The document discusses the orientation of substituents on benzene rings. It explains that orientation is determined by the stability of the carbonium ion intermediate formed during electrophilic aromatic substitution. Electron-donating groups stabilize ortho- and para-substituted carbonium ions more than meta-substituted ions, directing substitution to the ortho and para positions. Electron-withdrawing groups have the opposite effect, destabilizing ortho- and para-substituted ions more and directing substitution to the meta position. Halogens are an exception, directing to ortho and para positions due to their electron-withdrawing inductive effect and electron-donating resonance effect.

1. BENZENE-
ORIENTATION

2. 2
Orientation
This is the factor that becomes important to know
whether the disubstituted product will be ortho, para or
meta product. As a mono-substituted benzene ring has
two equivalent ortho sites, two equivalent meta sites
and a unique para site, three possible constitutional
isomers may be formed in such a substitution. If
reaction occurs equally well at all available sites, the
expected statistical mixture of isomeric products would
be 40% ortho, 40% meta and 20% para. But this is not
so.

3. Theory of Orientation Based on
Stability of Carbonium Ion
1. Electron release or withdrawal through
inductive effect: It has been pointed out
that the electron-releasing group
stabilizes the carbonium ion. It is an
activating group that activates all
positions of the benzene ring, even the
meta positions. However, the ortho and
para positions are activated much more
than the meta.

4. 4
Further an electron-withdrawing group
destabilizes the carbonium ion. The deactivating
group deactivates all positions in the ring, even
the positions meta to it. It directs meta simply
because it deactivates the ortho and para
positions even more than the meta.
Thus in both types of orientations, the
effects of any group, whether activating or
deactivating, is strongest at the ortho and para
positions.

5. 5
Ortho- and para-directing groups with +1 effect
(carbonium ions formed by attack at para, ortho
and meta positions in toluene) :
Let us compare the carbonium ions formed by the
attack of para, ortho and meta positions in toluene. In
structures II and VI, the positive charge is located on
the carbon atom to which —CH 3 is attached.
Although —CH 3 releases electrons to all positions of
the ring, it does so more strongly to the carbon atom
near it. Consequently structures II and VI are
particularly stable.

6. 6
Because of the contribution from structure II and
VI, the hybrid carbonium ions resulting from
attack at the para and ortho positions are more
stable than the carbonium ion resulting from
attack at a meta position. in toluene, ortho and
para substitution is thus faster than meta
substitution because electron release is more
effective during attack at positions ortho and
para to it.

7. “
7

8. 8
b. Meta-directing groups with −I and −M
effects (carbonium ions formed by attack at
para, ortho and meta positions of
nitrobenzene): On comparing the carbonium
ion formed by attack at the para, ortho and meta
positions of nitrobenzene (containing —NO 2 , a
deactivating group), we find that each of these is
a hybrid of three structures, I–III for para, IV–VI
for ortho and VII–IX for meta.

9. 9
In structures II and VI, the positive charge is
located on the carbon atom to which NO 2 group
is attached. Although —NO 2 withdraws
electrons from all positions, it does so most from
the carbon atom nearest to it and hence this
carbon atom, which is already positive, has little
tendency to accommodate the positive charge of
the carbonium ion. Thus these structures are
particularly unstable and do little to stabilize the
ions.

10. 10
The carbonium ions for para and ortho
attack are virtually a hybrid of only two
structures I, II, and IV, V. These carbonium
ions are, therefore, less stable than the
carbonium ion resulting from attack at the
meta position. The attack at meta positions
give hybrid of three structures and in which
the positive charge is accommodated by
three carbon atoms.

12. Thus in nitrobenzene,
ortho, para substitution is
slower than meta
substitution because
electron withdrawal by —
NO 2 is more effective
during attack at the
position ortho and para to
it.

13. 13
Electron release through resonance
Ortho and para directing groups having −I and +M
effects (carbonium ions formed by attack at para, ortho
and meta positions of aniline):So far we have discussed
the electron releaseand electron withdrawal through the
inductive effects, i.e. due to the electronegativity of the
group concerned. Certain groups (—NH 2 , —OH and
their derivatives) are powerful activators towards
electrophilic aromatic substitution though they contain
electronegative atoms and thus possess inductive
effect (electron-withdrawing).

14. 14
Therefore, it is imperative that electron-release
should be through some other way than through
their inductive effects. This is believed to be
done by the resonance effect. The effects of —
NH 2 and —OH on electrophilic substitution can
be explained by assuming that nitrogen and
oxygen can share more than a pair of electrons
with the ring and can accommodate a positive
charge.

15. 15
The carbonium ion formed by attack at para- and
ortho- to the —NH 2 group of aniline is
considered to be hybrid not only of structures I–
III and V–VII, with positive charges located at
carbons of two rings but also of structures IV and
VIII in which positive charge is carried by
nitrogen. Structures IV and VIII are especially
stable since in these structures, every atom
(except hydrogen) has a complete octet of
electrons.

16. 16

17. 17
These carbonium ions (IV and
VIII) are much more stable
than the one obtained from
attack at benzene itself or at
meta to the —NH 2 group of
aniline (IX–XI structures). In
none of these structures,
structures like IV or VIII are
possible.

19. 19
Thus substitution in aniline occurs faster than
substitution in benzene and occurs mostly at
ortho and para positions to —NH 2 groups.
Similarly orientation at ortho- and para-positions
of —OH group and derivatives of —NH 2 and
—OH groups can be explained by similar
structures as shown by para and ortho attack
(structures IV and VIII).

20. 20
Effect of halogens ( carbonium ions formed by
attack at para, ortho and meta positions of
chlorobenzene ): Halogens are a class in themselves
in the sense that they are deactivating and yet ortho
and para directing. The halogens have a (−I) effect, i.e.
they are electron withdrawing and have (+M) effect, i.e.
they have electron-releasing resonance effect. To
understand their orientation infuence, let us compare
the structure of the carbonium ions formed by the
attack of the electrophile at the para and ortho position
to the attack at the meta position.

21. 21

22. 22
Thanks
!