Organic Chemistry

2-Methylbutane is more likely than pentane to lose a methyl radical, because, when it
does, a secondarycarbocation is formed. In contrast, when pentane loses a methyl
radical, a less stable primarycarbocation is formed.

PROBLEM 1

How could you distinguish the mass spectrum of 2,2-dimethylpropane from those of

pentane and 2-methylbutane?

PROBLEM 2

What mzvalue is most likely for the base peak in the mass spectrum of 3-methylpentane?

PROBLEM 3 SOLVED

The mass spectra of two very stable cycloalkanes both show a molecular ion peak at

One spectrum shows a base peak at the other shows a base peak at

Identify the cycloalkanes.

SOLUTION The molecular formula for a cycloalkane is Because the molecular

mass of both cycloalkanes is 98, their molecular formulas must be

A base peak of 69 means the loss of an ethyl substituent

whereas a base peak of 83 means the loss of a methyl substituent Be-

cause the cycloalkanes are known to be very stable, we can rule out cycloalkanes with three

or four-membered rings. A seven-carbon cycloalkane with a base peak signifying the loss of

an ethyl substituent must be ethylcyclopentane. A seven-carbon cycloalkane with a base

peak signifying the loss of a methyl substituent must be methylcyclohexane.

PROBLEM 4

The “nitrogen rule”states that if a compound has an odd-mass molecular ion, the compound

contains an odd number of nitrogen atoms.

a. Calculate the value for the molecular ion of the following compounds:

1. 
   
   
   
   2. b. Explain why the nitrogen rule holds.
      c. State the rule in terms of an even-mass molecular ion.
   
   
   
   

CH 3 CH 2 CH 2 CH 2 NH 2 H 2 NCH 2 CH 2 CH 2 NH 2

m>z

198 - 83 = 152.

84 + 14 = 982. 198 - 69 = 292 ,

C 7 H 1417 * 12 =

CnH 2 n.

m>z=83.

m>z=98. m>z=69,

>

+ +

+

molecular ion

m/z = 72

m/z = 57

[CH 3 CHCH 2 CH 3 ] CH 3 CHCH 2 CH 3 CH 3

CH 3

Section 13.2 The Mass Spectrum • Fragmentation 487

CH 3 CH 2

CH 2 CH 3 CH 3

+

++

+

CH 3

electron

beam

electron

beam

ethyl- m/z^ =^69 m/z = 83

cyclopentane

methyl-

cyclohexane

0 20406080100

m/z

100

50

0

Relative abundance

15

27

29

43

57

72

CH 3 CHCH 2 CH 3

CH 3

>Figure 13.3

The mass spectrum of

2-methylbutane.