Question 1

Why do aldehydes and ketones undergo nucleophilic addition rather than electrophilic addition?

Accepted Answer

The oxygen atom is more electronegative than carbon, so it pulls the π electrons of the C=O bond toward itself. This leaves the carbonyl carbon with a partial positive charge (δ+), making it attractive to electron-rich species. Nucleophiles therefore attack the carbon — the exact opposite of alkenes, where the electron-rich double bond invites electrophiles.

Question 2

Why are aldehydes more reactive than ketones towards nucleophilic addition?

Accepted Answer

For two independent reasons: (1) steric hindrance — ketones carry two alkyl groups around the carbonyl carbon versus one (or none) in aldehydes, so the incoming nucleophile faces more crowding; (2) the +I effect — two alkyl groups push electron density toward the carbon, reducing its partial positive charge. Formaldehyde, with no alkyl groups, is the most reactive of the simple aliphatic examples.

Question 3

What is the hybridisation of the carbonyl carbon before and after nucleophilic attack?

Accepted Answer

Before attack the carbonyl carbon is sp² hybridised and trigonal planar, with bond angles of approximately 120°. After the nucleophile adds, it becomes sp³ hybridised and tetrahedral, with bond angles of approximately 109.5°. The π bond breaks and its electron pair moves onto the oxygen, giving a tetrahedral alkoxide intermediate.

Question 4

What is a tetrahedral alkoxide intermediate?

Accepted Answer

It is the species formed immediately after the nucleophile bonds to the carbonyl carbon. The carbon has folded from planar sp² into tetrahedral sp³ geometry, and the former π electrons sit on the oxygen as a full negative charge (O⁻). This intermediate is then protonated to give the neutral addition product, such as a cyanohydrin from HCN or an alcohol from a Grignard reagent.

Question 5

How do Tollens' and Fehling's tests distinguish aldehydes from ketones?

Accepted Answer

Tollens' reagent gives a silver mirror with all aldehydes, both aliphatic and aromatic. Fehling's solution gives a red-brown precipitate of Cu₂O with aliphatic aldehydes only — aromatic aldehydes do not respond. Ketones give neither test. The iodoform test is given by methyl ketones and acetaldehyde, but also by ethanol and methyl secondary alcohols, so it is not exclusive to carbonyl compounds.

The Carbonyl: Attack From Above, Not Across

Practice

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