The bromination of acetone is acid-catalyzed: catalyst CH,COCH, + B, CH, COCH,Br

Question

The bromination of acetone is acid-catalyzed: catalyst CH,COCH, + B, CH, COCH,Br + H+ + Br The rate of disappearance of bromine was measured for several different concentrations of acetone, bromine, and H* ions at a certain temperature: Rate of (CH3COCH3] [Br2] [H+] disappearance (M) (M) (M of Br2 (MWs) 0.30 0.0500.050 5.7 x 10-5 0.30 0.10 0.050 5.7 x 10-5 0.30 0.0500.10 1.2 x 10-4 0.40 0.050 0.20 3.1x 10-4 0.40 0.050 O.050 7.6 x 10-5 (1) (2) (4) (5) (a) What is the rate law for the reaction? (b) Determine the rate constant. (c) The following mechanism has been proposed for the reaction: *OH CH3–-CH3 + H3O+ = CH3–C–CH3 +H,0 (fast equilibrium) CH-C-CH3+ H2O CH3–C=CH2+H;0* (slow) CH-SCH,+Br2 CH3–-CHBr + HBr (fast) Show that the rate law deduced from the mechanism is consistent withthat shown in part (a).

Explanation / Answer

Ans.

a) The rate law of this reaction iS

rate = K [3HCCOCH3][H+]

b) rate constant is

k = A exp(–Ea/RT)

c) According to the above mechansim rate shoud be

rate = K [3HCCOCH3][Br2][H+]

However order with respect to Br2 is Zero order becuase of acetone and acid are present in large excess compared to bromine

Therefore

rate = K [3HCCOCH3][H+]

Hence a solution is good agrrement with c solution

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