Ch12 : Substituent Effects

Chapter 12 : Reactions of Arenes. Electrophilic Aromatic Substitution

Substituent Effects(contd.)

The following table shows the effect substituents have on both the rate and orientation of electrophilic aromatic substitution reactions.

Study Tip:

This is a VERY important table ! It is worth knowing.... your best understanding will come if you learn HOW it works.
It's application goes way beyond electrophilic aromatic substitution reactions.

Key concepts to review ? Resonance and electronegativity

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:

The H atom is the standard and is regarded as having no effect.
Activating groups increase the rate
Deactivating groups decrease the rate

EDG = electron donating group
EDG can be recognised by lone pairs on the atom adjacent to the π system, eg: -OCH₃
except -R, -Ar or -vinyl (hyperconjugation, π electrons)

EWG = electron withdrawing group
EWG can be recognised either by the atom adjacent to the π system having several bonds to more electronegative atoms, or,

₂

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 π system making it more nucleophilic
EWG remove electron density from the π system making it less nucleophilic.