Question

What identifying characteristics would be present in the mass spectrum of a compound containing two bromine atoms?

Answer 1

There should be M, M+2, and M+4 peaks with relative intensities of 1:2:1.

Explanation:

For example, see the mass spectrum of 1,3-dibromopropane.

Natural bromine is a mixture of 50.50 % `""^79"Br"` and 49.50 % `""^81"Br"`.

Thus, the molecule could contain two `"Br-79"` atoms, one `"Br-79"` and one `"Br-81"` atom, or two `"Br-81"` atoms.

This will give rise to three molecular ion peaks, each separated by two mass units.

We can calculate the relative size of each peak.

M peak:

Probability of `2 "Br-79" = 0.5050^2 "(one way)" = 0.2550`

M+2 peak:

Probability of `"1 Br-79 and 1 Br-81" = 2(0.5050)(0.4950) "(two ways)" = 0.5000`

M+4 peak:

Probability of `"2 Br-81" = 0.4950^2 "(one way)" = 0.2450`

If we set the relative probability of the largest peak at 100, the relative intensities become

`"M:M+2:M+4" = 51:100:49 ≈ 1:2:1`

Other feature:

You should see a peak at M-79 (loss of `"Br"`).

Compare the peak at 121 in the spectrum of 1,3-dibromopropane,