Knowing what to look for with respect to isotopic clusters can aid in interpreting mass sp...

Knowing what to look for with respect to isotopic clusters can aid in interpreting mass spectra. How many peaks would you expect to see for the molecular ion in each of the following compounds? At what m/z values would these peaks appear? (Disregard the small peaks due to 13C and 2H.)

(a) p-Dichlorobenzene

(b) o-Dichlorobenzene

(c) p-Dibromobenzene

(d) p-Bromochlorobenzene

Sample Solution (a) The two isotopes of chlorine are 35Cl and 37Cl. There will be three isotopically different forms of p-dichlorobenzene present. They have the structures shown as follows. Each one will give an M+ peak at a different value of m/z.