Chapter 18.10, Problem 1LTS

Interpretation Introduction

Interpretation:

In the given compound below

The following observations have to be determined –

• The aromatic ring is whether activated or deactivated
• The strength of activation/deactivation
• Directing effects of the substituents

Concept Introduction:

• Aromatic compounds undergo special type of reactions namely electrophilic substitution reactions. Electrophilic substitution reactions are more prominent in aromatic compounds than in aliphatic compounds and hence they are precisely known as Aromatic electrophilic substitution reactions. An electrophile is a species that is deficient of electrons.
• Though benzene, the simplest form of aromatic compound is considered as highly stable its electrophilic substitution reactions are due to its 6$\text{π}$ electrons.
• The efficiency of the aromatic ring to take part in electrophilic substitution reactions depends upon the availability of its $\text{π}$ electrons. If the $\text{π}$ electrons are readily available then a new bond between Carbon and the electrophile is formed readily on the expense of those $\text{π}$ electrons.
• Various types of aromatic compounds undergo electrophilic substitution reactions. Except in the case of an aromatic ring without any substituent on the ring, the reactivity of many other aromatic compounds varies based on their substituents.
• In such cases the availability of the $\text{π}$ electrons present in the aromatic ring depends on the electronic effects caused by the substituents on the ring. An electrophile is a species that is always in the need of electrons. It is always attracted towards the region where the electron density is more.
• If the presence of substituents enhances the electron density of the ring then the reaction between the aromatic ring and the electrophile is triggered readily. Then the aromatic 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 ability of the $\text{π}$ electrons to bond with the electrophile is diminished and the aromatic ring becomes 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.
• It can be more explained in this way – electron donating groups increase the electron density of the ring and can be termed as activators.  Electron withdrawing groups decrease the electron density of the ring and are termed as deactivators. These are collectively known as the electronic effects caused by the substituents. Halogens are exception that though they are electron withdrawing groups they are weak deactivators.
• The classification of the 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.