Answer
Grignard reagents add to aldehydes to give, after protonation, secondary alcohols.
The two alkyl groups of the secondary alcohol are the alkyl group from the Grignard reagent (highlighted in red) and the alkyl group that was bonded to the carbonyl group of the aldehyde (highlighted in blue).
Work Step by Step
There are two ways to synthesize 4-methylhexan-2-ol using Grignard reagent.
The first way is to use acetaldehyde and (2-methylbutyl)magnesium bromide as a Grignard reagent.
Another way is to use 3-methylpentanal, and as a Grignard reagent, we can use methylmagnesium bromide.
In both ways, we get the same product, but a new C-C bond is formed at the two different sites indicated in the figures.
* Solvent is not indicated in the figures, but these reactions take place in ether or THF.
