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

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Answer 1 · 2016-08-15T21:46:55

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.

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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,