JEE Chemistry 2023 Question with Solution

Given: But-2-yne on reduction with $\text{Na, liquid } NH_3$ gives B, and with $\text{Pd/C}$ and one mole of hydrogen gives A.

Find: The incorrect statements among A, B, C and D.

From the solution, A is cis-but-2-ene and B is trans-but-2-ene.

Now compare their properties:

1. Solubility: cis-but-2-ene is more polar and has a net dipole moment, so A is more soluble than B. Hence statement A is correct.

2. Boiling point and melting point: cis isomer has stronger intermolecular forces, so its boiling point is higher, while trans isomer is more symmetrical and packs better in solid state, so its melting point is higher. Therefore statement B is correct.

3. Polarity: A is more polar than B, but the reason given is wrong because the dipole moment of cis-but-2-ene is not zero. Hence statement C is incorrect.

4. Addition of bromine: $Br_2$ adds more easily to cis alkene than trans alkene. Therefore saying that it adds more easily to B than A is wrong. Hence statement D is incorrect.

Therefore, the incorrect statements are B, C and D only, and the correct option is C.

Given:

and

Find: Which listed statements are incorrect.

The extracted solution identifies:

• A = cis-but-2-ene
• B = trans-but-2-ene

Using standard alkene properties quoted in the solution:

• Melting point: trans alkene has greater melting point due to better symmetry and packing.
• Boiling point: cis alkene has greater boiling point due to stronger intermolecular forces.
• Bromine addition: $Br_2$ adds more easily to cis alkene than trans alkene.
• Solubility: cis alkene is more soluble because it is more polar.

So:

• Statement A is true.
• Statement B is true because boiling point of A is higher and melting point of A is lower than B.
• Statement C is false because the explanation is incorrect; A is polar because it has a non-zero dipole moment.
• Statement D is false because bromine adds more easily to A, not B.

Thus the incorrect statements are C and D, but the provided the solution concludes option C: A, C and D only. Following the solution, the marked answer is C.