Question

1. Explain the fragment patterns, isotope peaks, peak abundances, (The molecular formula for the compound is: C₂H₂Cl₂).

Mass/Charge	Relative Abundances
12	2.7
13	3
14	0.6
24	4
25	15
26	34
27	0.7
31	0.3
35	7
36	1.9
37	2.3
38	0.7
47	6.5
47.5	0.2
48	5.9
49	4.2
50	1.8
51	0.7
59	2.6
60	24
61	100
62	9.9
63	32
64	0.7
95	1.5
96	67
97	2.4
98	43
99	1
100	7
101	0.1

3:1 Ratio = Chlorine present

M⁺ peak

M⁺² peak

M⁺⁴ peak

Answer 1

The compound of molecular formula C₂H₂Cl₂ has molecular weight (m/z) = 96, 98 and 100. It is due to that the molecular ion peaks (M+ , M+2 and M+4) each contain two chlorine atom. Which is mainly because of the chlorine atom has isotopes, ³⁵Cl and ³⁷Cl .

In case of one chlorine atom in the structure, one can observe the two lines in the mass spectrum with the peak heights or the abundance of the peaks are in the ratio of 3:1. That reflects the fact that chlorine contains 3 times as much of the ³⁵Cl isotope as the ³⁷Cl one.

In case of two chlorine atoms in the structure, one can observe the three line in the mass spectrum with the peak heights or the abundance of the peaks are in the ratio of 9:6:1. It can be further explained as

For C₂H₂Cl₂

26 = C₂H₂

35, 37 = Cl

Therefore

26 + 35 + 35 = 96 = M = confirms the molecular ion peak with relative abundance is 67

26 + 35 + 37 = 98 = M+2  ion peak with relative abundance is 43

26 + 37 + 37 = 100 = M+4 ion peak with relative abundance is 7

The fragment m/z = 61 confirms the ion peak with 100% abundance due to the structure as [CH=CH-Cl]⁺