Chapter 19.11, Problem 3LTS

Interpretation Introduction

Interpretation:

In the given compound below

                                  

The position that is most likely to be substituted by electrophile has to be identified.

Concept Introduction:

• In the family of Aromatic compounds, Electrophilic substitution reaction is a type of reaction in which a particular group or atom in a compound is replaced by electrophile. An electrophile is a species that is deficient of electrons.

• The position that the electrophile occupies in the aromatic ring system depends on various factors like – presence of substituents, activation and deactivation of the ring etc.

• Activation or deactivation of the ring towards electrophilic substitution reaction depends upon the electron density of the ring which is influenced by the electronic effects caused by the substituents present on the aromatic ring.

• The pi electrons of the aromatic system must be readily available so that the reaction proceeds easily. Presence of many bulk substituents and/or the presence of strong electron withdrawing groups diminishes the ability of the delocalizing pi electrons to bond with the electrophile.

• If the presence of substituents enhances the electron density of the ring then the reaction between the benzene ring and the electrophile is triggered readily. Then the ring is said to be activated towards electrophilic substitution reactions.

• If the presence of substituents causes electron drain in the ring then the $\text{π}$ electrons are not available to bond with electrophile. The benzene ring becomes devoid of electrons and deactivated towards electrophilic substitution reactions.

• Thus the substituents can be widely of two types – activators and deactivators. Activators are the substituents that enhance the electron density of the ring. Deactivators are the substituents that suppress the electron density of the benzene ring.

The classification of activators and deactivators can be summarized as follows

Activators			Deactivators
Strong	Moderate	Weak	Strong	Moderate	Weak
$\text{-OH}$	$\text{-NHCOR}$	$\text{-R}$	${\text{-NO}}_{\text{2}}$	$\text{-COR}$	$\text{-X}$(HALOGENS)
${\text{-NH}}_2$	$\text{-NROR}$		${\text{-NR}}_{\text{3}}$	$\text{-COH}$
${\text{-O}}^{\text{-}}$	$\text{-OR}$		${\text{-CX}}_{\text{3}}$	$\text{-COOR}$
$\text{-NHR}$	$\text{-O-C=OR}$			$\text{-COOH}$
${\text{-NR}}_{\text{2}}$				${\text{-CONH}}_{\text{2}}$
				${\text{-SO}}_{\text{3}}\text{H}$
				$\text{-CN}$

• The substituents influence the position that is occupied by the electrophile. Activators enhance the electron density of the ring at the ortho- and para- position to them and direct the electrophile to occupy ortho and para positions.

• Deactivators decrease the electron density of the ring at the ortho- and para- position to them and direct the electrophile to occupy meta position. Exception is Halogens which are weak deactivators yet they are ortho para directors.