Several carbonyl groups, which one does ethylene glycol protect?

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


[![enter image description here](https://i.stack.imgur.com/iJEHe.png)](https://i.stack.imgur.com/iJEHe.png)


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](https://i.stack.imgur.com/C5Cjq.jpg)](https://i.stack.imgur.com/C5Cjq.jpg)


*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](https://i.stack.imgur.com/48Gu1.jpg)](https://i.stack.imgur.com/48Gu1.jpg)


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


### References


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](https://chem.libretexts.org/?title=Core/Organic_Chemistry/Aldehydes_and_Ketones/Reactivity_of_Aldehydes_%26_Ketones)


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