Question

What is reactivity towards NSR order?
(A) $ {{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{ - X}} $
(B) $ {{\text{C}}_{\text{6}}}{{\text{D}}_{\text{5}}}{\text{ - X}} $
(C) $ {{\text{C}}_{\text{6}}}{{\text{T}}_{\text{5}}}{\text{ - X}} $
(D) none

Answer 1

Hint: In the above question, we have to find out which of the given options is more reactive towards nucleophilic substitution reactions. So, we can answer it according to the inductive effect. Inductive effect is inversely proportional to the reactivity towards NSR.

Complete Step by step complete solution
We know that aryl compounds are less vulnerable to nucleophilic substitution reaction. Nucleophilic substitution reactions can occur with aryl halides if strong electron‐withdrawing groups (deactivators) are located at ortho and/or para position with respect to the carbon atom that's attached to the halogen.
But here, no electron withdrawing groups are present. So, the answer lies on the inductive effect of hydrogen, deuterium and tritium.
Inductive effect depends upon the distance. Hence, tritium has a stronger inductive effect than deuterium which in turn has more inductive effect than protium. Hence, $ {{\text{C}}_{\text{6}}}{{\text{T}}_{\text{5}}}{\text{ - X}} $ is least reactivity towards nucleophilic substitution reaction and $ {{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{ - X}} $ is the most reactivity towards nucleophilic substitution reaction.
Hence, reactivity towards NSR order is $ {{\text{C}}_{\text{6}}}{{\text{T}}_{\text{5}}}{\text{ - X}} < {{\text{C}}_{\text{6}}}{{\text{D}}_{\text{5}}}{\text{ - X}} < {{\text{C}}_{\text{6}}}{{\text{H}}_{\text{5}}}{\text{ - X}} $ .

Note:
Because of resonating structure, aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides. Due to resonance, C-X bond acquires partial double bond character and becomes shorter and stronger and cannot be easily replaced by nucleophiles.
Other reasons for the low reactivity of aryl halides are:
Difference in the hybridization states of carbon atoms in C−X bond. In alkyl halides, the carbon atom is $ {\text{s}}{{\text{p}}^{\text{3}}} $ hybridized whereas in aryl halides, it is $ {\text{s}}{{\text{p}}^2} $ hybridized and hence, more electronegative. Hence, C−X bond in aryl halides is more difficult to break. Polarity of the C−X bond in aryl halides is lower than that in alkyl halides. Polarity is directly proportional to the NSR reactivity. Hence, lower polarity results in lower reactivity.