NEET Practice Questions (MCQs) with Answers & Solutions

According to the NCERT text, 'C— Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.' Also, 'The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C— X bond more tightly than sp3-hybridised carbon in haloalkane with less s-chararcter. Thus, C— Cl bond length in haloalkane is 177pm while in haloarene is 169 pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.'

The NCERT states, 'The presence of an electron withdrawing group (-\text{NO}_2) at ortho- and para-positions increases the reactivity of haloarenes.' It further explains, 'As shown, the presence of nitro group at ortho- and para-positions withdraws the electron density from the benzene ring and thus facilitates the attack of the nucleophile on haloarene. The carbanion thus formed is stabilised through resonance.'

The NCERT text mentions, 'Halogen atom besides being slightly deactivating is o, p -directing; therefore, further substitution occurs at ortho- and para-positions with respect to the halogen atom.' It also states, 'Further , the halogen atom because of its –I effect has some tendency to withdraw electrons from the benzene ring. As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.'

The NCERT specifies, 'The effect is pronounced when (-NO2) group is introduced at ortho- and para- positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position.' This indicates that a nitro group at the ortho-position would increase reactivity.

The NCERT states, 'The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C— X bond more tightly than sp3-hybridised carbon in haloalkane with less s-chararcter. Thus, C— Cl bond length in haloalkane is 177pm while in haloarene is 169 pm.'

The NCERT mentions, 'Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres.'

The NCERT lists, 'Haloarenes undergo the usual electrophilic reactions of the benzene ring such as halogenation, nitration, sulphonation and Friedel-Crafts reactions.' Wurtz reaction involves coupling of alkyl halides and is not an electrophilic substitution of the benzene ring.

According to the NCERT, 'Due to resonance, the electron density increases more at ortho- and para-positions than at meta-positions.'

The NCERT states, 'Instability of phenyl cation: In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, SN1 mechanism is ruled out.'

The NCERT says, 'As a result, the ring gets somewhat deactivated as compared to benzene and hence the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.'