Acetone, bromination

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

Enol of acetone Bromine Conjugate acid of bromoacetone... 

Halc ens are absorbed into organic solvents, such as tetrachloromethane ( carbon tetrachloride ) or hexane. In these non-polar solvents chlorine is colourless, bromine is red and iodine is violet. In polar organic solvents such as ethanol ( alcohol ) and propanone ( acetone ), bromine and iodine give brownish solutions. 

Like bromine, iodine is soluble in organic solvents, for example chloroform, which can be used to extract it from an aqueous solution. The iodine imparts a characteristic purple colour to the organic layer this is used as a test for iodine (p. 349). NB Brown solutions are formed when iodine dissolves in ether, alcohol, and acetone. In chloroform and benzene a purple solution is formed, whilst a violet solution is produced in carbon disulphide and some hydrocarbons. These colours arise due to charge transfer (p. 60) to and from the iodine and the solvent organic molecules. 

Another teat, which indicates the reactivity of the halogen atom (chlorine and bromine), is based upon the fact that sodium chloride and sodium bromide are sparingly soluble in pure acetone ... 

It consists in treating a solution of sodium iodide in pure acetone with the organic compound. The reaction is probably of the S 2 type involving a bimolecular attack of the iodide ion upon the carbon atom carrying the chlorine or bromine the order of reactivities of halides is primary > secondary > tertiary and Br > Cl. 

Apply the test to compounds which contain chlorine or bromine. If the compound is a solid, dissolve 0 1 g. in the minimum volume of pure, dry acetone. To 1 ml. of the sodium iodide acetone reagent add 2 drops of the compound (if a hquid) or the acetone solution (if a sohd). Shake and allow to stand at room temperature for 3 minutes. Note whether a precipitate is formed and also whether the solution acquires a reddish-brown colour (liberation of iodine). If no change takes place at rocrm temperature, place the test-tube in a beaker of water at 50°. After 5 minutes, cool to room temperature, and observe whether a reaction has occurred. 

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. 

From acetone in water and concentrated hydrochloric acid by addition of bromine. Hughes, Watson, and Yates, J. Chem. Soc. 1931, 3322. 

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. 

A solution of 16jS-methyl-l la,17a,21-trihydroxy-5j5-pregnane-3,20-dione 21-acetate (52), 45 g, in dioxane (297 ml) is cooled to 15° and treated over a 5 min period with a solution of bromine (34.2 g) in dioxane (594 ml) precooled to 18°. After 2 min a solution of sodium acetate (60 g) in water (600 ml) is added and the mixture poured into ice water (8 liters). The precipitate is filtered off, washed to neutrality with water, and dried to give the crude dibromide (53), 55.7 g mp 125-126° (dec.) [aJu 58°. A mixture of dibromide (53), 55.5 g, lithium bromide (27.8 g), lithium carbonate (27.8 g) and DMF (1.11 liters) is refluxed under rapid stirring for 6 hr. The mixture is concentrated under vacuum to about 250 ml, poured into ice water (8 liters) containing hydrochloric acid (250 ml), and extracted with methylene dichloride. The extracts are washed to neutrality with water and evaporated to dryness. The residue is dissolved in acetone, evaporated to dryness under reduced pressure, redissolved in acetone and crystallized by the additon of hexane. This gives the dienone (54) 24.4 g, mp 236-239°. 

Acetoxyandrost-5-en-17-one (59) is converted into the ethylene ketal (60) by treatment with ethylene glycol, triethylorthoformate and p-toluenesulfonic acid. The ketal is brominated with pyridinium bromide perbromide in THF and then treated with sodium iodide to remove bromine from the 5 and 6 positions. This gives the 16a-bromo compound (61) which is hydrolyzed in methanol to the free alcohol (62). Dehydrobromination is effected with potassium Fbutoxide in DMSO to give the -compound (63). Acid catalyzed hydrolysis of the ketal in aqueous acetone gives the title compound (64). ... 

Preparation of 3a-Hydroxy-5) -pregn-17(20)-en-21-oic Acid . A solution of 15 g of 3a-acetoxy-5jS-pregnan-20-one in 290 ml of glacial acetic acid is treated with 13 g of bromine at room temperature. After complete addition of bromine the reaction mixture is heated at 40-50° for 30 min, and the product precipitated with water and filtered. The product is taken up in ethyl acetate (500-600 ml) and the resulting solution washed with dilute aqueous potassium bicarbonate. The solvent is concentrated in vacuo and the product crystallized from acetone to give 16g of dibromide mp, 173-175°. 

A 3-ketone in a 17a-hydroxypregnane-3,20-dione reacts selectively with acetone cyanohydrin to give the 3-cyanohydrin. This can be brominated at C-21 without affecting the A-ring, followed by removal of the protecting... 

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. 

The immediate outcome of the Hantzsch synthesis is the dihydropyridine which requires a subsequent oxidation step to generate the pyridine core. Classically, this has been accomplished with nitric acid. Alternative reagents include oxygen, sodium nitrite, ferric nitrate/cupric nitrate, bromine/sodium acetate, chromium trioxide, sulfur, potassium permanganate, chloranil, DDQ, Pd/C and DBU. More recently, ceric ammonium nitrate (CAN) has been found to be an efficient reagent to carry out this transformation. When 100 was treated with 2 equivalents of CAN in aqueous acetone, the reaction to 101 was complete in 10 minutes at room temperature and in excellent yield. 

Citraptene, CjiHjoO, is the odourless solid constituent of the expressed oil of lemon. After repeated recrystallisations from acetone and methyl alcohol, it forms needles melting at 146° to 147°. By treatment with bromine in chloroform solution it yields a dibromide, CjoHjdBr O, melting at 250° to 260°. It appears to be a dimethyl-oxycoumarin of the constitution—... 

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