed by acidification), acidic KMnO4 or chromic acid turns an alkyl side chain on a benzene ring into a –COOH group, whatever the length of the chain.
• Toluene gives benzoic acid: C6H5CH3 + 3[O] → C6H5COOH + H2O. Ethylbenzene also gives benzoic acid, because the extra carbon is lost.
• The carbon attached to the ring must carry at least one hydrogen atom. Primary and secondary alkyl groups are oxidised, but a tertiary group such as tert-butyl is not affected.

From Nitriles and Amides

• Nitriles are hydrolysed to carboxylic acids in the presence of an acid (H⁺) or a base (OH⁻) as catalyst. The reaction passes through an amide: RCN + H2O → RCONH2, then RCONH2 + H2O → RCOOH + NH3. Under mild conditions the reaction can be stopped at the amide.
• In acid the overall change is RCN + 2H2O + H⁺ → RCOOH + NH4⁺. For example, ethanenitrile (CH3CN) gives ethanoic acid and benzonitrile (C6H5CN) gives benzoic acid.
• Because nitriles are made from alkyl halides and KCN, this route adds one carbon atom: CH3CH2Br + KCN → CH3CH2CN + KBr, and hydrolysis of CH3CH2CN gives CH3CH2COOH (propanoic acid).

From Grignard Reagents

• A Grignard reagent, RMgX, is made from an alkyl or aryl halide and magnesium in dry ether. It adds to carbon dioxide (often solid CO2, dry ice) to give a salt, which dilute acid converts into the carboxylic acid: RMgX + CO2 → RCOOMgX, then RCOOMgX + H2O (H⁺) → RCOOH + Mg(OH)X.
• For example, CH3MgBr gives ethanoic acid and C6H5MgBr gives benzoic acid.
• The carbon of CO2 becomes the carboxyl carbon, so this route also adds one carbon atom to the halide.
• Everything must be kept dry, because water destroys the reagent: RMgX + H2O → RH + Mg(OH)X.

From Acyl Chlorides, Anhydrides and Esters

• Acyl chlorides are hydrolysed by water: RCOCl + H2O → RCOOH + HCl. With aqueous alkali the carboxylate salt forms, and acidification gives the acid.
• Anhydrides are hydrolysed to two molecules of acid: (CH3CO)2O + H2O → 2CH3COOH.
• Esters are hydrolysed by dilute acid in a reversible reaction, RCOOR′ + H2O ⇌ RCOOH + R′OH, or more completely by alkali (saponification): RCOOR′ + NaOH → RCOONa + R′OH, after which RCOONa + HCl → RCOOH + NaCl. Ethyl benzoate, for example, gives sodium benzoate and then benzoic acid.

Applications

• Industry makes ethanoic acid on a large scale by combining methanol with carbon monoxide over a rhodium or iridium catalyst: CH3OH + CO → CH3COOH.
• Vinegar is made when bacteria (Acetobacter) oxidise the ethanol in fermented liquids to ethanoic acid.
• In the laboratory, choose the route by counting carbons: oxidation for the same number, the nitrile or Grignard route for one more, and side-chain oxidation for an aromatic acid from an alkylbenzene.