Organic Chemistry

Section 10.8 Benzylic Halides, Allylic Halides, Vinylic Halides, and Aryl Halides 383

10.8 Benzylic Halides, Allylic Halides,

Vinylic Halides, and Aryl Halides

Our discussion of substitution reactions, to this point, has been limited to methyl
halides and primary, secondary, and tertiary alkyl halides. But what about benzylic,
allylic, vinylic, and aryl halides? Let’s first consider benzylic and allylic halides. Ben-
zylic and allylic halides readily undergo reactions unless they are tertiary. Tertiary
benzylic and tertiary allylic halides, like other tertiary halides, are unreactive in
reactions because of steric hindrance.

Benzylic and allylic halides readily undergo reactions because they form
relatively stable carbocations. While primary alkyl halides (such as and
) cannot undergo reactions because their carbocations are too
unstable, primary benzylic and primary allylic halides readily undergo reactions
because their carbocations are stabilized by electron delocalization (Section 7.7).

If the resonance contributors of the allylic carbocation intermediate have different
groups bonded to their carbons, two substitution products will be obtained.

Vinylic halides and aryl halides do not undergo or reactions. They do not
undergo reactions because, as the nucleophile approaches the back side of the
carbon, it is repelled by the electron cloud of the double bond or the aromatic ring.

a vinylic halide an aryl halide

nucleophile

CC

Cl

H H

R

Br

nucleophile

a nucleophile is repelled

by the π electron cloud

p

SN 2 sp^2

SN 2 SN 1

sp^2

SN 1

CH 3 CH 2 CH 2 Br SN 1

CH 3 CH 2 Br

SN 1

benzyl chloride

1-bromo-2-butene

an allylic halide

2-buten-1-ol

benzyl methyl ether

CH 2 Cl + CH 3 O−

CH 3 CH CHCH 2 Br + HO− CH 3 CH CHCH 2 OH + Br−

CH 2 OCH 3 + Cl−

SN2 conditions

SN2 conditions

SN 2

SN 2

SN (^1) CH 3 OH
CH 2 Cl CH 2

• Cl−
  CH 2 OCH 3 + H+
  SN 1
  CH 2 CH CH 2


• Br−
  H 2 O
  CH 2 CHCH 2 Br CH 2 CHCH 2 CH 2 CHCH 2 OH + H+


• 
  

  ++

  
  


• 
  


• 
  

  CH 3 CH CHCH 2 Br CH 3 CH CHCH 2

  
  


• 
  

  CH 3 CHCH CH 2
  CH 3 CH CHCH 2 OH CH 3 CHCH CH 2
  H+

  
  


• Br−
  H 2 O
  OH + H+
  H 2 O
  SN 1
  Benzylic and allylic halides undergo
  SN 1 and reactions.SN 2
  Vinylic and aryl halides undergo
  neither nor reactions.SN 1 SN 2
  3-D Molecule:
  Benzyl cation
  3-D Molecule:
  Allyl cation