Chapter 12 :  Reactions of Arenes. Electrophilic Aromatic Substitution

Substituent Effects(contd.)

There are two main electronic effects that substituents can exert:

RESONANCE effects are those that occur through the pi system and can be represented by resonance structures. These can be either electron donating (e.g. -OCH₃) where pi electrons are pushed toward the arene or electron withdrawing (e.g. -C=O) where pi electrons are drawn away from the arene.

INDUCTIVE effects are those that occur through the sigma system due to electronegativity effects.  These too can be either electron donating electron donating (e.g. -Me) where sigma electrons are pushed toward the arene or electron withdrawing (e.g. -CF₃, +NR₃) where sigma electrons are drawn away from the arene.

A simplified approach to understanding substituent effects is provided, based on the "isolated molecule approach".  The text (as do most others) uses the more rigourous approach of drawing the resonance structures for each of the intermediate carbocations formed by attack at each of the o-, m-  and p- positions and looking at how the initial substituent influences the stability of the system.

We are going to break down the types of substituents into various subgroups based on the structural features of the substituent immediately adjacent to the aromatic ring:

• type 1 = substituents with lone pairs (e.g. -OCH₃, -NH₂) on the atoms adjacent to the pisystem.
  
• type 2 = substituents that are CH systems (i.e. -alkyl, -vinyl or -aryl).
• type 3 = substituents that are C=C systems (i.e. -vinyl or -aryl).
• type 4 = substituents with π bonds to electronegative atoms (e.g. -C=O, -CF₃, -NO₂)
• type 5 = substituents with several bonds to electronegative atoms (e.g.  -CF₃)
• type 6 = substituents that are halogen systems (i.e. -F, -Cl, -Br, -I)

© Dr. Ian Hunt, Department of Chemistry