Q12-45E - Organic Chemistry (Mcmurry)

Question

Grignard reagents undergo a general and very useful reaction with ketones. Methylmagnesium bromide, for example, reacts with cyclohexanone to yield a product with the formula ${\mathbf{C}}_{\mathbf{7}}{\mathbf{H}}_{\mathbf{14}}\mathbf{O}$. What is the structure of this product if it has an IR absorption at $\mathbf{3400}{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$?

Step-by-Step Solution

Answer

Step 1: Reaction of carbonyl compound with Grignard reagent

Grignard reagents react with carbonyl compounds to generate primary, secondary and tertiary alcohols. The reaction of aldehyde and ketone with Grignard reagent generates secondary and tertiary alcohol respectively.

Step 2: Infrared spectrum

Carbonyl compounds can be easily determined from the NMR spectra as they exhibit peak in the region 1670 to ${\text{1780 cm}}^{\text{ - 1}}$. Saturated open chain ketones and six membered ketones exhibit a peak in the region  ${\text{1715 cm}}^{\text{ - 1}}$.

Step 3: Proposing the structure from the IR data

The IR absorption shows that the compound has a IR absorption value at ${\text{3400 cm}}^{\text{ - 1}}$. The reaction of cyclohexanone with methyl magnesium bromide forms 1-methylcyclohexanol (alcohol).

The IR absorption value at ${\text{3400 cm}}^{\text{ - 1}}$ indicates the hydroxyl group in 1-methylcyclohexanol.

The reaction can be given as:

                       Cyclohexanone with methyl magnesium bromide

 

The structure of the compound can be given as: