Question

Which of the following is the most correct electron displacement for a nucleophilic reaction to take place?
A)
 

B)

C)

D)

Answer 1

Hint: Refer to the mechanism of reverse hyperconjugation. Reverse hyperconjugation is most commonly observed in the case of $\alpha - {\text{Halo alkenes}}$. In this phenomenon, there is movement of electron density from the filled $\pi - $ or p-orbital to the neighbouring empty ${\sigma ^*} - orbital$.

Complete answer:
Firstly, observe that we are given a particular type of system in all the options and it is as shown below:

This above structure belongs to the class of $\alpha - {\text{Halo alkenes}}$. Reverse hyperconjugation phenomenon is observed in $\alpha - {\text{Halo alkenes}}$, where the ${\sigma ^*} - orbital$ is located on the carbon-halogen bond. In reverse hyperconjugation, there is movement of electron density from the filled $\pi - $ or p-orbital to the neighbouring ${\sigma ^*}$-orbital. Therefore, in case of $\alpha - {\text{Halo alkenes}}$, there is delocalization of electron density from the double bond (i.e., filled $\pi - orbital$ electron density) towards the electrophilic carbon (i.e., neighbouring ${\sigma ^*}$-orbital). The electrophilic carbon or carbocation is generated when a halogen group leaves with its shared pair of electrons. Simply, we can say that delocalization of electrons occurs towards the halogen group through the hyperconjugative mechanism in $\alpha - {\text{Halo alkenes}}$.
We can express the reverse hyperconjugation mechanism in the given $\alpha - {\text{Halo alkene}}$ as:

Thus among the given options, only in option A, reverse hyperconjugation mechanism is directed.

Hence, the correct option is A.

Note:
Reverse hyperconjugation phenomenon is also known as negative hyperconjugation. This is attributed to the fact that in negative hyperconjugation, the electron density flows in the opposite direction (from filled p-orbital to empty ${\sigma ^*} - orbital$) than it does in the more common hyperconjugation phenomenon (from $\sigma - orbital$ to empty $\pi - orbital$). Negative hyperconjugation stabilizes the molecule or transition state. It causes the elongation of the $\sigma - bond$ by adding electron density to its anti-bonding orbital.