492 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy
m/z100500
0 10203040506070809010011012 0Relative abundance4143(^5787)
(^101116)
CH 3 CH 2 CH O CHCH 3
CH 3 CH 3
Figure 13.6N
The mass spectrum of sec-butyl
isopropyl ether.
b. A bond is cleaved homolyticallyat the position because it leads to a
relatively stable cation in which the positive charge is shared by two atoms
(a carbon and an oxygen). The alkyl group leading to the most stable radical
is one most easily cleaved. Thus, the peak at is more abundant than
the one at even though the compound has three methyl groups
bonded to carbons that can be cleaved to produce a peak at
because a primary radical is more stable than a methyl radical.
PROBLEM 8
The mass spectra of 1-methoxybutane, 2-methoxybutane, and 2-methoxy-2-methylpropane
are shown in Figure 13.7. Match the compounds with the spectra.
Alcohols
The molecular ions obtained from alcohols fragment so readily that few of them
survive to reach the collector. As a result, the mass spectra of alcohols show small
molecular ion peaks. Notice the small molecular ion peak at in the mass
spectrum of 2-hexanol (Figure 13.8).
Like alkyl halides and ethers, alcohols undergo cleavage. Consequently, the mass
spectrum of 2-hexanol shows a base peak at m>z= 45 ( cleavage leading to a morea
a
m>z= 102
CH 3 CH 2 CH O CHCH 3
CH 3 CH 3
- cleavage
m/z = 87
CH O CHCH 3 CH 3 CH 2
CH 3 CH 3
CH 3 CH 2 CH O CHCH 3
CH 3 CH 3
- cleavage
m/z = 101
CH 3 CH 2 CH O CHCH 3 CH 3
CH 3
CH 3 CH 2 CH O CHCH 3
CH 3 CH 3
- cleavage
m/z = 101
CH 3 CH 2 CH O CHCH 3 CH 3
CH 3
carbon
a m>z=101,
m>z=101,
m>z= 87
3-D Molecules: C¬C a
Methyl propyl ether;
Methyl propyl ether radical
cation
m/z = 116
CH 3 CH 2 CH O CHCH 3
−e−
CH 3 CH 3
CH 3 CH 2 CH O CHCH 3
CH 3 CH 3 +
m/z = 57
CH 3 CH 2 CH O CHCH 3
CH 3 CH 3
- m/z = 43
CH 3 CH 2 CH O CHCH 3
sec-butyl isopropyl ether CH^3 CH^3
- m/z = 43