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Chapter 12 :
Reactions of Arenes. Electrophilic Aromatic Substitution |
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Substituent Effects (contd.)
Here is a table that shows the effect of substituents on a benzene ring
have on both the rate and orientation of electrophilic aromatic substitution
reactions.
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These effects are a combination of RESONANCE and INDUCTIVE
effects (see next page)
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
effects:
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The H atom is the standard and is regarded as having no effect.
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Activating groups increase the rate
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Deactivating groups decrease the rate
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EDG = electron donating group
-
EDG can be recognised by lone pairs on the atom adjacent to the
p
system, eg: -OMe
-
except -R, -Ar or -vinyl (hyperconjugation, p
electrons)
-
EWG = electron withdrawing group
-
EWG can be recognised either by the atom adjacent to the
p
system
having several bonds to more electronegative atoms, or,
having a formal +ve or d +ve charge, eg:
-CO2R, -NO2
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EDG / activating groups direct ortho / para
-
EWG / deactivating groups direct meta
-
except halogens (-X) which are deactivating BUT direct
ortho
/ para
-
EDG add electron density to the p system
making it more nucleophilic
-
EWG remove electron density from the p
system making it less nucleophilic
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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, once as
an EDG and once as an EWG ?
Why are amines (-NH) better activators than alcohols (-OH) ?
