Halkoarenes & Haloarenes MCQs for NEET — Chemistry Questions with Answers

The context explicitly states: 'The above methods are not applicable for the preparation of aryl halides because the carbon-oxygen bond in phenols has a partial double bond character and is difficult to break being stronger than a single bond.'

The context mentions: 'Propene yields two products, however only one predominates as per Markovnikov’s rule.' (Unit 13, Class XI), which implies that the halogen adds to the carbon atom of the double bond that has the greater number of alkyl substituents (or fewer hydrogen atoms).

The context describes: 'addition of bromine in CCl4 to an alkene resulting in discharge of reddish brown colour of bromine constitutes an important method for the detection of double bond in a molecule.'

Haloarenes contain a halogen atom(s) attached to sp$^2$ hybridised carbon atom(s) of an aryl group.

Halogen atoms are more electronegative than carbon, therefore, carbon-halogen bond of alkyl halide is polarised. This electronegativity difference is the primary factor influencing the polarity.

The sp$^2$ hybridised carbon with a greater s-character (in haloarenes) is more electronegative and can hold the electron pair of the C-X bond more tightly than sp$^3$-hybridised carbon (in haloalkanes) with less s-character. This results in a stronger and shorter bond in haloarenes.

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.

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.'