Question

A laboratory student added 1-bromobutane to a flask containing dry ether and magnesium turnings. An exothermic reaction resulted, and the ether boiled vigorously for several minutes. Then she added acetone to the reaction mixture and the ether boiled even more vigorously. She added dilute acid to the mixture and separated the layers. She evaporated the ether layer, and distilled a liquid that boiled at $\mathbf{143}{}^{\mathbf{\circ }}\mathbf{C}$. role="math" localid="1649504068494" $\mathbf{GC}-\mathbf{MS}$analysis of the distillate showed one major product with a few minor impurities. The mass spectrum of the major product is shown here.

(a) Draw out the reactions that took place and show the product that was formed.

(b) Explain why the molecular ion is not visible in the mass spectrum, and show what ions are likely to be responsible for the strong peaks at $\mathit{m}\mathbf{/}\mathit{z}$, 59 and 101.

Short answer

(a) The product formed is $\text{2 - methylhexan - 2 - ol}$.

(b) The molecular ion is not visible in the mass spectrum because the ions formed are stabilized by resonance. The ions responsible for the strong peaks at$m/z$ 59 and 101 are as shown below.

Step-by-step solution

Molecular ion

When high-energy electron hits an organic compound, it removes one of the electrons from the molecule and hence, produces positively charged ions known as the molecular ions.

Formation of Molecular ion

The molecular ion not only acts as a cation but also as a free radical because of the presence of an odd number of electrons. Hence, it can be called as enol radical cation.

Fragmentation of alcohols

Alcohols typically fragment subsequent to the carbinol carbon atom to form a resonance stabilized carbocation.

Reaction part and the ions responsible for strong peaks at   59 and 101.

(a) $\text{1 - bromobutane}$, on treatment with and ether forms a Grignard product (bytylmagnesium bromide), which on further treatment with acetone and hydrolysis yield$\text{2 - methylhexan - 2 - ol}$.

Reaction

The fragmentation of to form peaks at peaks at and are as shown below.

Fragmentation of $\text{2 - methylhexan - 2 - ol}$

(b) Alcohols commonly fragment next to carbinol carbon atom to form a resonance stabilized cation. The molecular ion peak is not visible in the mass spectrum for localid="1649503861319" $\text{2 - methylhexan - 2 - ol}$since the strong peaks formed at localid="1649503809885" $m/z$and 101 are stabilized by resonance. The ions responsible for the strong peaks at localid="1649503837365" $m/z$59 and 101 are as shown below.

Ions Responsible for Strong Peaks