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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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

    Then show how to use it

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

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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.

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.

Memory hook

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

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

How CBSE asks it

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

Avoid common mistakes

Common confusion

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

Common wrong answer

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

Exam tip

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

Quick check

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

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

Answer writing and exam use

1-mark answer

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-mark answer

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

3-mark answer

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. 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. For CH3CHO + HCN, CN- attacks the carbonyl carbon of ethanal and the oxygen is protonated. The product is CH3CH(OH)CN, a cyanohydrin. Asked as mechanism steps, reactivity order of aldehydes and ketones, reagent-product matching, aldol product formation, or reason-based questions. Writing CH3CH2CN for ethanal with HCN; this misses the -OH group that remains after nucleophilic addition.
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