Organic Chemistry

(85 kcal/mol) bonds have similar strengths, and the species that is formed is a relative-

ly stable cation, since its positive charge is shared by two atoms:

Notice that cleavage does not occur in alkyl bromides be-

cause the bond is much weaker than the bond.

PROBLEM 7

Sketch the mass spectrum of 1-chloropropane.

Ethers

The mass spectrum of sec-butyl isopropyl ether is shown in Figure 13.6. The fragmen-

tation pattern of an ether is similar to that of an alkyl halide.

1. Electron bombardment dislodges one of the lone-pair electrons from oxygen.


2. Fragmentation of the resulting molecular ion occurs in two principal ways:
   a. A bond is cleaved heterolytically, with the electrons going to the more
   electronegative oxygen atom.

C¬O

m/z = 78

+

+

(^35) Cl
−e−
−e−
m/z = 43
CH 3 CH +

• CH 3


• 
  

  CH 3
  CH 3 CH
  CH 3
  CH 3 CH Cl
  2-chloropropane
  CH 3
  m/z = 80
  37 +
  CH 3 CH Cl
  CH 3
  m/z = 78

  
  


• 35
  CH 3 CH Cl +
  CH 3
  m/z = 80


• 37
  CH 3 CH Cl
  CH 3
  m/z = 63
  35 +
  CH 3 CH Cl +
  m/z = 65
  37 +
  CH 3 CH Cl
  CH^35 Cl
  3 CH
  CH 3
  +^37
  (^35) Cl


• 
  

  (^37) Cl
  heterolytic
  cleavage
  homolytic
  cleavage
  C¬C C¬Br
  CH 3 CH“Cl^ a
  (^) · •≠+.
  CH 3 +CH¬Cl^ ¶

  
  

• ≠

Section 13.5 Fragmentation at Functional Groups 491

CH 3 CHCH 3

Cl

m/z

100

50

0

0 102030405060708090100

Relative abundance^41

43

63

65

78

80

>Figure 13.5

The mass spectrum of

2-chloropropane.

Recall that an arrowhead with

one barb represents the movement

of one electron.

3-D Molecules:

Isopropyl cation;

2-Chloropropane;

2-Chloropropane radical

cation

Tutorial:

Fragmentation of ethers