Nucleophilic addition to carbonyls
The carbon of a C=O group is partly positive, so nucleophiles attack it, pushing the pi electrons onto oxygen. This gives an alkoxide, which picks up H⁺ to become an alcohol. Aldehydes react faster than ketones.
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Analogy
A door with a weak lock: the carbonyl carbon is exposed to anyone who pushes.
Example
H⁻ attacks the C=O carbonOxygen becomes O⁻Add water or acid: O–HProduct: propan-2-ol
In practice
Sodium borohydride adds hydride to a ketone to make a secondary alcohol, a key step in making many medicines.
Practice
Flashcards
- What does NaBH₄ do to a ketone?Reduces it to a secondary alcohol.
Questions people ask
Why are aldehydes more reactive than ketones?
A ketone has two alkyl groups that donate electron density to the carbonyl carbon and make it less positive. They also crowd the carbon, blocking attack. An aldehyde has only one alkyl group and a small hydrogen, so its carbon is more electrophilic and easier to reach.
What do LiAlH₄ and NaBH₄ do?
Both are sources of hydride (H⁻) that reduce carbonyl compounds to alcohols. NaBH₄ is mild and reduces aldehydes and ketones. LiAlH₄ is stronger and also reduces esters and carboxylic acids. LiAlH₄ reacts violently with water, so it needs dry solvents.