Return to Contents
Chapter 12 : 
Reactions of Arenes. Electrophilic Aromatic Substitution		 Ch 12 contents

Substituent Effects(contd.)

Study Tip:
Learn to recognise the types of substituents and then break it down into the following :
(1) Electron donors activate and direct ortho and para.
(2) Electron withdrawing groups deactivate and direct meta.
(3) Except halogens.... they deactivate but direct ortho and para.

First let's cover the electronic effects :

1.
Substituents with lone pairs (e.g. -OCH3, -NH2) on the atoms adjacent to the p system are electron donating groups (EDG) - they activate the aromatic ring by increasing the electron density on the ring through a resonance donating effect.  The resonance effect only allows electron density to be positioned at the ortho- and para- positions. Hence these sites are more nucleophilic, and the system tends to react with electrophiles at these ortho- and para- sites.  This is most easily seen by pushing the curly arrows, see below :

 schematic diagram to show sites of attack with an electron donating group present

resonance donation of an electron donating group

2.
Alkyl substituents  (e.g. -CH3, -CH2CH3) are also electron donating groups - they activate the aromatic ring by increasing the electron density on the ring through an inductive donating effect.  This is the same effect that allows alkyl groups to stabilise simple carbocations.  They overall effect is similar to that described above.
3.
Substituents with C=C  (e.g. -vinyl or  -aryl) are also electron donating groups - they activate the aromatic ring by a resonance donating effect.  This is a similar effect to that for type 1 except that the electrons are from a bonded pair not a lone pair.


4.
 Substituents with pi bonds to electronegative atoms (e.g. -C=O, -NO2) adjacent to the pi system are electron withdrawing groups (EWG) - they deactivate the aromatic ring by decreasing the electron density on the ring through a resonance withdrawing effect.  The resonance decreases the electron density at the ortho- and para- positions. Hence these sites are less nucleophilic, and so the system tends to react with electrophiles at the meta sites. schematic diagram to show sites of attack with an electron withdrawing group present

resonance withdrawal of an electron donating group
5.
 Substituents with several bonds to electronegative atoms (e.g. -CF3) adjacent to the pi system are electron withdrawing groups (EWG) - they deactivate the aromatic ring by decreasing the electron density on the ring through a inductive withdrawing effect.  The net overall effect is similar to that described above for other electron withdrawing groups.


6. Halogen substituents are a little unusual in that they are deactivating but still direct ortho- / para-. The reason is that they are both inductive electron withdrawing (due to their electronegativity) but they are also resonance donating (lone pair donation).  The inductive effect lowers the reactivity of the starting material but the resonance effect controls the regiochemistry due the stability of the intermediate carbocations.

What about other effects ?
Other than the electronic effects due to resonance and induction described above, substitutents can also influence product distributions in another way. For example consider the following experimental data on the nitration of various alkyl benzenes :  

steric effects in electrophilic aromatic substitution

-R ortho-% meta-% para-%
-CH3 58
 4
 37
-CH2CH3 45
 6
 49
-CH(CH3)2 30
 8
 62
-C(CH3)3 16
 11
 73

QUESTION : Can you explain the trend in this data ? ANSWER


Other thought provoking questions.....

previous page
 next page
organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary