Mark D. Waddington a n d J. E. Meany Central Washington university Ellensburg. Washington 98926
1
The General Base Catalyzed Enolization of Acetone An undergraduate kinetics experiment
Althourh kinetic e x ~ e r i m e n tas.m.e a r in lahoratorv manuals at varinus levels of sophistication, few exist which demonstrate the i m ~ o r t a n t heno omen on of general catalvsis bv bases and/or.acids. he present experimental mod& "is.-the iodination of acetone employs a simple spectrophotometric technique and allows the instructor t o familiarize students with the determination of reaction mechanism hy kinetic methods. The system can further he used todemonstrate more advanced theories associated with homogeneous acid-base catalysis. T h e experiment descrihed helow is suitahle for students of organic o r physical chemistry and requires a minimum of lal~oratoryequipment. Used for student pairs a t our institution, a single :l-hr lahoratory period was f w n d sufficient for the completion of the measurements. T h e position of equilibrium for the enolization of acetone is too far (WICOCH,,--CHI=C(OH)CH:I
(1)
t o the left to allow direct measurement of the reaction progress ( 1 1. It is known, however, that the halogenation of ketones in aqueous solution is first-order in ketone and zero-order in halogen ( 2 ) .Moreover, the rates of chlorination, hromination, and iodination are all identical ( 2 ) .Such evidence indicates t h a t t h e rate contrnllingstep for the halogenation of acetone is the transfnrmatinn of the ketone into its e n d form, which subsequently undergoes rapid reactinn with halogen ( 3 )
Thus, 1.0 measure the rate of halngenation of acetone is t o measure the rate ~rf'enolieation I -1 - -~[CH:+COCH:II L, = -2- k,,h,dlCH:,COCH:rl (41 dl dl T h e haloyenations nf enolizahle ketones are suhject t o catalysis (41 hoth by general acids
+ howIOH-1 h,,~,,,!= ho+ k~,o+lH:lO+l
+ ~ H A I H A+]k~-[A-l ( 7 )
where ko, t h e "spontaneous" rate constant, refers t o t h e catalytic contribution hy the solvent, water. For the enolization of acetone in phnsphate huffers around neutral pH, it can he shown t h a t t h e catalytic contrihutions hy H:
