Q:

Why do orthoesters react with Grignard reagents, but acetals and ketals don't?

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. 

A:

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.

