Question

Alkyl groups act as _________ when attached to a $\pi$-system.

• A. electron acceptors
• B. electron donors
• C. hydrogen donors
• D. do not interact with $\pi$-system

Answer 1

The correct option is B electron donors

Alkyl is an electron-releasing group. It acts as an electron-donor species.

The carbocations in which carbon atom has sextet configuration (and hence electron deficient) can receive a pair of electrons from the alkyl group.

In alkyl halide, the polarity of bond $(C-X)$ has a partial positive charge on the carbon atom and hence the carbon atom becomes an electrophile centre at which a nucleophile can attack.

When alkyl group is attached to $\pi$-system like in ethanal due to greater $EN$ of $O$ than $C,\pi \overrightarrow{e}s$ are withdrawn by oxygen through electromeric effect and $C-1$ acquires a positive charge and oxygen atom acquires a negative charge. So to overcome the deficiency of $\overrightarrow{e}$ on $C-1,\left(C{H}_3\right)$ group attached to it gives $\sigma \overrightarrow{e}s$ to $C-1$ and thus $\left(C{H}_3\right)$ group acts as an $\overrightarrow{e}$ donor via inductive effect. This can also be explained by hyperconjugation.