Organic Chemistry

494 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

m/z

CH 3 CH 2 CH 2 CH 2 CHCH 3

OH

100

80

60

40

20

0

0 20 40 60 80 100 120

Relative abundance

45

69

31 57 8487

27

m/z = 102

Figure 13.8N
The mass spectrum of 2-hexanol.

Tutorial:

Fragmentation of alcohols

In all the fragmentations we have seen so far, only one bond is broken. An impor-

tant fragmentation occurs in alcohols, however, that involves breaking two bonds. Two

bonds break because the fragmentation forms a stable water molecule. The water that

is eliminated comes from the OH group of the alcohol and a hydrogen. Thus,

alcohols show a fragmentation peak at because of loss of water.

Notice that alkyl halides, ethers, and alcohols have the following fragmentation

behavior in common:

1. A bond between carbon and a more electronegativeatom (a halogen or an
   oxygen) breaks heterolytically.


2. A bond between carbon and an atom of similar electronegativity(a carbon or a
   hydrogen) breaks homolytically.


3. The bonds most likely to break are the weakest bonds and those that lead to for-
   mation of the most stable cation. (Look for fragmentation that results in a cation
   with a positive charge shared by two atoms.)

PROBLEM 9

Primary alcohols have a strong peak at What fragment is responsible for this peak?

Ketones

The mass spectrum of a ketone generally has an intense molecular ion peak. Ketones

fragment homolytically at the bond adjacent to the bond, which results

in the formation of a cation with a positive charge shared by two atoms. The alkyl

group leading to the more stable radical is the one that is more easily cleaved.

If one of the alkyl groups attached to the carbonyl carbon has a hydrogen, a

cleavage known as a McLafferty rearrangementmay occur. In this rearrangement,

g

+ +

2-pentanone m/z = 86

CH 3 CH 2 CH 2 CCH 3

−e−

O

CH 3 CH 2 CH 2 CCH 3

+

m/z = 43

O CH 3 CH 2 CH 2 CH 3 CO

m/z = 71

CH 3 CH 2 CH 2 CCH 3

+

O +

C¬C C“O

m>z=31.

+

m/z = (102 − 18) = 84

CH 3 CH 2 CHCH 2 CHCH 3

OH

CH 3 CH 2 CHCH 2 CHCH 3

H +

+ H 2 O

m>z=M - 18

g

3-D Molecules:

2-Hexanone;

2-Hexanone radical cation;

Acetone enol radical cation