Question

In the mass spectrum of the following compounds, which is the tallest—the peak at m/z= 57 or the peak at m/z= 71?

a. 3-methylpentane

b. 2-methylpentane

Short answer

a. The peak at m/Z value equal to 57 is more intense than the peak at m/z value equal to 71.

b. The peak at m/z equal to 71 is more intense than the peak at m/z equal to 57.

Step-by-step solution

Step-by-step-solutionStep 1: Mass spectrum

The molecular mass, formula and structural features of a compound can be understood using the mass spectrum. The relative abundance of a positively charged fragment is plotted against the m/z value in the mass spectrum.

Identifying the tallest peak in the spectrum of the compounds

a.

3-Methylpentane can lose an ethyl radical forming a secondary carbocation and a primary radical than a methyl radical forming secondary carbocation and a methyl radical. 3-methylpentane possesses two pathways to lose an ethyl radical. Hence the peak at m/Z value equal to 57 is more intense than the peak at m/z value equal to 71.

Structure of 3-methylpentane

b.

2-Methylpentane comprise two pathways to lose a methyl radical, forming a secondary carbocation and a methyl radical in each pathway. It cannot form a secondary carbocation by losing an ethyl radical. The loss of an ethyl radical forms a primary carbocation and a primary radical. Hence, it is more apt to lose a methyl radical than an ethyl radical. Hence the peak at m/z equal to 71 is more intense than the peak at m/z equal to 57.

Structure of 2-methylpentane