Before You Begin
Question #43987
The Question
Consider the acid dissociation of the glycine cation (, Acid I) and cyanoacetic acid (, Acid II) at . Let and be their respective acid dissociation constants for the proton. Define the ratio of their dissociation constants as . Let be the ratio in pure water (dielectric constant ), where the values are and , respectively. Let be the ratio in an aqueous ethanol mixture (). Assuming no change in the primary structural conformations, which of the following mathematically represents the correct relationship between and ?
Options
- A.
- B.
- C.
- D.
Check Your Answer
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
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 ).
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 and groups actually increases as the dielectric constant of the solvent decreases, further mitigating the shift for the amino acid.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Aldehydes, Ketones, and Carboxylic Acids - Class 12 |
| Concept | Carboxylic Acid Strength |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |