Question

Temperature, Question 7 (13 marks) Acetone is brominated in acidic solution according to the following reaction: CH3COCH3(aq) + Br2(aq) - CH3COCH_Br(aq) + H" (aq) + Br" (aq) Student A proposed the following four-step mechanism for the decomposition of Bra: CH3COCH3 + H + CH3C(OH)CH3 ki, ki CH,C(OH)CH3 -CH:C(OH)-CH2 + H+ ka, slow CH;C(OH)=CH2 + Br2 - CH3C(OH)CH2Br* + Br k3 CH:C(OH)CH2Br* - CH3COCH2Br + H+ k4 Deduce the rate law for the above reaction by assuming a pre-equilibrium (7 marks) (b) Which of the above species from the mechanism is/are catalyst(s)? Explain your choice. (2 marks) (c) Which of the above species from the mechanism is/are (an) intermediate(s)? Define the term intermediate using the steady-state approximation to elaborate. (4 marks) Page 3 of 6

Answer 1

(b) CH3COCH3 + Br2+ H+ → CH3COCH2Br + Br–+ H+

Reaction is catalysed by H+

(c) only the protonated acetone ion CH3COCH4+ is

the intermediate

(a)

K2 is the re action constant of the slowest rate determining step Rate K₂ [CH₃ CO CH₂t] o equilibrium reaction From the CH3CO CH3 + H+ ki CH3Ć (OH) CH3 K, TCH ₂ CO CH₂] [r]=K, ICH₃C(OH] CH- [CH₃C (OH) CH₂] = k1 [CH₃COCH 3 ] [et] ky substituting (1) in Rate = Kik. [CH₃COCH ₃] [HT] K-1