1. When a monosubstituted benzene undergoes electrophilic substitution, the position of the incoming group (orientation) and reaction rate (reactivity) are determined by the original substituent. 2. Substituents are classified as ortho/para-directing and activating, meta-directing and deactivating, or ortho/para-directing and deactivating. 3. Orientation and reactivity can be explained by resonance structures of possible carbocation intermediates that form during the reaction. The most stable carbocation will determine the preferred position for incoming groups.

1. Orientation in Mono substituted ring systems
Aromatic Electrophilic Substitution
Presented By :- Dalpat singh
MSc 2nd Sem.
Submitted to :- Dr. M.R.K. Sherwani

2. Introduction :-
• When a monosubstituted benzene undergoes an electrophilic substitution , the position
taken up by the incoming group (The Orientation) and the rate of reaction ( the reactivity )
are determined by the substituent already present on the benzene ring.
• On the basis of various substituents can be divided into three categories :-
1. Ortho, para-directing and activating groups : if an electron donating group is already present
on the benzene ring then the incoming group is attached to the ortho and para positions.
these type of substituted benzene rings are more reactive then the benzene itself. This is
called activation of benzene ring. Ex :- NR2 , NHR , NH2 ,OH ,OR ,NHCOOR etc.
2. Meta-directing and deactivating Groups :- if an electron withdrawing group is already
present on the benzene ring then the incoming group is attached to the meta positions.
These type of substituted benzene rings are less reactive towards the electrophilic
substitution than benzene itself. This is called deactivation of benzene ring systems.
Ex :- NO2, NR3 , CF3, CN , CHO ,COR , etc.
3. Ortho, para-directing and deactivating groups :- halogens are Ortho and para directing but
they deactivate the benzene ring for further electrophilic substitution.

3. Explanation based on charge distribution :
1. Ortho, Para-directing and activating Groups :
(a) when an atom having a lone pair of electron directly attached to the ring then the electron
density on ortho and para position increased through conjugation.
(b) Aryl groups are also ortho and para directing and activating groups because they
increase the electron density on o- and p- positions as follows :-

4. (c) in case of alkyl groups the electron density on ortho and para positions is increased by
hyperconjugative effect. Thus, they are ortho and para directing and activating groups.
2. Meta-directing and deactivating groups :
(a) when an electron withdrawing group directly attached to the ring then the electron density on
ortho and para position decreased through conjugation, thus, m-positions have a relatively high
electron density. Therefore, further electrophilic substitution is made more difficult by the presence
of a m-directing group.

5. (b) The m-directing and deactivating groups having a positively charged atom for
example : NR+
3, NH+
3 etc. also attract electrons from the ortho and para positions through
strong inductive effect.
(c) The m-directing and deactivating nature of CCL3 group is due to electron attraction
form the o- and p- positions through –I as well as hyperconjugative effect.

6. 3. Ortho and para directing and deactivating groups : Halogens are Ortho and para directing
but they deactivate the benzene ring for further electrophilic substitution. This is because there
are two opposing effects. They are deactivators because they inductively withdraw electrons
from the ring more strongly than they electrons by resonance.
Explanation based on carbocation stability:
The effect of substituents on orientation and reactivity can be best explained by writing all the possible
resonating structure of the carbocation (arenium ion) intermediate for each of the three possible reaction
courses.

7. 1. Ortho, Para directing and activating group : (a) lone pair containing atom attached to ring

8. (b) When alkyl groups are attached to the ring :

9. 2. Meta directing and deactivating group :

10. 3. Ortho, Para directing
and deactivating group :
(a) Ortho :
(b) Para :
(c) Meta :

11. Thank you