2. Aldehydes & Ketones
The carbonyl carbon is electrophilic — nucleophiles attack it, and the product depends on the nucleophile. Aldehydes are more reactive than ketones, and only aldehydes can be oxidized further.
Nucleophilic Addition
- Hydride (NaBH4, LiAlH4): → alcohol.
- Grignard (RMgBr): → alcohol (aldehyde → secondary, ketone → tertiary).
- HCN: → cyanohydrin.
- Alcohols (H+): → hemiacetal, then acetal.
- Amines: → imine (Schiff base).
Example: Acetaldehyde + 2 methanol (acid catalyst) → 1,1-dimethoxyethane (an acetal). Acetals are protecting groups for carbonyls — stable to base and nucleophiles, cleaved by aqueous acid.
The Wittig Reaction
A phosphorus ylide converts a carbonyl into an alkene with the double bond exactly where the C=O was.
Example: Cyclohexanone + $\text{Ph}_3\text{P=CH}_2$ → methylenecyclohexane.
Oxidation: Tollens' Test
Aldehydes oxidize to carboxylic acids with Tollens' reagent (ammoniacal AgNO3), depositing a silver mirror. Ketones do not react — the classic aldehyde/ketone distinction.
Practice Problems
- What does LiAlH4 reduction of propanal give?
- What is the product of acetone + HCN?
- What happens when benzaldehyde is treated with Tollens' reagent?
- What alkene forms from formaldehyde + Ph3P=CHCH3 (Wittig)?
Answers: 1. 1-propanol 2. 2-hydroxy-2-methylpropanenitrile (the cyanohydrin) 3. Benzoic acid, plus a silver mirror 4. Propene