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Carbonyl Group Chemistry

Aldehydes and Ketones

These are formed by oxidation of alcohols. Aldehydes can be further oxidized to carboxylic acids, which is the basis for Fehling’s and Tollen’s tests.

Oxidation of alcohols to form aldehydes and ketones

They can be reduced back to alcohols using NaBH4. Reduction occurs via nucleophilic addition of a hydride ion (H⁻).

Reduction mechanism of an aldehyde to alcohol

Similar mechanisms apply for addition of cyanide (CN⁻), potentially forming a racemic mixture due to attack from either side of the C=O bond.

Testing Aldehydes and Ketones

Test Procedure Result
Tollen’s Reagent Mix with aqueous ammonia and silver nitrate, warm gently. Aldehyde: silver mirror forms. Ketone: no reaction.
Fehling’s Solution Mix and heat solution. Aldehyde: red precipitate. Ketone: no change.

Carboxylic Acids and Esters

Carboxylic acids contain both a carbonyl and hydroxyl group on the same carbon. They are weak acids and react with sodium hydrogen carbonate to release CO₂.

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Esters are formed by reacting a carboxylic acid with an alcohol using an acid catalyst. Their structure is shown below:

General ester structure diagram

Naming Esters: The alcohol provides the prefix (e.g., ethyl), and the acid provides the suffix (e.g., ethanoate), forming names like ethyl ethanoate.

Uses: Esters are used as flavourings, solvents, and plasticisers.

Hydrolysis of Esters

Esters can be broken down by alkalis to form an alcohol and a carboxylate salt:

CH₃COOC₂H₅ + NaOH → C₂H₅OH + CH₃COONa

This is the process used in making soap from fats, which are natural esters.

Acylation Reactions

Acyl chlorides and acid anhydrides react with nucleophiles via a substitution-elimination mechanism:

Acylation reaction mechanism

Industrial Note: Ethanoic anhydride is preferred over ethanoyl chloride in manufacturing (e.g. aspirin) due to cost, safety, and no HCl by-product.