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Aldehydes & Ketones
The carbonyl group is the most important electrophile in organic chemistry. You will learn why it is attacked by nucleophiles, then draw the products of hydration, acetal formation, imine and enamine formation, cyanide addition, hydride and Grignard reactions, and the Wittig reaction.
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What this chapter covers
- The Carbonyl Group: Structure & Electrophilicity. Every aldehyde and ketone reaction starts with one fact: the carbonyl carbon is electron-poor and a nucleophile attacks it.
- Hydrates, Hemiacetals & Acetals. Oxygen nucleophiles add reversibly to carbonyls; acetals give you a way to hide a ketone or aldehyde while you react elsewhere.
- Nitrogen Nucleophiles & Cyanide. Imines, enamines, and cyanohydrins appear in biochemistry and synthesis questions alike; know what forms from which nucleophile.
- Hydride Reductions & Grignard Reagents. NaBH₄, LiAlH₄, and Grignard reagents are the three big carbonyl nucleophiles on the MCAT; know what each one can and cannot touch.
- The Wittig Reaction. The Wittig reaction turns a C=O into a C=C at exactly the position of the carbonyl; it is the standard way to make an alkene from an aldehyde or ketone.
Key points to know
- The carbonyl carbon is electrophilic; oxygen is the nucleophilic/basic end
- Reactivity: formaldehyde > aldehyde > ketone (less hindered, less electron donation)
- Hydrate: two OH on one carbon; favored only for very electrophilic carbonyls
- Hemiacetal: OH and OR; stable mostly when cyclic (sugars)
- Primary amine + C=O → imine (–H₂O), optimum pH about 4–5
- Secondary amine + C=O → enamine (–H₂O)
- NaBH₄ reduces only aldehydes/ketones; LiAlH₄ also reduces esters, acids, amides
- Grignard adds R⁻ to C=O: aldehyde → 2° alcohol, ketone → 3° alcohol, formaldehyde → 1°
Try a practice question
Which compound is the most reactive toward nucleophilic addition?
- mol:C=O{formaldehyde}
- mol:CC=O{acetaldehyde}
- mol:CC(=O)C{acetone}
- mol:CC(=O)c1ccccc1{acetophenone}
Show the answer
A. Formaldehyde has no alkyl groups, so it is the least hindered and has no electron donation into the carbonyl carbon. Acetophenone is the least reactive because of two donating groups, one of them conjugated.
Common questions
What does this chapter cover?
Aldehydes & Ketones covers The Carbonyl Group: Structure & Electrophilicity, Hydrates, Hemiacetals & Acetals, Nitrogen Nucleophiles & Cyanide, Hydride Reductions & Grignard Reagents, The Wittig Reaction.
What should I know first?
The carbonyl carbon is electrophilic; oxygen is the nucleophilic/basic end Reactivity: formaldehyde > aldehyde > ketone (less hindered, less electron donation) Hydrate: two OH on one carbon; favored only for very electrophilic carbonyls
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