The bromination of acetone in dilute aqueous solution proceeds according to the…
CHEMISTRY · Multi-part · A Level · Chemistry · Introduction to organic chemistry · Introduction to organic chemistry
Problem
The bromination of acetone in dilute aqueous solution proceeds according to the overall reaction
CH₃COCH₃ + Br₂ → CH₃COCH₂Br + HBr.
At pH 3.0 and 25 °C, the empirical rate law is
rate = k[acetone][Br₂][H⁺].
Low-temperature NMR spectroscopy of a reaction mixture rapidly quenched after 10 % conversion shows a resonance attributable to the enol form of acetone, CH₂=C(OH)CH₃. No peaks associated with a free bromonium ion are detected.
Two stepwise mechanisms are under consideration.
Mechanism X
1. CH₃COCH₃ + H⁺ ⇌ CH₂=C(OH)CH₃ + H⁺ (fast equilibrium)
2. CH₂=C(OH)CH₃ + Br₂ → CH₃COCH₂Br + HBr (slow)
Mechanism Y
1. Br₂ + H⁺ → Br⁺ + HBr (fast)
2. CH₃COCH₃ + Br⁺ → CH₃COCH₂Br + H⁺ (slow)
Assume activities equal concentrations and that water is present in large excess.
(a) Derive the rate law predicted by Mechanism X, eliminating the concentration of any reaction intermediates.
(b) Derive the rate law predicted by Mechanism Y.
(c) Compare your derived rate laws with the experimental rate law. Indicate which mechanism is kinetically consistent.
(d) Using the NMR evidence, evaluate which mechanism is more plausible. Provide a consolidated argument that integrates both kinetic and spectroscopic data.
(e) Propose a pH-dependent experiment that could further test the preferred mechanism. Specify the observation that would confirm your mechanistic assignment.