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Question #43987

Difficulty: 8/10 | 🟢 Free

The Question

Consider the acid dissociation of the glycine cation (\ce+NH3−CH2−COOH\ce{^+NH3-CH2-COOH}, Acid I) and cyanoacetic acid (\ceNC−CH2−COOH\ce{NC-CH2-COOH}, Acid II) at 298 K298\ K. Let Ka(I)K_{a}(\text{I}) and Ka(II)K_{a}(\text{II}) be their respective acid dissociation constants for the \ce−COOH\ce{-COOH} proton. Define the ratio of their dissociation constants as R=Ka(I)Ka(II)R = \frac{K_{a}(\text{I})}{K_{a}(\text{II})}. Let RWR_W be the ratio in pure water (dielectric constant ϵ≈78\epsilon \approx 78), where the pKapK_a values are 2.342.34 and 2.452.45, respectively. Let RER_E be the ratio in an 80%80\% aqueous ethanol mixture (ϵ≈30\epsilon \approx 30). Assuming no change in the primary structural conformations, which of the following mathematically represents the correct relationship between RWR_W and RER_E?

Options

  • A.

    RE>RW>1R_E > R_W > 1

  • B.

    RW>RE>1R_W > R_E > 1

  • C.

    1>RE>RW1 > R_E > R_W

  • D.

    RE≈RW>1R_E \approx R_W > 1

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Carboxylic Acid Strength|Amino Acid Zwitterion pH Sensitivity|Inductive Effect Trends

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' of the electrostatic field effect versus pure inductive effects under varying solvent polarities.

Students typically assume that zwitterions are highly unstable in non-polar solvents, leading them to erroneously conclude that the acidity of the glycine cation will plummet (predicting RW>RE>1R_W > R_E > 1).

They fail to recognize two hidden variables: (1) The dissociation of a cationic acid is isocoulombic, suffering a much smaller desolvation penalty than the charge-separating dissociation of a neutral acid. (2) The internal electrostatic stabilization between the \ce+NH3\ce{^+NH3} and \ce−COO−\ce{-COO^-} groups actually increases as the dielectric constant of the solvent decreases, further mitigating the pKapK_a shift for the amino acid.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterAldehydes, Ketones, and Carboxylic Acids - Class 12
ConceptCarboxylic Acid Strength
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree