according to PCI

1. Pharmaceutical Organic
Chemistry – II
Effects of substituents on
Orientation of mono-substituted
Benzene compounds
By : Ishika Choudhary
b.Pharma ; 3rd sem.
Roll no. 51
Enroll no. AJU190857

2. Substitution in
mono-substituted benzenes :
O All hydrogen atoms in the benzene ring are
equivalent. Therefore , only one
monosubstitution product (C6H5-S) is
possible.

3. O A second substituent, E, can occupy any
of the remaining five positions :
O If substitution was random,
proportion of disubstituted
products would be :
O Ortho = 40%
O Meta = 40%
O Para = 20 %
O However this distribution is
never observed actually. The
proportion of products formed,
in fact, is determined by the
nature of the first substituent
on the ring rather than on any
mathematical probabilities.

4. Two types of influence of
substitution :
O Directive or Orientation
effect : the first substituent
(G) may direct the next
incoming substituent (E) to
ortho, meta, or para position ,
depending on the nature of
the first substituent. This is
called the directive or the
orientation (orient= to
arrange) effect.

5. OActivity effect :
OThe substituent already present
may activate or deactivate the
benzene ring toward further
substitution. These effects are
called the Activity effects.

6. Orientation effects of mono-
substituted benzene :
O Ortho – Para directing effect : certain
substituents direct the second substituent to the
ortho and para positions simultaneously. These
are called ortho-para directors.
O Example, nitration of phenol.

7. O Meta – Directing Effects : the substituents
which direct the second incoming substituent
primarily to the meta-position, are referred to
as meta-directors.
O Example, nitration of nitrobenzene.

9. The key points to be
remembered in this topic :
O The nature of groups already on an aromatic ring
affect both the reactivity and orientation of future
substitution.
O Ortho-para directors direct future substitution to
the ortho and para positions.
O Meta directors direct future substitution to the
meta position.