Question

Assertion : The rate of nitration of benzene is less than hexa deuteron benzene.
Because
Reason : Deuterium is an isotope of hydrogen.
a.) Both Assertion and Reason are correct and Reason is the correct explanation for assertion.
b.) Both Assertion and Reason are correct and Reason is not the correct explanation for assertion.
c.) Assertion is correct but Reason is incorrect.
d.) Assertion is incorrect and Reason is correct.

Answer 1

Hint: In the nitration of benzene, the step in which the leaving group leaves is the fast one and not the rate determining step. Instead the attack of benzene rings on electrophile is the rate determining step. Further, the hydrogen and deuterium both have the same atomic number and different mass number.

Complete step by step answer :
For this we need to know the mechanism of nitration of benzene. On what factors the rate of nitration of benzene depends and which one is the rate determining step in the mechanism.
So, let us see the mechanism of nitration of benzene first. The mechanism is -
Step 1:- The first step involves the generation of nitronium ion which is an electrophile by reaction of conc. Sulphuric acid and nitric acid. This step is fast and occurs very easily.
Step 2 :- This step involves the attack of nitronium ion on the benzene ring. The benzene ring is highly electron rich and thus attacks the nitronium ion. This step is slow and is the rate determining step.
Step 3 :- The hydrogen or deuterium whichever atom is present gets abstracted. This step is also fast. So, the above mechanism clearly displays that whether the leaving atom is hydrogen or deuterium, it does not matter because the third step is not rate determining.
Thus, the assertion is wrong.
The reason that deuterium is the isotope of hydrogen is correct. Yes, it is true that deuterium is the isotope of hydrogen.

Note: The mixture of conc. Sulphuric acid and nitric acid are called nitrating mixtures. This step is fast because both are concentrated in nature and have very high energy. During this step, a large amount of energy is generated in the form of heat.