KCET · Chemistry · General Organic Chemistry

Mesomeric effect involves

A delocalisation of \(\pi\)-electrons
B delocalisation of \(\sigma\)-electrons
C partial displacement of electrons
D delocalisation of \(\pi\) and \(\sigma\)-electrons

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Answer & Solution

Correct Answer

(A) delocalisation of \(\pi\)-electrons

Step-by-step Solution

Detailed explanation

Resonance or mesomeric effect is defined as, the polarity produced in the molecule by the interaction of two \(\pi\)-bonds or between a \(\pi\)-bond and a lone pair of electrons present on an adjacent atom. There are two types of mesomeric effect.
\(+M\) effect is observed when direction of electron displacement is away from an atom or substituent group attached to the conjugated system e.g., - halogen, \(-\mathrm{OH},-\mathrm{NH}_{2}\) etc. On the other hand, - M effect is observed when the transfer of electrons is towards the atom or substituent group attached to the conjugted systems, e.g., \(-\mathrm{NO}_{2},-\mathrm{COOH}\) etc.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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