Question

Which of the following has the shortest C-Cl bond ?

• A. $\mathrm{Cl}-\mathrm{CH}={\mathrm{CH}}_2$
• B. $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_3$
• C. $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{OCH}}_3$
• D. $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$

Answer 1

The correct option is D

$\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$

The explanation for the correct answer

(D) $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$

1. $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$ has the shortest $\mathrm{C}-\mathrm{Cl}$ bond due to extended conjugation.
2. The extended conjugation in this compound is due to presence of ${\mathrm{NO}}_2$ group which have the electronegative atoms like Oxygen and Nitrogen that causes conjugation.
3. Conjugation decreases the bond length of $\mathrm{C}-\mathrm{Cl}$.
4. Thus, $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$ has the shortest $\mathrm{C}-\mathrm{Cl}$ bond.

The explanation of incorrect answers

(A) $\mathrm{Cl}-\mathrm{CH}={\mathrm{CH}}_2$

1. In $\mathrm{Cl}-\mathrm{CH}={\mathrm{CH}}_2$, no conjugation of bonds occur.
2. Due to which it has no effect on its $\mathrm{C}-\mathrm{Cl}$ bond.
3. So, it has not the shortest $\mathrm{C}-\mathrm{Cl}$ bond.
4. Thus, it is an incorrect option.

(B) $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_3$

1. In $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_3$, not any group present which affect the $\mathrm{C}-\mathrm{Cl}$ bond.
2. Due to which not any conjugation of bonds occur and hence it has not the shortest $\mathrm{C}-\mathrm{Cl}$ bond.
3. Thus, it is an incorrect option.

(C) $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{OCH}}_3$

1. In $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{OCH}}_3$, the conjugation occurs but the conjugation is less than $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$.
2. This is due to the fact that Nitrogen and Oxygen are more electronegative and thus causes the conjugation much easier than $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{OCH}}_3$.
3. Its $\mathrm{C}-\mathrm{Cl}$ bond is shorter but not shorter than$\mathrm{C}-\mathrm{Cl}$ bond of $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$.
4. Thus, it is an incorrect option.

Thus, $\mathrm{Cl}-\mathrm{CH}=\mathrm{CH}-{\mathrm{NO}}_2$ has the shortest $\mathrm{C}-\mathrm{Cl}$ bond. Therefore, option D is correct.