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Question #44175

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The Question

Consider the following three alkenes: (I) \ce{CH2=CH2} (II) \textit{cis}-\ce{CH3-CH=CH-CH3} (III) \textit{cis}-\ce{(CH3)3C-CH=CH-C(CH3)3}

Let $d_1, d_2, d_3$ be their respective \ce{C=C} bond lengths. Let $E_1, E_2, E_3$ be the activation energies required for thermal rotation around the \ce{C=C} bond (\textit{cis}-\textit{trans} isomerization). Let $H_1, H_2, H_3$ be the magnitudes of their standard enthalpies of hydrogenation ( $|\Delta H_{hydro}|$ ), assuming the resulting alkanes adopt their most stable conformations.

Which of the following correctly represents the order of these physical quantities?

Options

A.
$d_3 > d_2 > d_1$ ; $E_1 > E_2 > E_3$ ; $H_3 > H_1 > H_2$
B.
$d_1 > d_2 > d_3$ ; $E_3 > E_2 > E_1$ ; $H_1 > H_2 > H_3$
C.
$d_3 > d_2 > d_1$ ; $E_1 > E_2 > E_3$ ; $H_1 > H_2 > H_3$
D.
$d_3 > d_2 > d_1$ ; $E_3 > E_2 > E_1$ ; $H_3 > H_2 > H_1$

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Bond Energy vs Bond Length|Activation Energy Trap|Reaction Progress Coordinate

Why This Question Is Tricky

Common Mistakes

This problem tests the 'Activation Energy Trap' and the breakdown of textbook thermodynamic rules under extreme steric hindrance.

Students typically memorize that 'more substituted alkenes have shorter/stronger double bonds and lower heats of hydrogenation.' However, bulky substituents in a cis configuration create severe ground-state steric strain.

This artificially lengthens the bond, lowers the kinetic barrier for bond rotation by destabilizing the ground state relative to the transition state, and makes the heat of hydrogenation anomalously high due to the massive release of strain energy upon forming the flexible alkane.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Bond Energy vs Bond Length
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free