We know that ethylene glycol is used to protect carbonyl functional groups. But, consider this:

enter image description here

Here, we have two carbonyl groups, but only one equivalent of ethylene glycol. Obviously, only one carbonyl group will get protected in this case. But will it be the $\ce{C}^1$ or $\ce{C}^2$ carbonyl? And why?


Several factors influence the overall rate of a reaction under various conditions. Among the crucial factors are:

  1. structural features of the carbonyl compound;
  2. the role of protons or other Lewis acids in activating the carbonyl group toward nucleophilic attack;
  3. the reactivity of the nucleophilic species and its influence on subsequent steps; and
  4. the stability of the tetrahedral intermediate and the extent to which it proceeds to product rather than reverting to starting material. (Ref.1)

Limiting ourselves to current molecule, aldehydes are more reactive than ketones. Electron-withdrawing substituents render the carbonyl group more electrophilic.

reactivity chart

Image credit: Organic Chemistry, Structure and Function Seventh Ed by Peter Vollhardt, University of California at Berkeley, Neil Schore, University of California at Davis

Because aldehydes form acetals more readily than ketones, we can protect an aldehyde selectively in the presence of a ketone. The following example shows the reduction of a ketone in the presence of a more reactive aldehyde (Ref.2):

enter image description here

With reference to your question, C-2 is more reactive then C-1 carbonyl group for the above reasons.


  1. Advanced Organic Chemistry, Fifth Edition, Part A: Structure and Mechanisms by Francis A. Carey and Richard J. Sundberg, University of Virginia
  2. Organic Chemistry, Eighth Edition, by L. G. Wade Jr., Whitman College
  3. Reactivity of Aldehydes & Ketones - Chem LibreTexts
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