Reduction of bromine

The hydrobromic acid sulphuric acid solution may be prepared by the reduction of bromine with sulphurous acid (Section 11,49, 1) distillation of the reaction product is unnecessary ... 

Reduction of bromine by TI(I) is important in the bromide-catalysed isotopic exchange between TI(I) and Tl(III). A potentiometric examination revealed that the reaction was first-order both in T1(I) and in Bt2 with k2 = 7.2 x 10 1. mole . sec (25 °C) and acid-independent. This is in passable agreement with... 

Hydrobromic Acid. In the sulfur-dioxide reduction of bromine, it should be noted that the proportion of water used depends upon whether the reduction mixture is to be distilled for the preparation of 48 per cent hydrobromic acid, or whether it is to be used directly for the manufacture of alkyl bromides. 

Alkyl Bromides General Discussion. When an alcohol is heated with aqueous 48 per cent hydrobromic acid, a partial conversion takes place into the corresponding bromide. The reaction is, however, more rapid and more complete in the presence of sulfuric acid. Although the constant boiling hydrobromic acid obtainable on the market may be used in all the above experiments, its preparation by the sulfur-dioxide reduction of bromine will be considerably cheaper and equally convenient, provided a cylinder of sulfur dioxide is available. For use in the preparation of alkyl bromides, distillation of the bromine-sulfur dioxide reduction mixture is superfluous. 

Watanabe I, Kawano M, Tatsukawa R (1993) Consumption trend and environmental research on brominated flame retardants in Japan and the formation of polyhalogenated dibenzofurans at the metal reclamation factory. Document distributed at the OECD Workshop on the Risk Reduction of Brominated Flame Retardants, Neuchatel, Switzerland... 

Hydrogen bromide can also be made by the reduction of bromine with hydrogen sulfide ... 

Attempts by Smith and ToUens " to oxidize n-fructose with bromine by the method of Clowes and Tollens were unsuccessful a double compound of D-fructose and calcium bromide was obtained. No yield of this product was given. The reaction mixture of D-fructose, calcium carbonate, water and bromine was shaken until all the bromine had dissolved, after which the double compound was separated from the concentrated solution. The calcium bromide was formed from the interaction of calcium carbonate with hydrogen bromide in the reaction solution. The hydrogen bromide could be formed in two ways by the hydrolysis of bromine in water (reaction 1, page 134) or by the reduction of bromine during the oxidation of D-fructose. The former is the more probable explanation, but a blank run without the ketose was not attempted. 

A convenient laboratory preparation involves the reduction of bromine or iodine with red phosphorus and water ... 

Hydrobromic acid. Method 1 (from bromine and sulphur dioxide). A mixture of 600 g. (or 188-6 ml.) of bromine, 250 ml. of water and 760 g. of crushed ice is placed in a 1 6 litre round-bottomed flask and a rapid stream of sulphur dioxide (from a siphon of the liquefied gas) is passed into the flask, care being taken that the outlet of the gas-delivery tube is below the surface of the bromine layer. The rate of flow of the gas is adjusted so that it is completely absorbed. It is advisable to cool the flask in ice and also to shake the contents from time to time. The reduction is complete when the mixture assumes a uniform yellowish-brown or yellow colour, which is unaffected by further introduction of sulphur dioxide excess of the latter gas should be avoided as it will be... 

Introduction of a 3-bromosubstituent onto thiophene is accompHshed by initial tribromination, followed by reduction of the a-bromines by treatment with zinc/acetic acid, thereby utilizing only one of three bromines introduced. The so-called halogen dance sequence of reactions, whereby bromothiophenes are treated with base, causing proton abstraction and rearrangement of bromine to the produce the most-stable anion, has also been used to introduce a bromine atom at position 3. The formation of 3-bromotbiopbene [872-31-1] from this sequence of reactions (17) is an efficient use of bromine. Vapor-phase techniques have also been proposed to achieve this halogen migration (18), but with less specificity. Table 3 summarizes properties of some brominated thiophenes. 

Bromine occurs ia the form of bromide ia seawater and ia natural brine deposits (see Chemicals frombrine). Chloride is also present. In all current methods of bromine production, chlorine, which has a higher reduction potential than bromine, is used to oxidize bromide to bromine. 

Another method for preparing alkah bromates is by electrolysis of bromine in alkah solutions. Anodes coated with Pb02 are used and a small amount of dichromate is added to prevent reduction of BrO at the cathode (76). 

Apart from the nuclear bromination observed (Section 2.15.13.1) in the attempted radical bromination of a side-chain methyl group leading to (396), which may or may not have involved radical intermediates, the only other reaction of interest in this section is a light-induced reduction of certain hydroxypyrido[3,4-f)]pyrazines or their 0x0 tautomers analogous to that well-known in the pteridine field (63JCS5156). Related one-electron reduction products of laser photolysis experiments with 1 -deazaflavins have been described (79MI21502). 

Use of the trapping agent is recommended as the most efficient method for running acyloin condensations for many reasons. Among them are (a) the work-up is very simple filter and distil (b) the bis-(silyloxy)olefin is usually easier to store than the free acyloin and is readily purified by redistillation (c) unwanted base-catalyzed side reactions during reduction are completely avoided and (d) the bis-(silyloxy)olefin can be easily converted directly into the diketone by treatment with 1 mole of bromine in carbon tetrachloride.Other reactions are described in Riihlmann s review and in Organic Reactions ... 

The reduction of 1 l-keto-9 -H steroids gives an lla-ol, due to the fact that ring B or C must be in the boat form. The presence of bromine at the 2a-, 5a- and 12a-positions does not afifect greatly the isomer composition resulting from reduction of 3-, 6- and 11-ketones respectively. When in a 7-keto steroid the bromine is in the 6a-position a mixture of la- and 7 -ols results when the bromine is in the 6 -position only the 7jS-hydroxy compound is obtained. " The presence of bromine in the 16a- or 16 -positions seems to reduce the amount of 17 -alcohol formed. ... 

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