Question

Question:Reaction of pentanoyl chloride $\left({\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2{\mathbf{CH}}_2\mathbf{COCl}\right)$ with lithium dimethyl cuprate $\left[\mathbf{LiCu}{\left({\mathbf{CH}}_3\right)}_2\right]$forms a compound J that has a molecular ion in its mass spectrum at 100, as well as fragments at m/z= 85, 57, and 43 (base). The IR spectrum of J has strong peaks at 2962 and 1718 cm–1. Propose a structure for J.

Short answer

Answer

Structure of J:

Step-by-step solution

Reaction with lithium dimethyl cuprate

Lithium dimethyl cuprate is also known as the Gilman reagent. It acts as a reagent for conjugate addition in $\mathbf{\alpha }\mathbf{,}\mathbf{\beta }$-unsaturated ketones and nucleophilic reagents for alkyl halides.

When pentanoyl chloride reacts with the Gilman reagent, the ${{\text{S}}_{\text{N}}}^{\text{2}}$ attack occurs at the carbonyl carbon center, and chlorine acts as a leaving reagent. 2-Hexanone is formed as a product.

Reaction of pentanoyl chloride with the Gilman reagent

Determination of the structure of J

According to the reaction mentioned before, the structure of J can be as follows.

Possible structure of J

The molecular weight of J is 100, which is similar to the molecular ion of J.

The fragments that form upon the cleavage of 2-hexanone are given below.

Fragments of 2-hexanone

Thus, the fragments of 2-hexanone also have similar base peaks, as mentioned in the above data.

IR spectroscopy of 2-hexanone

The IR spectrum of J shows the two peaks: ${\text{2962\hspace{0.17em}cm}}^{\text{-1}}{\text{,1718\hspace{0.17em}cm}}^{\text{-1}}$.

Comparing the peaks with the structure of 2-hexanone, we see that there will be a peak at around ${\text{1700\hspace{0.17em}cm}}^{\text{-1}}$, owing to the ketone functionality. The C-H bond stretching will occur at around ${\text{3000-2800\hspace{0.17em}cm}}^{\text{-1}}$.

The above comparison also justifies that J is 2-hexanone.