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ChemistryMediumClass 12

Nucleophilic Addition — Mechanism with HCN, RMgX

Aldehydes, Ketones & Carboxylic Acids

17

JEE Qs

8%

Hard

60

min

Master the electron-pushing mechanisms and understand how steric and electronic factors dictate the reactivity and product types for nucleophilic addition reactions.

🧮 Key Formulas

General Nucleophilic Addition: R_2C=O + Nu^- / Nu-H --> R_2C(O^-)Nu --> R_2C(OH)Nu
Cyanohydrin Formation: R_2C=O + HCN --> R_2C(OH)CN
Grignard Addition: R_2C=O + R'MgX --> R_2C(OMgX)R' --(H_2O, H^+)--> R_2C(OH)R'

✅ Key Points for JEE

  • 1The carbonyl carbon is sp2 hybridized and electrophilic due to the polarity of the C=O bond, making it susceptible to nucleophilic attack, which occurs perpendicular to the carbonyl plane.
  • 2The mechanism involves nucleophilic attack on the carbonyl carbon, leading to the formation of a tetrahedral alkoxide intermediate, followed by protonation.
  • 3Reactivity of aldehydes and ketones towards nucleophilic addition is governed by steric hindrance (formaldehyde > aldehydes > ketones) and electronic factors (electron-withdrawing groups increase reactivity, electron-donating groups decrease it).
  • 4Grignard reagents (RMgX) act as strong nucleophiles (carbanion equivalents) and strong bases; they react with aldehydes/ketones to form primary, secondary, or tertiary alcohols after hydrolysis.
  • 5Addition of HCN to aldehydes/ketones, often catalyzed by a base to generate cyanide ions (CN-), forms cyanohydrins, which are valuable synthetic intermediates.

⚠️ Common Mistakes

  • ✕Incorrectly showing the movement of electrons (arrow pushing) during the nucleophilic attack and subsequent protonation steps in the mechanism.
  • ✕Forgetting the crucial acidic workup (hydrolysis) step required to protonate the intermediate alkoxide after the addition of a Grignard reagent, leading to incorrect product representation.
  • ✕Failing to correctly compare the relative reactivity of different aldehydes and ketones, overlooking the combined effects of steric hindrance and electronic factors.
  • ✕Incorrectly identifying the degree (primary, secondary, or tertiary) of the alcohol formed from the reaction of a specific aldehyde or ketone with a Grignard reagent.

NCERT Chapters

  • Class 12 Chemistry Ch 12: Aldehydes, Ketones and Carboxylic Acids