Acetone bromination rate

Erlenmeyer was first to consider ends as hypothetical primary intermediates in a paper published in 1880 on the dehydration of glycols.1 Ketones are inert towards electrophilic reagents, in contrast to their highly reactive end tautomers. However, the equilibrium concentrations of simple ends are generally quite low. That of 2-propenol, for example, amounts to only a few parts per billion in aqueous solutions of acetone. Nevertheless, many important reactions of ketones proceed via the more reactive ends, and enolization is then generally rate-determining. Such a mechanism was put forth in 1905 by Lapworth,2 who showed that the bromination rate of acetone in aqueous acid was independent of bromine concentration and concluded that the reaction is initiated by acid-catalyzed enolization, followed by fast trapping of the end by bromine (Scheme 1). This was the first time that a mechanistic hypothesis was put forth on the basis of an observed rate law. More recent work... 

The rate of disappearance of bromine was measured for several different concentrations of acetone, bromine, and ions at a certain temperature ... 

To a mixture of 250 ml of ether and 3 moles of freshly distilled acrolein ivere added about 3 moles of bromine at a rate such that the temperature could easily be maintained between -30 and -90°C (bath of dry-ice-acetone or liquid Nj). After persisting of the browncolour, the temperature was allowed to rise to 0°C. Freshly distilled ethyl orthoformate (3.25 moles) and 96% ethanol (30 ml) were added. 

Oxidations usually proceed in the dark at or below room temperature in a variety of solvents ranging from aqueous bicarbonate to anhydrous benzene-pyridine. Base is quite commonly used to consume the hydrogen halide produced in the reaction, as this prevents the formation of high concentrations of bromine (or chlorine) by a secondary process. The reaction time varies from a few minutes to 24 hours or more depending on the nature of the reagent and the substrate. Thus one finds that NBS or NBA when used in aqueous acetone or dioxane are very mild, selective reagents. The rate of these oxidations is noticeably enhanced when Fbutyl alcohol is used as a solvent. In general, saturated, primary alcohols are inert and methanol is often used as a solvent. 

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... 

The rate equation is first-order in acetone, first-order in hydroxide, but it is independent of (i.e., zero order in) the halogen X2. Moreover, the rate is the same whether X2 is chlorine, bromine, or iodine. These results can only mean that the transition state of the rds contains the elements of acetone and hydroxide, but not of the halogen, which must enter the product in a fast reaction following the rds. Scheme VI satisfies these kinetic requirements. 

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... 

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,... 

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). 

A. 2,4-Dibromo-3-pentanone (1). A three-necked, 250-ml. flask is fitted with a stirrer, a dropping funnel, and a condenser protected by a calcium chloride drying tube. Bromine (160 g., 1 mole) is added rapidly to a stirred solution of 45 g. (0.52 mole) of 3-pentanone (Note 1) and 1 ml. of phosphorus tribromide maintained between —10° and 0° with a dry ice-acetone bath in an efficient hood. Toward the end of the reaction very large amounts of hydrogen bromide are evolved, and the rate of addition must be controlled to allow the hood to exhaust the gas. Alternatively, a... 

The problems arising from the uncertainty in the values of Kmol can be avoided by changing to compounds for which the enol is the bulk component of the keto-enol equilibrium. This is true for the compounds [ 1 ]-[4] for which the enol content in aqueous solution and the p7sTa-value is also given (Bell and Davis, 1965). The corresponding rate coefficients for bromination by molecular bromine are given in Table 9. The values of k (eqn 36) show little variation with the reactivity of the enol and are similar (106—107 mol-1 s-1 dm3) to that reported above for acetone. These values appear curiously low for reaction on encounter. 

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). 

Sat. bromine water was added in aliquots of 15mL to a suspension of 2 -deoxyguanosine (5.00 g, 18.7 mmol) in HjO (30 mL) at a rate that permitted the yellow color of the mixture to disappear between each addition. The colorless solid was then quickly collected by filtration and successively washed with cold HjO (30 mL) and acetone (30 mL). The crude product was recrystallized (H O), filtered, and dried yield 3.70 g (57%) mp 208 °C (dec.). 

Some vanadium-containing bromoperoxidases, such as that from A. nodosum, exhibit a remarkable chemical stability (19, 29, 32). For example, the enzyme from A. nodosum remains fully active in media containing appreciable amounts of methanol, ethanol, propanol, or butanol (up to 60% v/v). In these experiments the effect of solvent on the initial rate of bromination was studied. When the enzyme was stored in acetone, methanol, and ethanol, stability was observed for more than one month. Also, when the enzyme was kept under turnover conditions... 

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