Question

Question: Consider the following four structures, followed by mass spectral data. Match each structure with its characteristic molecular ion or fragment. In each case, give a likely structure of the ion responsible for the base peak

(a) base peak at 105

(b) base peak at 72

(c) doublet at 198 and 200, base peak at 91

(d) base peak at 91, large peak at 43

Short answer

Answer

(a) The base peak at 105 is observed for structure 2.

(b) The base peak at 72 is observed for Structure 4.

(c) The structure 1 has the molecular ion peak doublet at 198 and 200. The base peak is observed at 91.

(d) The base peak at 91 and the large peak at 43 are observed for structure 3.

Step-by-step solution

Mass spectroscopy

The structural elucidation of an organic compound can be done by using mass spectroscopy. The positively charged species will show different path in mass spectroscopy based on their m/z value. The fragmentation of a compound leads to the formation of a radical ion and from radical ion various cationic species will be generated.

Subpart (a)

The base peak is the one which hass 100% abundance in mass spectroscopy.

The structure with 100% abundance at 105 m/z is structure 2.

Subpart (b)

The base peak at 72 is observed for Structure 4

.

The fragmentation is as shown below:

Fragmentation of structure 4

Subpart (c)

The structure 1 has the molecular ion peak doublet at 198 and 200. The base peak is observed at 91.

Since Br- exists in two isotopic forms each of mass 79 m/z and 81 m/z.

Due to that a doublet is observed for the molecular ion peak at 198 m/z and 200 m/z.

The fragmentation for the base peak is as shown below:

Fragmentation in structure 1

Subpart (d)

The base peak at 91 and the large peak at 43 are observed for structure 3

The fragmentation is as shown below:

Fragmentation in structure 3