Question

a) Aryl halides are less reactive in nucleophilic substitution reactions.

i) Write any two reasons for less reactivity,

ii) Give one example for nucleophilic substitution reactions of aryl halides.

b) Write a method for the preparation of alkyl halides.

c) Which of the following is not a polyhalogen compound?

a) Chloroform

b) Freon

c) Carbon tetrachloride

d) Chlorobenzene

Answer 1

i) 1. Resonance Effect- In aryl halides the electron pairs on halogen atom are in conjunction with π-electrons of the ring and different resonating structures are possible, 

e.g. resonance in chlorobenzene:

The C – X bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in aryl halides is dicult than in alkyl halides and therefore, they are less reactive towards nucleophilic substitution reaction. 

2. Dierence in hybridisation of carbon atom in C-Xbond: In alkyl halides, the carbon atom attached to halogen is sp hybridised while in the case of aryl halides, the carbon atom attached to halogen is sp hybridised.

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 alkyl halides with less s-character. Thus, the C – X bond length in aryl halides is less than that in alkyl halides. Since it is difficult to break a shorter bond than a longer bond, therefore, aryl halides are less reactive than alkyl halides towards nucleophilic substitution reaction.

3. Instability of phenyl cation – In the case of aryl halides, the phenyl cation formed as a result of self-ionization will not be stabilised by resonance and therefore, S_N1 mechanism is ruled out.

4. Because of the possible repulsion, it is less likely for the electron-rich nucleophile to approach electron-rich arenes. (any two reasons)

ii) Chlorobenzene when heated in aqueous sodium hydroxide solution at a temperature of 623 K and a pressure of 300 atmospheres followed by acidification gets converted to phenol.

b) Alkyl halides can be prepared by treating alcohols with concentrated halogen acid in presence of anhydrous zinc chloride as a catalyst.

c) d) Chlorobenzene