Electromeric effect:

Options

A. Comes into play at the demand of attacking reagent
B. Involves displacement of electrons in a sigma bond
C. Comes into play in the molecule when at least one atom has unshared pair of electrons
D. Invovles the distortion of the electron cloud.

Answer

### Step-by-Step Solution:

1. **Definition of Electromeric Effect**:
The electromeric effect is defined as the complete transfer of pi electrons to one of the atoms in a molecule due to the influence of an attacking reagent (such as an electrophile or nucleophile).

2. **Understanding the Mechanism**:
- When an electrophile approaches a molecule with a double bond, such as CH2=CH2 (ethylene), the pi electrons of the double bond are transferred towards the electrophile, creating a positive charge on one of the carbon atoms.
- Conversely, when a nucleophile approaches, the pi electrons shift towards the nucleophile, resulting in a negative charge on one of the carbon atoms.

3. **Analyzing the Options**:
- **Option A**: "Comes into play at the demand of attacking reagent" - This is true as the electromeric effect occurs due to the presence of an attacking reagent.
- **Option B**: "Involves displacement of electrons in a sigma bond" - This is incorrect because the electromeric effect specifically involves pi electrons, not sigma bonds.
- **Option C**: "Comes into play in the molecule when at least one atom has an unshared pair of electrons" - This is not a requirement for the electromeric effect to occur.
- **Option D**: "Involves the distortion of the electron cloud" - While there is some distortion, the key feature is the complete transfer of pi electrons, not just distortion.

4. **Conclusion**:
Based on the analysis of the options, the correct answer is **Option A**. The electromeric effect is indeed influenced by the attacking reagent.