Question

The bromination of acetone is acid catalyzed:

H+ catalyst

CH₃COCH₃ + Br₂ = CH₃COCH₂Br + H+ + Br–

The rates of reaction were determined by measuring the rates of disappearance of bromine using different concentrations of acetone, bromine, and H⁺ ions, and the following data were obtained at a certain temperature.

Rate of disappearance

Expt# [CH₃COCH₃] [Br₂] [H⁺] of Br₂ (mol · L–1 · s–1)

(M) (M) (M) _

1 0.30 0.050 0.050 5.9 x 10–5

2 0.30 0.10 0.050 5.9 x 10–5

3 0.30 0.050 0.10 1.2 x 10–4

4 0.15 0.050 0.050 2.9 x 10–5

———————————————————————————

(a) Determine the rate law for the reaction and calculate the rate constant.

(b) The following mechanism has been proposed for the reaction:

O OH⁺

║ ║

Step-1: CH₃-C-CH₃ + H3O⁺ ⇌ CH₃-C-CH₃ + H₂O (fast equilibrium)

OH

│

Step-2: (CH₃)₂C═OH+ + H₂O = CH₃-C═CH₂ + H₃O⁺ (slow)

OH O

│ ║

Step-3: CH₃-C═CH₂ + Br₂ = CH3-C-CH₂Br + HBr (fast)

Show that the rate law deduced from the mechanism is consistent with that shown in part (a).

(Answer: (a) Rate = k[CH3COCH3][H3O⁺]; k = 3.9 x 10–3 M^–1 ∙ s^–1)

Answer 1

Steady state approximation is used to determine rate law in a multistep reaction mechanism. It assumes that the rate of change of concentration of intermediates in a multi-step reaction are constant. d[C]/dt is 0 in this case.