Question 67

But-2-yne and hydrogen (one mole each) are separately treated with (i) Pd/C and (ii) Na/ liq. NH3 to give the products X and Y respectively.

Identify the incorrect statements.
A. X and Y are stereoisomers.
B. Dipole moment of X is zero.
C. Boiling point of X is higher than Y.
D. X and Y react with $$O_{3}/Zn+H_{2}O$$ to give different products.
Choose the correct answer from the options given below :

Solution

A. X and Y are stereoisomers: True. They are geometrical isomers (cis and trans).

B. Dipole moment of X is zero: False. In cis-but-2-ene (X), the bond dipoles of the $$C-CH_3$$ groups reinforce each other, giving a net dipole moment ($$\mu \neq 0$$). In trans-but-2-ene (Y), the dipoles cancel out, making its dipole moment zero.

C. Boiling point of X is higher than Y: True. Boiling point is directly proportional to the dipole moment. Since the cis isomer (X) is more polar, it has stronger intermolecular forces and a higher boiling point.

D. X and Y react with $$O_3/Zn + H_2O$$ to give different products: False.

Reaction Name: Reductive Ozonolysis.
Both cis and trans isomers of But-2-ene break at the double bond to yield two molecules of Ethanal ($$CH_3CHO$$). The connectivity is the same, so the products are identical.

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