Here is a table that shows the effect of
substituents on a benzene ring
have on the rate and orientation of electrophilic aromatic substitution
||These effects are a
combination of RESONANCE and INDUCTIVE
effects (see review in Ch12)
The effects are also important in other reactions and properties (e.g.
acidity of the substituted benzoic acids).
Here are some general pointers
for recognising the substituent
- The H atom
is the standard and is regarded as having no effect.
groups increase the rate
groups decrease the
- EDG = electron
- EDG can
be recognised by lone pairs
on the atom adjacent to the p
system, eg: -OMe
-R, -Ar or -vinyl
- EWG = electron
- EWG can
be recognised either
by the atom adjacent to the p
having several bonds to more electronegative atoms, or,
having a formal +ve or d +ve charge, eg:
- EDG /
activating groups direct ortho
- EWG /
deactivating groups direct meta
halogens (-X) which
are deactivating BUT direct ortho
- EDG add
electron density to the p system
making it more nucleophilic
remove electron density from
system making it less nucleophilic
Thought provoking questions.....
- Why are esters (-OCOR)
and amides (-NHCOR) less activating than ethers
(-OR) and amines (-NH) ?
- Why do esters and amides
appear in the table twice, onceas an
once as an EWG ?
- Why are amines (-NH)
better activators than alcohols (-OH) ?
|| © Dr.
Ian Hunt, Department of