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Question #43982
The Question
Consider the reaction of 2-(trifluoromethyl)pentanedioic acid with one equivalent of \ce{Br2} in the presence of catalytic red phosphorus at 353 K, followed by aqueous workup. Which of the following represents the major organic product formed and the correct kinetic rationale for its regioselectivity?
Options
- A.
2-bromo-4-(trifluoromethyl)pentanedioic acid; enolization at the C4 position is kinetically favored because the resulting enol intermediate avoids the severe inductive destabilization caused by the \ce{-CF3} group.
- B.
2-bromo-2-(trifluoromethyl)pentanedioic acid; enolization at the C2 position is kinetically favored because the -hydrogen at C2 is significantly more acidic due to the strong effect of the \ce{-CF3} group.
- C.
2-bromo-4-(trifluoromethyl)pentanedioic acid; the \ce{-CF3} group sterically hinders the formation of the acyl bromide at C1, directing the reaction exclusively to the C5 carboxylic acid group.
- D.
2-bromo-2-(trifluoromethyl)pentanedioic acid; the intermediate enol at C2 is thermodynamically stabilized by intramolecular hydrogen bonding between the enol hydroxyl and the fluorine atoms.
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
Alpha-Hydrogen Acidity|Hunsdiecker/Hell-Volhard-Zelinsky|Inductive Effect Trends
Why This Question Is Tricky
Common Mistakes
This problem tests the 'Reactivity vs.
Stability' contradiction in the HVZ mechanism.
Students often memorize that 'HVZ relies on alpha-hydrogen acidity' and fall into the trap of selecting the most acidic proton (Option B).
The hidden variable is the mechanism's rate-determining step: the formation of the enol intermediate.
A strong electron-withdrawing group (EWG) like -CF3 increases ground-state acidity but severely destabilizes the sp2-hybridized enol transition state via inductive pull, making the more acidic proton kinetically inert towards halogenation.
Quick Facts About This Question
Question Information
| Subject | Chemistry |
| Chapter | Aldehydes, Ketones, and Carboxylic Acids - Class 12 |
| Concept | Alpha-Hydrogen Acidity |
| Difficulty | 8 |
| Cognitive Load | 7 |
| Target Solve Time | 160 sec |
| Access | Free |