Iodination of acetone

The following data were collected during a kinetic study of the iodination of acetone by measuring the concentration of unreacted I2 in solution. ... 

Differential temperature method. A differential method has been applied to a study of the iodination of acetone, a pseudo-zeroth-order reaction when [(CHj)2CO] [I2].26 It allows the determination of AW to much higher accuracy than otherwise. The reaction rate is expressed mathematically as... 

The iodine solution is added dropwise at such a rate that the entire quantity of iodine is added over a period of 45-74 minutes. If the iodine is added too rapidly, iodination of acetone will take place. Caution. Iodoacetone species are extremely powerful lacrimators. During the addition of the iodine solution, a very slow flow of nitrogen should be maintained (1-5 bubbles per minute). The resultant product should consist of a dark brown solution and unreacted iron (150-mole % excess). This excess iron powder is necessary for the over-all reaction, for reducing the iodination of acetone, and for improving the yield of product. 

Iodination of acetone is slowed by increasing magnetic field strength.149... 

Recently, the present author (Toullec and Verny-Doussin, 1980) obtained a concordant value for the equilibrium constant of (65 L = H) for [83, n — 3] from kinetic data on amine-catalysed iodination of acetone at low iodine concentrations. The principle of this determination is similar to that described for estimation of the keto-enol equilibrium constants by the kinetic halogenation method (see p. 48). At low iodine concentrations ( 10-6 mol dm-3), the enamine pathway is preferred [see (61)] and iodination of the enamine is rate-limiting. Rate measurements, in a pH range in which only the monoprotonated and diprotonated forms of the diamine exist, made it possible to determine the second-order rate constants k u which include the equilibrium constants, for interconversion of the ketone and protonated enamine... 

Leeds most famous chemical alumna was May Sybil Leslie.60 Leslie was born on 14 August 1887 in Yorkshire and studied chemistry at the University of Leeds. She graduated with first class honours in 1908, and was awarded an M.Sc. for research with Harry M. Dawson01 the following year on the kinetics of the iodination of acetone, work that has since become a classic in its field.62 In that same year, 1909, Leslie was awarded a scholarship, which she decided to use to work with Marie Curie63 in Paris. Her letters from Paris to Smithells are among the few accounts of life in the early Curie laboratory.04... 

Many similar examples of the application of the Bronsted relation may be cited, including the mutarotation of glucose, the iodination of acetone, the bromination of aceto-acetic ester, and the dehydration of 1,1-dihy-droxyethane to acetaldehyde- and many other closely related compounds (see R. P. Bell, loc, cit). It is usually found that on going from one acid to another structurally very different, the two acids will not fit the same... 

The catalysis has been the subject of careful study by a number of different laboratories, and it is found that the rate of iodination of acetone in aqueous solution of acetate buffer is given by (concentrations in moles/liter)... 

Here, too, the kinetics show the rate of halogenation to be independent of halogen concentration, but dependent upon ketone concentration and, this time, acid concentration. Here, too, we find the remarkable identity of rate constants for apparently different reactions fpr bromination and iodination of acetone, and exchange of its hydrogens for deuterium for iodination and racemization of phenyl sec-butyl ketone. 

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 same. Later he found that iodination of acetone proceeded at the same rate as chlorination... 

A prerequisite that the mechanism in a series should be the same is a linear free-energy correlation if a member of the series takes a different pathway then the alternative mechanism will be more favourable, will predominate over the normal and will be diagnosed as a positive deviant point. An example of this behaviour is the demonstration that general acid catalysis by primary amines of the iodination of acetone differs from that by carboxylic acids (Fig. 24) [3a] simple proton transfer is identified with the carboxylic acid catalysis and it is thought that amines utihse Schiff s base formation (Eqn. 116) leading to a more favourable path. 

This is a very common pattern in acid catalysis. In the first step the acid transfers a proton to the substrate, which in the second step transfers another proton, the product being formed either simultaneously or in a subsequent step. In basic catalysis (which also occurs with the iodination of acetone), the substrate molecule first transfers a proton to the basic catalyst, and in a second step accepts a proton at another position. 

Conclusive evidence for catalysis by species other than hydrogen or hydroxide ions was first obtained by J.W. Dawson (51-52) of the University of Leeds. His most important work was on the conversion of the keto form of acetone into its enol form, a process that can be studied by measuring the rate of iodination of acetone, the iodine adding rapidly to the enol form. Dawson first showed that undissociated acids bring about acid catalysis and spoke of the dual... 

Experiment 3.3. Determination of the rate constant and kinetic order of iodination of acetone in water... 

Some reactions may be catalyzed by any acid or any base present, and the kinetics may then be extremely complex. For example, if the rate of iodination of acetone (Figure 23.5) is measured in... 

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