How we prepare aldehyde from alcohol using Grignard reagent ?

Question

The number of carbon in aldehyde should be more (at least 2) than in alcohol. Cyclohexanol to C6H10 CH2 CHO ?

Answer 1

Boy, that's a tricky one.....

The first problem is that aldehydes are highly reactive towards Gringnard reagents.  The second problem is Grignards don't react with alcohols.  The second problem is the most straightforward.  You can convert a hydroxyl to a chloro (or better yet bromo) with something like PCl5 or thionyl chloride in ether or THF and then add Mg to form the Grignard reagent.  

Now the second part of the reaction is the tricky part.  If you want to add a one carbon aldehyde, you react your Grignard with formic acid.  To minimize self-condensation, you would drip the freshly formed Grignard reagent into a huge excess of formic acid.   This way, the Grignard reagent would quickly react with formic acid, and not have much chance to find a cyclohexane aldehyde to react with.  If you want a longer aldehyde, you need a bifunctional molecule, one with an aldehyde and a ketone.  Make an acetal out of the aldehyde with something which reacts only with aldehydes, maybe ethylene glycol, then react the ketone with the Grignard regent.  When you hydrolize the Mg salt to the -OH (with H3O+) you will also knock off the acetal to free up your aldehyde.  I know it sounds like a lot of steps, but in practice its really pretty easy.

Good luck.

Wilds