Grignard reagents react with aldehydes and ketones to form alcohols, with a new carbon-carbon bond.The δ− carbon of the Grignard reagent acts as a nucleophile and attacks the δ+ carbon of the C=O group (nucleophilic addition). This forms an intermediate magnesium alkoxide, which is then hydrolysed with dilute acid to give the alcohol. The reaction is carried out in dry ether.R'MgX + R2C=O → R2R'C-OMgX → R2R'C-OH The type of alcohol depends on the carbonyl compound:Methanal, HCHO, gives a primary alcohol.
Other aldehydes, RCHO, give secondary alcohols.
Ketones, RCOR, give tertiary alcohols.This is because the carbon of the C=O group becomes the carbon carrying the OH group, and it gains one extra carbon group from the Grignard reagent. In methanal it had no carbon groups, so it ends up with one; in an aldehyde it had one and ends up with two; in a ketone it had two and ends up with three.Worked examples, all using methylmagnesium bromide, CH3MgBr, then dilute acid:With methanal: CH3CH2OH, ethanol (primary).
With ethanal: CH3CH(OH)CH3, propan-2-ol (secondary).
With propanone: (CH3)3COH, 2-methylpropan-2-ol (tertiary).To plan a route, find the carbon carrying the OH in the target alcohol. Break one of its C-C bonds: one piece becomes the Grignard reagent and the other becomes the carbonyl compound. For example, propan-1-ol, CH3CH2CH2OH, can be made from ethylmagnesium bromide and methanal.
Other aldehydes, RCHO, give secondary alcohols.
Ketones, RCOR, give tertiary alcohols.This is because the carbon of the C=O group becomes the carbon carrying the OH group, and it gains one extra carbon group from the Grignard reagent. In methanal it had no carbon groups, so it ends up with one; in an aldehyde it had one and ends up with two; in a ketone it had two and ends up with three.Worked examples, all using methylmagnesium bromide, CH3MgBr, then dilute acid:With methanal: CH3CH2OH, ethanol (primary).
With ethanal: CH3CH(OH)CH3, propan-2-ol (secondary).
With propanone: (CH3)3COH, 2-methylpropan-2-ol (tertiary).To plan a route, find the carbon carrying the OH in the target alcohol. Break one of its C-C bonds: one piece becomes the Grignard reagent and the other becomes the carbonyl compound. For example, propan-1-ol, CH3CH2CH2OH, can be made from ethylmagnesium bromide and methanal.