[shafaifer]

Say you have a molecule, consisting of the two functional groups alcohol and ester (2-hydroethyl acetate for example), that is brought into a mixture with the Grignard reagent CH₃MgBr. My question is if the Grignard reagent will be protonated by the alcohol instead of reacting with the ester and form a ketone for instance. Just generally you should "protect this alcohol", isn't correct?

Best regards

[pgk]

Exactly, you need to protect the hydroxyl group, otherwise the Grignard reagent will be deactivated.
Theoretically, it could be possible to use the double amount of the Grignard reagent, in order to achieve the addition on the ester group, too. But in practice, this does not work well and leads to low yields.
For your own education, Grignard reaction with esters does not stop to the ketone, but it goes further to the formation of tertiary alcohols. Ketones can be prepared from esters by reaction with organocadmium reagents.
Please, take a better look to the Grignard reaction.

[shafaifer]

Thank you very much for your education, by all means, is it not possible to withdraw the ketone from further reaction into a tertiary alcohol?

[pgk]

No, due to reaction kinetics. The one reaction is faster than the other. Which one?

[shafaifer]

I guess the 3° alcohol.

[pgk]

Exactly.

[kriggy]

Thank you very much for your education, by all means, is it not possible to withdraw the ketone from further reaction into a tertiary alcohol?

If you want an aldehyde, I seen that orthoesters yield  aldehydes when reacting with Grignards. It would however, require some setup to make the orthoester in your molecule