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Aldehydes, Ketones and Carboxylic Acids - Mechanism of nucleophilic addition

Grade 12ICSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The carbonyl carbon in aldehydes and ketones is sp2sp^2 hybridized and electrophilic due to the polar nature of the >C=O>C=O bond, where oxygen carries a partial negative charge (δ−\delta^-) and carbon carries a partial positive charge (δ+\delta^+).

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In Step 1 of the mechanism, a nucleophile (Nu−Nu^-) attacks the electrophilic carbonyl carbon from a direction approximately perpendicular to the plane of the sp2sp^2 hybridized orbitals. This changes the hybridization of the carbon atom from sp2sp^2 to sp3sp^3.

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The attack results in the formation of a tetrahedral alkoxide intermediate where the negative charge resides on the oxygen atom.

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In Step 2, the intermediate captures a proton (H+H^+) from the reaction medium or the reagent itself to produce a neutral addition product.

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Reactivity trends: Aldehydes are generally more reactive than ketones toward nucleophilic addition due to: 1. Steric reasons (ketones have two bulky groups), and 2. Electronic reasons (two alkyl groups in ketones reduce the electrophilicity of the carbon more than one alkyl group in aldehydes via the +I+I effect).

📐Formulae

>C=O+Nu−→Slow>C(Nu)−O−>C=O + Nu^- \xrightarrow{\text{Slow}} >C(Nu)-O^-

>C(Nu)−O−+H+→Fast>C(Nu)−OH>C(Nu)-O^- + H^+ \xrightarrow{\text{Fast}} >C(Nu)-OH

CH3CHO+HCN⇌CH3CH(OH)CNCH_3CHO + HCN \rightleftharpoons CH_3CH(OH)CN

R2C=O+NaHSO3⇌R2C(OH)SO3NaR_2C=O + NaHSO_3 \rightleftharpoons R_2C(OH)SO_3Na

💡Examples

Problem 1:

Explain the formation of cyanohydrin from Ethanal (CH3CHOCH_3CHO) using the nucleophilic addition mechanism.

Solution:

  1. Generation of nucleophile: HCN+OH−→CN−+H2OHCN + OH^- \rightarrow CN^- + H_2O.
  2. Attack: The CN−CN^- ion attacks the carbonyl carbon of CH3CHOCH_3CHO to form the intermediate [CH3CH(CN)O]−[CH_3CH(CN)O]^-.
  3. Protonation: [CH3CH(CN)O]−+H2O→CH3CH(OH)CN+OH−[CH_3CH(CN)O]^- + H_2O \rightarrow CH_3CH(OH)CN + OH^-.

Explanation:

The cyanide ion (CN−CN^-) acts as a strong nucleophile. The base (OH−OH^-) is required to catalyze the reaction by generating CN−CN^- from the weak acid HCNHCN.

Problem 2:

Arrange the following in increasing order of reactivity towards nucleophilic addition: CH3COCH3CH_3COCH_3, CH3CHOCH_3CHO, HCHOHCHO.

Solution:

CH3COCH3<CH3CHO<HCHOCH_3COCH_3 < CH_3CHO < HCHO

Explanation:

Formaldehyde (HCHOHCHO) is most reactive as it has no alkyl groups to cause steric hindrance or reduce the positive charge on carbon via the +I+I effect. Acetone (CH3COCH3CH_3COCH_3) is least reactive due to the presence of two electron-releasing methyl groups.