Analytical Chemistry

Thus, for C 7 H 7 NO, the number of sites is 5 and would be a possible structure.

Table 9.12 Intensities of isotope peaks (relative to the parent peak) for combinations of bromine and chlorine

Halogen present %

M + 2

%

M + 4

%

M + 6

%

M + 8

%

M + 10

%

M + 12

Br 97.7

Br 2 195.0 95.5

Br 3 293.0 286.0 93.4

Cl 32.6

Cl 2 65.3 10.6

Cl 3 99.8 31.9 3.47

Cl 4 131.0 63.9 14.0 1.15

Cl 5 163.0 106.0 34.7 5.66 0.37

Cl 6 196.0 161.0 69.4 17.0 2.23 0.11

BrCl 130.0 31.9

Br 2 Cl 228.0 159.0 31.2

Cl 2 Br 163.0 74.4 10.4

rings, double bonds and triple bonds, the latter counting as two double bonds. The formula for
calculating the number of unsaturated sites is:

A complete structural analysis can sometimes be accomplished by a study of the fragmentation pattern.
Some general rules for the modes of fragmentation expected from particular types of compound are
formulated on the basis of the concepts of physical and organic chemistry. These are:

(1) The relative intensity of the parent peak decreases with increasing molecular weight in a homologous
series and as the degree of chainbranching increases, i.e. it is greatest for short, straight-chain
compounds. Large parent peaks are observed for aromatic and other cyclic structures and for molecules
containing double bonds.

(2) Cleavage of bonds is more likely to occur at branches in a chain, the largest substituent at a branch

being eliminated as a radical. Saturated rings with side chains tend to cleave at the α-bond leaving the
positive charge on the ring fragment. Alkyl substituted aromatics cleave at the β-bond to give a
prominent peak at m/z 91 from the tropylium ion: