What is the correct decreasing order of reactivity of the following carbonyl compounds toward nucleophilic addition reactions: Formaldehyde (I), Acetaldehyde (II), Acetone (III), and Di-tert-butyl ketone (IV)?

Answer and explanation

Correct answer: C. I > II > III > IV

  1. Step 1: Nucleophilic addition to a carbonyl group involves attack of a nucleophile on the electrophilic carbonyl carbon ().
  2. Step 2: Electronic effect: Electron-donating alkyl groups ( inductive effect) disperse the partial positive charge on the carbonyl carbon, decreasing its electrophilicity ().
  3. Step 3: Steric effect: Bulky alkyl groups crowd the transition state, hindering the nucleophile from attacking.
  4. Step 4: Formaldehyde (, I) has zero alkyl groups and is most reactive; acetaldehyde (, II) has one methyl group; acetone (, III) has two methyl groups; di-tert-butyl ketone (IV) has massive steric shielding, making it least reactive: I > II > III > IV.
Answer
I > II > III > IV
Why not the others: 'IV > III > II > I' inverts the reactivity series; 'III > II > I > IV' incorrectly ranks ketones ahead of aldehydes; 'II > I > III > IV' places acetaldehyde ahead of formaldehyde.

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