Question

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

Answer 1

Answer: (A)

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 $+M$ effect is observed when direction of electron displacement is away from an atom or substituent group attached to the conjugated system e.g. - halogen, $- OH _{1}- NH _{2}$ 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., $- NO _{2},- COOH$ etc.