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Question #44167

Difficulty: 8/10 | 🟢 Free

The Question

Consider the hydration equilibria of propan-2-one (Acetone) and 1,1,1,3,3,3-hexafluoropropan-2-one (HFA) in aqueous solution at 298 K. Generally, ketone hydration is disfavored due to steric hindrance (B-strain) in the sp3sp^3 hybridized gem-diol. Consequently, the standard Gibbs free energy of hydration for acetone is +17.1 kJ mol−1+17.1 \text{ kJ mol}^{-1}. However, for HFA, the massive inductive withdrawal (-I) of the fluorine atoms and the relief of sp2sp^2 dipole repulsion completely overcome the steric B-strain, yielding a standard Gibbs free energy of hydration of −34.2 kJ mol−1-34.2 \text{ kJ mol}^{-1}. What is the ratio of their hydration equilibrium constants, KHFAKAcetone\frac{K_{HFA}}{K_{Acetone}}, at 298 K? (Given: 2.303RT=5.7 kJ mol−12.303 RT = 5.7 \text{ kJ mol}^{-1} at 298 K)

Options

  • A.

    10910^9

  • B.

    10610^6

  • C.

    10−910^{-9}

  • D.

    10310^3

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Hydrogen Bonding Competition|Hydrate Stability vs. Carbonyl|Steric Hindrance (B-Strain)

Why This Question Is Tricky

Common Mistakes

Students often apply the 'steric hindrance' (B-strain) heuristic universally, assuming bulky groups (like −CF3-CF_3) will always destabilize the sp3sp^3 hybridized gem-diol relative to the sp2sp^2 carbonyl.

However, in poly-fluorinated ketones like HFA, the intense inductive withdrawal (-I) drastically increases the electrophilicity of the carbonyl carbon.

Furthermore, hydration relieves the severe electrostatic repulsion between the highly polarized C=OC=O and C−FC-F bonds present in the planar sp2sp^2 state.

This electronic stabilization completely eclipses the steric B-strain, inverting the expected thermodynamic stability and serving as a classic exception to the rule.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterChemical Bonding and Molecular Structure - Class 11 Part 1
ConceptHydrogen Bonding Competition
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree