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Nucleophilic Addition at the Carbonyl Group

Nucleophilic addition is the reaction in which a nucleophile attacks the electron-deficient carbonyl carbon, followed by protonation or further reaction to form an addition product.

In one line: Carbonyl addition keeps the carbon skeleton and changes C=O into C-OH plus Nu.

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Understand it

The C=O bond is polar, so the carbonyl carbon behaves as an electrophilic centre. Nucleophiles such as CN-, HSO3-, alcohols, and ammonia derivatives attack this carbon. Aldehydes usually react faster than ketones because they have less steric hindrance and weaker electron donation from alkyl groups.

Example

Ethanal reacts with HCN to form 2-hydroxypropanenitrile. Aldehydes and ketones also react with hydroxylamine to form oximes.

Rule to remember

General pattern: R2C=O + Nu- followed by protonation → R2C(OH)-Nu.

With HCN, product is cyanohydrin; with NH2OH, product is oxime; with 2,4-DNP, product is hydrazone derivative; aldehydes with dilute alkali and alpha-hydrogen can undergo aldol condensation.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1For CH3CHO + HCN, CN- attacks the carbonyl carbon of ethanal and the oxygen is protonated.
  2. 2The product is CH3CH(OH)CN, a cyanohydrin.

Method you can reuse

Check the reagent for the active nucleophile, mark the carbonyl carbon as the attack centre, break the C=O π bond towards oxygen, attach the nucleophile to carbon, then protonate oxygen if needed.

Visual tip

A mechanism arrow diagram is useful to show nucleophile attack on carbonyl carbon and movement of π electrons to oxygen.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is ethanal generally more reactive than propanone towards nucleophilic addition?

Think first, then show answer
Ethanal has less steric hindrance and less electron donation to the carbonyl carbon than propanone, so its carbonyl carbon is more easily attacked.

Common wrong answer

Writing CH3CH2CN for ethanal with HCN; this misses the -OH group that remains after nucleophilic addition.

Don't mix this up

Students often show attack on oxygen instead of carbon, or forget that acid-base conditions affect the active nucleophile concentration.

Exam tip

In product prediction, keep the original carbonyl carbon and convert C=O into C-OH plus the new group attached to carbon.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as mechanism steps, reactivity order of aldehydes and ketones, reagent-product matching, aldol product formation, or reason-based questions.
  1. 1

    Enough for 1 mark

    Nucleophilic addition is the reaction in which a nucleophile attacks the electron-deficient carbonyl carbon, followed by protonation or further reaction to form an addition product.

  2. 2

    Add for 2 marks

    Example: Ethanal reacts with HCN to form 2-hydroxypropanenitrile. Aldehydes and ketones also react with hydroxylamine to form oximes.

  3. 3

    Add for 3 marks

    The C=O bond is polar, so the carbonyl carbon behaves as an electrophilic centre.

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