Chapter 18.10, Problem 16PTS

Interpretation Introduction

Interpretation:

Form the given compounds, the aromatic ring present whether activated or deactivated, the strength of activation/deactivation and the directing effects of the substituent present.  On each ring of the compounds have to be determined and illustrated.

Concept Introduction:

• A ring system is said to be aromatic if it obeys Huckel’s rule of Aromaticity. Accordingly aromatic ring is cyclic, planar and has (4n+2)$\text{π}$ electrons. Benzene is the simplest aromatic ring system. Thus Benzene is cyclic, planar and has (4n+2)$\text{π}$ electrons that is 6$\text{π}$ electrons.
• Benzene has 6 $\text{C-C}$ bonds, 6 $\text{C-H}$ bonds and 3 $\text{C=C}$ bonds. The 3 $\text{C=C}$ bonds give rise to 6$\text{π}$ electrons.
• The 6$\text{π}$ electrons in benzene are not localized but they are in continuous state of motion that they are delocalized. This attributes to the special property in benzene ring called “resonance”. The phenomenon of resonance is the characteristic feature of the aromatic compounds.
• Benzene undergoes 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 is considered as highly stable its electrophilic substitution reactions are due to the 6$\text{π}$ electrons. In these reactions a new bond between Carbon and the electrophile is formed and a $\text{C-H}$ bond in benzene is broken.
• The efficiency of the benzene 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.
• There are various types of electrophilic substitution reactions that occur in benzene and its family. Various types of benzene compounds undergo these reactions. Except in the case of benzene where there is no substituent present on the ring, many other benzene derivatives have one or more than one substituent already present in the ring.
• In such cases the availability of the $\text{π}$ electrons present in the benzene ring 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 where the electron density is more.
• 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.
• It can be more explained in this way – electron donating groups increase the electron density of the ring and can be termed as activators whereas 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 exceptions that though they are electron withdrawing groups they are weak deactivators.
• The substituents influences the position must be 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. Halogens are ortho para directors.