A synthetic chemist enthusiastically heats a pure sample of 3-bromo-2,2-dimethylbutane in a strongly — Haloalkanes and Haloarenes Chemistry Question
Question
A synthetic chemist enthusiastically heats a pure sample of 3-bromo-2,2-dimethylbutane in a strongly aqueous ethanol solution, initiating a complex nucleophilic substitution. In the intricate mechanism successfully leading to the dominant major substitution product, exactly how many distinct carbon atoms are physically and directly bonded to the positively charged central carbon atom in the final, most thermodynamically stable rearranged carbocation intermediate?
💡 Solution & Explanation
The substrate is $CH_3-C(CH_3)_2-CH(Br)-CH_3$. Heterolytic cleavage yields the initial $2^\circ$ carbocation: $CH_3-C(CH_3)_2-C^+H-CH_3$. The adjacent quaternary carbon is highly sterically congested but possesses mobile methyl groups. A rapid 1,2-methyl shift transpires to relieve tension and generate a vastly more stable $3^\circ$ carbocation: $CH_3-C^+(CH_3)-CH(CH_3)_2$. Let's precisely analyze this final carbocation structure: the $C^+$ center is directly attached to 1) the left-side methyl group ($-CH_3$), 2) the top-side methyl group ($-CH_3$), and 3) the right-side isopropyl group ($-CH(CH_3)_2$). Thus, the positive carbon is directly bonded to exactly 3 adjacent carbon atoms.