Discussion
An organic compound exhibits the [M]+, [M+2]+ and [M+4]+ peaks in the intensity ratio 1:2:1 in the mass spectrum, and shows a singlet at δ 7.49 in the 1H NMR spectrum in CDCl3. The compound is:
Answer: B
The compound is 1,4-dibromobenzene.
Bromine has two isotopes, 79Br and 81Br, in almost equal amounts. A molecule with two bromines therefore shows M, M+2 and M+4 in the ratio 1 : 2 : 1. Two chlorines (35Cl : 37Cl about 3 : 1) give roughly 9 : 6 : 1 instead, which rules out both dichlorobenzenes.
A single 1H NMR signal means all four ring hydrogens are equivalent, which is true only of the para isomer. 1,2-Dibromobenzene has two sets of hydrogens (AA'BB'), giving two multiplets rather than a singlet.
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