Question

Which of the following has the minimum heat of dissociation?
a. \[{{(C{{H}_{3}})}_{3}}N\to B{{F}_{3}}\]
b. ${{(C{{H}_{3}})}_{3}}N\to B(C{{H}_{3}}){{F}_{2}}$
c. ${{(C{{H}_{3}})}_{3}}N\to B{{(C{{H}_{3}})}_{2}}F$
d. ${{(C{{H}_{3}})}_{3}}N\to B{{(C{{H}_{3}})}_{3}}$

Answer 1

Hint: As the +I effect increases the tendency to act as Lewis acid decreases and the coordinate bond strength decreases in the same order. As the strength decreases, the heat of dissociation decreases.

Complete Step by Step Solution:
Heat of dissociation is defined as the energy of reaction that arises due to the breaking of larger compounds into smaller atoms or fragments. We know higher is the stability of the compound, higher is the bond strength between them. As the bond strength increases, the bond length decreases and the energy needed to dissociate them increases. So, we can say that lesser the stability of the molecule, lesser is the energy required to break them into smaller atoms.

In the given question, as the $+I$effect of the alkyl groups increases the tendency to act as Lewis acid decreases and hence the stability decreases thereby the bond strength decreases and the heat of dissociation decreases. Hence, we can conclude that the compound ${{(C{{H}_{3}})}_{3}}N\to B{{F}_{3}}$ has the maximum heat of dissociation and the compound ${{(C{{H}_{3}})}_{3}}N\to B{{(C{{H}_{3}})}_{3}}$has the minimum heat of dissociation.

Therefore the correct answer is d. ${{(C{{H}_{3}})}_{3}}N\to B{{(C{{H}_{3}})}_{3}}$.

Additional information:
Some $-I$ groups are:
-CN, -OH, -COOH, -$N{{O}_{2}}$, etc.
Some $+I$ groups are:
All alkyl groups

Note: The thing needed to note here is the factor that affects the Lewis acid character, the higher the number of electron withdrawing groups, higher is the Lewis acid character and higher is the stability. Another thing is that the more the stability of the compound, the more difficult it is to break the bond between them.