Could someone please help me figure out the products of the following reaction?

enter image description here

Here are few possibilities I've come up with:

  1. The attacking reagent could the given vicinal diol, with its nucleophilic lone pair, attacking the electrophilic carbon of benzaldehyde. However, that doesn't seem to lead me anywhere.

  2. As the reaction is carried out in acidic medium, one of the -OH groups (or both?) could be protonated, to become better leaving groups leading to formation of a carbocation intermediate. What after that?

Chances are it's probably a nucleophilic addition, proceeding through a tetrahedral intermediate, than a carbocationic one.

It'd be great if someone could show a proper mechanism for the reaction, and explain why it should be so. Thanks a lot!

  • 1
    $\begingroup$ In my opinion, the answer should be (b). The reaction is the formation of acetal. $\endgroup$
    – S R Maiti
    Commented Dec 30, 2017 at 2:54
  • 1
    $\begingroup$ Yes, this is acetal formation $\endgroup$ Commented Dec 30, 2017 at 2:55

1 Answer 1


This reaction is similar to the one where you use ethylene glycol to form a temporary protecting group for an aldehyde that you do not want to react in further steps. The answer therefore should be (b).

  • 1
    $\begingroup$ I guess you and I know that you are correct but could you expand? e.g. showing OP where there mechanistic proposal was correct and how to move on from there. $\endgroup$
    – Jan
    Commented Dec 30, 2017 at 16:56

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