Question

1.How can you determine, based solely on isotopic pattern of molecular ion, whether molecule possess chlorine or bromine

2. What characteristic peak or fragmentation can you use to determine whether molecule possess iodine

Answer 1

AI) Chlorine naturally exists in 2 isotopic forms with unequal % abundance viz. Cl-35 with 75% and Cl-37 with 25% natural abundance.

Thus Mass spectra of compound containing 1 Cl atom shows 2 M and M+ 2 molecular ion peaks in the intensity ratio 3:1 i.e M+2 peak                                                  will be of intensity one third of that of M⁺ peak. Such an unequal intensity peak pattern can also be seen in Mass spectra of compound containing 2 or more Cl atoms. Such compounds will also have M+4, M+6, … peaks with different intensity.

AII) Bromine on the other hand exists in 2 isotopic forms with nearly same % natural abundance viz. Br-79 with 49% and Br-81 with 51% natural abundance.

Thus Mass spectra of compound containing Br will show M and M+ 2 peaks with almost equal intensity. This differentiates the bromo compound from chloro compound.

B) Aliphatic iodides gives the strongest molecular ion peak of the all aliphatic halides as Iodine is monoisotopic and hence there is no distinctive isotope peak for iodides.

The fragmentation peaks are much as chloro or bromo compound. M-128 peak due to loss of Iodine will be observed.