Steric Hindrance Issues

1
Steric Hindrance Issues

• Sites that are sterically hindered can be blocked
  and may undergo very little substitution. These
  include
• - a site that has two ortho substituents
• - a site that is ortho to one very bulky group

Example two ortho substituents
The site between CH3 and Cl is crowded.
2
Steric Hindrance Issues
Sites that are sterically hindered are be blocked
and may undergo very little substitution. These
include - a site that has two ortho
substituents - a site that is ortho to one
very bulky group
Example ortho to a bulky group (such as
isopropyl or t-butyl)
The site next to t-butyl is crowded.
3
Substitution Reactions - Practice

• Predict the major product for the following
  reactions.

4
16.3 Alkylation of Aromatic Rings

• EAS Reactions - use electrophiles like NO2 or
  Br

Can we add an alky group ( -R), via a carbocation
R?
5
Ring Alkylation Friedel-Crafts Reaction

• Aromatic substitution of an alkyl for an -H
• Catalyst AlCl3 Aluminum chloride promotes
  formation of the carbocation

6
Limitations of Friedel-Crafts Alkylation

• Only alkyl halides can be used (F, Cl, I, Br)
• Aryl halides and vinylic halides do not react
  (their carbocations are too hard to form)