Question

Assertion : Rates of nitration of benzene and hexadeuterobenzene are different.

Reason : C - H bond is stronger than C - D bond.

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true but R is not correct explanation of A

(c) A is true but R is false

(d) A and R are false

Answer 1

Correct option  (d) A and R are false

Explanation:

Rates of nitration of benzene and hexadeutero benzene are same because nitration depends upon availability of electrophile. C-D bond is stronger than C - H bond. Here changing a hydrogen atom to deuterium in C- H bond represents a 100% increase in mass. Here the mass of an atom affects the vibration frequency of the chemical bond that it forms, even if the electron configuration is nearly identical. Heavier atoms will lead to lower vibration frequencies or will have lower zero-point energy. With a lower zero-point energy, more energy must be supplied to break the bond, resulting in a higher activation energy for bond cleavage, thus C - D bond is stronger than C - H bond.