To prepare Butane -2-ol from $C{H}_{3}MgBr,$  the other compound used is

When CH₃MgBr reacts with an aldehyde, the Grignard reagent adds to the carbonyl carbon, followed by hydrolysis, resulting in the formation of an alcohol. For butan-2-ol, the target alcohol, the aldehyde used must have three carbon atoms (propanal) so that the addition of the methyl group from CH₃MgBr produces a four-carbon secondary alcohol.

Reaction:

CH₃MgBr + CH₃CH₂CHO → CH₃CH(OH)CH₂CH₃ (butan-2-ol)