Bromination, of acetone

In one of the earliest mechanistic investigations in organic chemistry, Arthur Lapworth discovered in 1904 that the rates of chlorination and bromination of acetone were the... 

Zeroth-order kinetics. The rate of bromination of acetone in acidic aqueous solution is governed by the enolization step. With [(CH3>2CO]o s> [B lo, the reaction rate is... 

This provides a valuable diagnostic tool for determination of mechanism. For example, in the bromination of acetone (12-4)... 

In general base catalysis, bases other than eOH are involved. Thus in the base catalysed bromination of acetone (c/. p. 295) in an acetate buffer it is found that,... 

One of the earliest observations relating to the possible occurrence of carbanions as reaction intermediates was that the bromination of acetone, in the presence of aqueous base, followed the rate law,... 

Dining bromination of acetone to bromoacetone, presence of a large excess of bromine must be avoided to prevent sudden and violent reaction. 

Thus it has been possible to show that in the bromination of acetone, a process which has been found to be unimolecular, not the normal keto-form, but the tautomeric enol-form reacts. The enol-form is present, in equilibrium with the keto-form, in amount too small to be measured. As soon as this amount has reacted a further quantity is formed and the process is repeated. That the reaction is unimolecular follows from the fact that it is the rate of rearrangement (I) which is measured, whilst the reaction of the enol with bromine (II) takes place with immeasurable rapidity (Lapworth). 

Thus, bromination of acetone may be achieved by using bromine in sodinm hydroxide solution, and this... 

For the acid-catalyzed ketone -> enol reaction of typical unhydrated ketones, AS appears to be close to the normal or collision theory value for a second-order reaction. For example, AS for the acid-catalyzed bromination of acetone is —12 e.u. (Rice and Kilpatrick, 1923). Since for normal keto-enol equilibria, AS0 is close to zero, the conclusion is that AS for the ketonization reaction would also be close to —12 e.u. 

A great number of data on inductive effects in the aliphatic series have been reported. Broadly speaking, results are in agreement with what is expected from the anionic and cationic characters of the transition states an electron-withdrawing substituent increases the rate of the base-promoted ionisation, whereas it retards that of acid-catalysed enolisation. For instance, a bromine atom at the exposition modifies the rate constants of acetate and HCl-catalysed bromination of acetone in water by factors of 4400 and 1/6.5, respectively (R. A. Cox and Warkentin, 1972 Watson and Yates, 1932). 

More recently, the present author examined the mechanism of bromination of acetone-acetone dimethyl acetal mixtures in methanol containing small... 

This is different and more complicated because it usually won t stop at the introduction of one halogen atom. If we go back to the bromination of acetone, the first step will now be a base-catalysed eno-lization to give the enolate ion instead of the enol. The enolate ion can attack a bromine molecule in a very similar way to the attack of the enol on bromine. The enolate will, of course, be even more reactive than the enol was (the enolate carries a negative charge). 

The direct bromination of aliphatic ketones occurs readily, often giving isomeric mixtures. Thus, methyl ethyl ketone and an equimolar quantity of bromine yield the a-btomomethyl (17%) and the a-bromoethyl (50%) isomers polybrominated products are also formed. On the other hand, only the tertiary hydrogen in methyl isopropyl ketone is substituted upon monobromination. By varying the conditions for the bromination of acetone, mono- or poly-substitution products may be formed e.g., bromo-acetone (44%), a,a -dibromoacetone (60%), and a,a,a -tribromoacetone (60%). ... 

Study of the bromination of acetone, done by A. Lapworth (of the University of... 

P cm 21.8 Show in detail how the enolization mechanism accounts for the following facts (a) the rate constants for acid-catalyzed hydrogen-deuterium exchange and bromination of acetone are identical (b) the rate constants for acid-catalyzed racemization and iodination of phenyl rec- tyl ketone are identical. 

Propose a mechanism that is consistent with the fact that the rate of the acidic bromination of acetone is independent of the concentration of bromine. 

---