JEE Main 2025 Apr 4 Shift 1, Chemistry Q18: Properties and Important Reactions of Aldehydes & Ketones

JEE Main2025Apr 4, Shift 1Chemistry

Given below are two statements.

Statement I : The dipole moment of $4 C H_{3} - 3 C H = 2 C H - 1 C H = O$ is greater than $4 C H_{3} - 3 C H_{2} - 2 C H_{2} - 1 C H = O$ .

Statement II : $C_{1} - C_{2}$ bond length of $4 C H_{3} - 3 C H = 2 C H - 1 C H = O$ is greater than $C_{1} - C_{2}$ bond length of $4 C H_{3} - 3 C H_{2} - 2 C H_{2} - 1 C H = O$ .

In the light of the above statements, choose the correct answer from the options given below:

A
Statement I is false but Statement II is true
B
Both Statement I and Statement II are false
C
Statement I is true but Statement II is false
D
Both Statement I and Statement II are true

Solution

The first compound is but-2-enal (an $α, β$ -unsaturated aldehyde, $C H_{3} - C H = C H - C H O$ ); the second is butanal ( $C H_{3} - C H_{2} - C H_{2} - C H O$ ).

Dipole moment: In but-2-enal, resonance pushes electron density from C3 onto the carbonyl oxygen, creating a substantial positive charge on C3 and a more negative oxygen. The separated charges are also farther apart along the conjugated chain. Since $μ = q \times d$ , both a larger $q$ and a larger $d$ make the dipole moment of but-2-enal larger than that of butanal. Statement I is true.

$C_{1} - C_{2}$ bond length: In but-2-enal, the $C_{1} - C_{2}$ bond has partial double-bond character because of the conjugated resonance form $C H_{3} - C + H - C H = C (O^{-}) H$ . Partial double-bond character shortens the bond. So $C_{1} - C_{2}$ in but-2-enal is shorter, not longer, than the pure single $C_{1} - C_{2}$ bond of butanal. Statement II is false.

$∴$ Option (3): I true, II false.

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JEE Main 2025 Apr 4 Shift 1, Chemistry Q18: Properties and Important Reactions of Aldehydes & Ketones

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