Question

Question:2,3-Dimethylbutane and 2,2-dimethylbutane have the same molecular ion in the mass spectrum, but only one of these isomers gives a significant fragment at m/z= 57.

(a) Whichisomer shows an intense peak at m/z= 57?

(b) Propose a structure for the ion that gives riseto this peak.

(c) The base peak in the mass spectrum of the other isomer occurs at m/z= 43.

What ion gives rise to this peak?

Short answer

Answer

(a) 2,2-dimethylbutane shows intense peak at m/z = 57.

Step-by-step solution

Fragmentation and determination of mass of the molecular ion

Fragmentation occurs when a bond is cleaved to form a radical and a cation. Through fragmentation, additional information regarding the organic molecule’s structure is known.

The given compounds are:

Structure of the two isomers

The mass of the molecular ion, that is, the molecular mass of both the compounds is 86.

Fragmentation of 2,3-dimethylbutane and 2,2-dimethylbutane

a.

The two compounds undergo fragmentation in the following ways:

Fragmentation of 2,3-dimethylbutane

The fragmentation of 2,3-dimethylbutane leads to the formation of the 2^o carbocation having m/z = 43.

Fragmentation of 2,2-dimethylbutane

The fragmentation of 2,2-dimethylbutane leads to the formation of 3^o carbocation having m/z= 57.

Thus, out of the two isomers, 2,2-dimethylbutane gives the intense peak at m/z = 57.

Determination of structure

(b) The ion that gives a peak at m/z = 57 is formed upon fragmentation of 2,2-dimethylbutane, and the peak is intense owing to the greater stability of the 3^o carbocation.

The structure of the ion that gives the intense peak is shown below:

$3^{\mathrm{o}}$carbocation

(c) The ion that gives a peak at m/z = 43 is given below: