Chapter 18.11, Problem 18PTS

Interpretation Introduction

Interpretation:

For each of the following compounds the position that is more prone to the substitution by electrophile in aromatic electrophilic substitution reactions has to be identified.

Concept Introduction:

• Benzene is an electron rich Aromatic compound and undergoes electrophilic substitution reactions. 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 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 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)
${\text{-NH}}_2$	$\text{-NROR}$		${\text{-NR}}_{\text{3}}$	$\text{-COH}$
${\text{-O}}^{\text{-}}$	$\text{-OR}$		${\text{-CX}}_{\text{3}}$	$\text{-COOR}$
$\text{-NHR}$	$\text{-O-C=OR}$			$\text{-COOH}$
${\text{-NR}}_{\text{2}}$				${\text{-CONH}}_{\text{2}}$
				${\text{-SO}}_{\text{3}}\text{H}$
				$\text{-CN}$