In: Chemistry
2. Ortho-dichlorobenzene has a molecular weight of 147.00 g/mol, but its MS has a very small relative abundance of the 147 peak. Instead, it has peaks at 146 (100% relative abundance), 148 (65%), and 150 (10.6%). The peak at 147 is about 6.6% as big as the 146 peak. What causes this distinctive molecular ion region for this compound? What peaks (note the plural) would you expect to see on the mass spectrum for the fragment (note the lack of plural) caused by loss of one chlorine atom (one of the C-Cl bonds breaking) during the ionization process?
Ortho dichloro benzene molecular weight is 147 g/mole. It is calculated as follows
C6H4Cl2 for Carbon Molecular weight of Carbon is 12 so 12X6 = 72
For Hydrogen Molecular weight of Hydrogen is 1 so 1X4 = 04
For Chlorine average Molecular weight of Chlorine is 35.453 i.e so 35.5X2 = 71
Total =147
However in nature chlorine is available in two isotopic forms they are 35Cl and 37Cl with relative abundance of 75.8% and 24.2% respectively. So we get molecular ion peak for 35Cl at m/z value 146 and M+H+ at m/z value 147 and we get molecular ion peak for 37Cl at m/z value 148 and M+H+ at m/z value 149 and combination of both isotopes at m/z value 150. During the fragmentation process after elimination of one chlorine atom we get molecular ion peak for 35Cl at m/z value 111 and M+H+ at m/z value 112 and we get molecular ion peak for 37Cl at m/z value 113 and M+H+ at m/z value 114 and combination of both isotopes at m/z value 115.