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My book mentions the synthesis of aldehydes and ketones using alkyl ortho esters. It's called Bodroux-Chichibabin aldehyde synthesis.

Now, later, the book mentions this:

R- displaces EtO-, giving an acetal or ketal, which are unreactive with Grignard reagents, so a secondary alcohol is not formed.

Why do orthoesters react with grignard reagents, but acetals and ketals don't? They have essentially the same functional groups, just two instead of three.

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Grignard reactions are performed in ether or THF. There you have only one -OR group on a carbon and, since it is the solvent, it is not reacting with it. This shows that not always a -OR group is substituted in a Grignard reaction.

When you have three -OR's on one carbon, the formal positive charge ($\delta+$) on it is higher than when there are only two -OR's. Therefore, the reaction proceeds on orthoesters and not on acetals or ketals.

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