Organic Chemistry

486 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

The way a molecular ion fragments
depends on the strength of its bonds
and the stability of the fragments.

The base peakis the peak with the greatest intensity, due to its having the greatest

relative abundance. The base peak is assigned a relative intensity of 100%, and the rel-

ative intensity of each of the other peaks is reported as a percentage of the base peak.

Mass spectra can be shown either as bar graphs or in tabular form.

A mass spectrum gives us structural information about the compound because the

mzvalues and the relative abundances of the fragments depend on the strength of the

molecular ion’s bonds and the stability of the fragments. Weak bonds break in prefer-

ence to strong bonds, and bonds that break to form more stable fragments break in

preference to those that form less stable fragments.

For example, the bonds in the molecular ion formed from pentane have

about the same strength. However, the bond is more likely to break than the

bond because fragmentation leads to a primarycarbocation

and a primaryradical, which together are more stable than the primarycarbocation

and methylradical (or primaryradical and methylcation) obtained from

fragmentation. fragmentation forms ions with or 29, and

fragmentation forms ions with or 15. The base peak of 43 in the

mass spectrum of pentane indicates the preference for fragmentation. (See

Sections 7.7 and 7.8 to review the relative stabilites of carbocations and radicals.)

A method commonly used to identify fragment ions is to determine the difference

between the mzvalue of a given fragment ion and that of the molecular ion. For ex-

ample, the ion with in the mass spectrum of pentane is 29 units smaller than

the molecular ion An ethyl radical has a molecular mass

of 29 (because the mass numbers of C and H are 12 and 1, respectively), so the peak at

43 can be attributed to the molecular ion minus an ethyl radical. Similarly, the peak

at can be attributed to the molecular ion minus a methyl radical.

Peaks at and are readily recognizable as being due to methyl and

ethyl cations, respectively. Appendix VI contains a table of common fragment ions

and a table of common fragments lost.

Peaks are commonly observed at mzvalues one and two units less than the mzvalues

of the carbocations because the carbocations can undergo further fragmentation—losing

one or two hydrogen atoms.

2-Methylbutane has the same molecular formula as pentane, so it, too, has a molec-

ular ion with (Figure 13.3). Its mass spectrum is similar to that of pentane,

with one notable exception: The peak at m>z= 571 M- 152 is much more intense.

m>z= 72

+ + +

m/z = 43 m/z = 42 m/z = 41

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

−H −H

> >

m>z= 15 m>z= 29

m>z= 571 M- 152

1 M- 29 = 432. (CH 3 CH 2 )

m>z= 43

>

1 2 3 4 5 +

+

molecular ion

m/z = 72

m/z = 43

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

+

CH 3 CH 2 CH 2 CH 2 CH 3

+

m/z = 29

+

CH 3 CH 2 CH 2 CH 2 CH 3

+

m/z = 57

+

CH 3 CH 2 CH 2 CH 2 CH 3

+

m/z = 15

+

CH 3 CH 2 CH 2 CH 2 CH 3

C-2¬C-3

C-1¬C-2 m>z= 57

C-2¬C-3 m>z= 43

C-1¬C-2

C-1¬C-2 C-2¬C-3

C-2¬C-3

C¬C

>

+ e−

molecular ion

m/z = 72

electron

CH beam

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

+

3-D Molecules:

Propane;

Propane radical cation