Question

How do Grignard reagents react with alcohols?

Answer 1

Grignard Reagent:

• The Grignard reagent is a combination of alkyl halides $\left(\mathrm{RX}\right)$ and magnesium $\left(\mathrm{Mg}\right)$.
• The molecular formula is $\mathrm{RMgX}$ (R - any alkyl group, X- halogens)

Preparation of Grignard Reagent:

• Alkyl halides react with Magnesium in the presence of dry ether to form a Grignard reagent.

$\underset{\mathrm{Alkyl}\mathrm{halide}}{\mathrm{RX}}+\hspace{0.17em}\underset{\mathrm{Magnesium}}{\mathrm{Mg}}\stackrel{\mathrm{dry}\mathrm{ether}}{\to }\underset{\mathrm{Grignard}\mathrm{reagent}}{\mathrm{RMgX}}$

Reaction with Alcohols:

• The Grignard reagent reacts with aldehydes and ketones to form alcohol.
• The first step of the reaction is the nucleophilic addition of Grignard reagent $\left(\mathrm{RMgX}\right)$ to the carbonyl group $(\mathrm{C}=\mathrm{O})$ to form an adduct.
• Hydrolysis of the adduct yields an alcohol$(\mathrm{R}-\mathrm{C}-\mathrm{OH})$.

• The overall reactions using different aldehydes and ketones are as follows:
• Methanal reacts with Grignard reagent to form Primary alcohols.

$\underset{\mathrm{Methanal}}{\mathrm{HCHO}}+\underset{\mathrm{Grignard}\mathrm{reagent}}{\mathrm{RMgX}}\to \underset{\mathrm{Adduct}}{{\mathrm{RCH}}_2\mathrm{OMgX}}\stackrel{{\mathrm{H}}_2\mathrm{O}}{\to }\underset{\mathrm{Primary}\mathrm{alcohol}}{{\mathrm{RCH}}_2\mathrm{OH}}+\mathrm{Mg}\left(\mathrm{OH}\right)\mathrm{X}$

• Aldehydes react with the Grignard reagent to form Secondary alcohols.

• Tertiary alcohols are formed by the reaction of ketones with the Grignard reagent.