JEE Chemistry 2026 Question with Solution

Given: But-2-yne is treated separately with $H_2$ and Pd/C to form $X$, and with $Na/\text{liq.}\,NH_3$ to form $Y$.

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

Alkynes undergo stereoselective partial reduction depending on the reagent used.

Now evaluate each statement:

1. Statement A: cis-but-2-ene and trans-but-2-ene are geometrical isomers, hence stereoisomers. So A is true.
2. Statement B: cis-but-2-ene has a non-zero dipole moment, so the statement "Dipole moment of X is zero" is false. Thus B is incorrect.
3. Statement C: The cis isomer is more polar and generally has a higher boiling point than the trans isomer. So C is true.
4. Statement D: Ozonolysis of both cis-but-2-ene and trans-but-2-ene gives the same carbonyl product, namely acetaldehyde.

Therefore D is incorrect.

So, the incorrect statements are B and D only. The correct option is D.

Given:

• Pd/C partial reduction gives syn-addition product.
• $Na/\text{liq.}\,NH_3$ gives anti-addition product.

Find: Which listed statements are incorrect.

Step 1: Identify $X$ and $Y$.

For an alkyne, partial hydrogenation with Pd/C gives the cis alkene, while reduction with $Na/\text{liq.}\,NH_3$ gives the trans alkene.

Hence:

• $X =$ cis-but-2-ene
• $Y =$ trans-but-2-ene

Step 2: Check stereochemistry.

Since cis and trans forms differ only in spatial arrangement, they are stereoisomers. So A is correct.

Step 3: Check dipole moment.

In cis-but-2-ene, bond dipoles do not cancel completely, therefore $\mu \ne 0$. In trans-but-2-ene, symmetry causes dipole cancellation, so $\mu = 0$ for $Y$, not for $X$. Hence B is incorrect.

Step 4: Compare boiling points.

Because cis-but-2-ene is more polar, intermolecular attraction is stronger, so its boiling point is higher than that of trans-but-2-ene. Thus C is correct.

Step 5: Ozonolysis products.

Both geometrical isomers have the same carbon skeleton. On ozonolysis, each double-bond carbon becomes an aldehyde carbon.

Thus both $X$ and $Y$ give the same product, not different products. Therefore D is incorrect.

Conclusion: The incorrect statements are B and D only, so the correct option is D.