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

Question

Madison Anderson · Accepted Answer

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

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,

Madison Anderson · Answer

undefined The mass spectrum would show two peaks corresponding to different isotopes of bromine, with a relative intensity ratio of 1:1. The molecular ion peak would be present at a higher mass, indicating the compound's molecular weight.