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.
They can be reduced back to alcohols using NaBH4. Reduction occurs via nucleophilic addition of a hydride ion (H⁻).
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:
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:
Industrial Note: Ethanoic anhydride is preferred over ethanoyl chloride in manufacturing (e.g. aspirin) due to cost, safety, and no HCl by-product.