Hydrocarbons reaction with bromine

Reaction with bromine. Unsaturated hydrocarbons react rapidly with bromine in a solution of carbon tetrachloride or cyclohexane. The reaction is the addition of the elements of bromine to the carbons of the multiple bonds. 

Although aromatic hydrocarbons are unsaturated, they have very different chemical properties to alkenes and allies. For example, benzene doesn t undergo an addition reaction with bromine despite having a double bond. 

Bromine ticlditioii thus serves to differentiate between two main groups of hydrocarbons the reaction is adaptable also to quantitative determinations (page 170) and as such is used extensively in quantitative analysis of certain classes of organic compounds. Only a few relatively unimportant hydrocarbons fail to respond to this test. On the other hand, among the unsaturated derivatives of hydrocarbons, there is considerable variation in the ease of reaction with bromine. 

J. F. Durana and J. D. McDonald, Infrared chemiluminescence studies of chlorine substitution reactions with brominated unsaturated hydrocarbons, J. Chem. Phys. 64 2518 (1976). 

Chakactkrisation of Unsaturatkd Aliphatic Hydrocarbons Unlike the saturated hydrocarbons, unsaturated aliphatic hydrocarbons are soluble in concentrated sulphuric acid and exhibit characteristic reactions with dUute potassium permanganate solution and with bromine. Nevertheless, no satisfactory derivatives have yet been developed for these hydrocarbons, and their characterisation must therefore be based upon a determination of their physical properties (boiling point, density and refractive index). The physical properties of a number of selected unsaturated hydrocarbons are collected in Table 111,11. 

The relative rates of reaction of ethane toluene and ethylbenzene with bromine atoms have been measured The most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one Arrange these hydrocarbons in order of decreasing reactivity... 

Additions of halogen fluorides to the more electrophilic perfluonnated olefins generally require different conditions Reactions of iodine fluoride, generated in situ from iodine and iodine pentafluoride [62 102 103, /05] or iodine, hydrogen fluoride, and parapeiiodic aud [104], with fluormated olefins (equations 8-10) are especially well studied because the perfluoroalkyl iodide products are useful precursors of surfactants and other fluorochemicals Somewhat higher temperatures are required compared with reactions with hydrocarbon olefins Additions of bromine fluoride, from bromine and bromine trifluonde, to perfluonnated olefins are also known [lOti]... 

Dibromoethane is a halogenated aliphatic hydrocarbon produced when gaseous ethylene comes in contact with bromine. The mixing of ethylene and bromine is accomplished in a variety of ways. One of the more common manufacturing processes involves a liquid-phase bromination of ethylene at 35°-85°C. After the bromination of ethylene, the mixture is neutralized to free acid and then purified by distillation. Other methods of 1,2-dibromoethane formation include the hydrobromination of acetylene and a reaction of 1,2-dibromoethane with water (Fishbein 1980 HSDB 1989). 

The reaction of unsaturated hydrocarbons with bromine is synthetically important. Unsaturated fatty acids have been electrobrominated in an acetic acid/acetonitrile medium and at platinum electrodes [128]. The mechanism and rate constant for the process with oleic, erucic, and linoleic acid were elucidated. The a-bromination of a,f-unsaturated ketones has been reported [129] on the basis of the electrolysis of a substrate/CF3COOH/CuBr/Et4NOTs/ MeCN reaction system. CuBr is present as a catalyst. 

Bierbach, A., I. Barnes, and K. H. Becker, Rate Coefficients for the Gas-Phase Reactions of Bromine Radicals with a Series of Alkenes, Dienes, and Aromatic Hydrocarbons at 298 + 2 K, lnt.. J. Chem. Kinet., 28, 565-577 (1996). 

Fullerenes can be derivatized by various means. For example, reaction with fluorine gas proceeds stepwise to the formation of colorless CeoFeo, which, according to the 19F nuclear magnetic resonance (NMR) spectrum, contains just one type of F site and so evidently retains a high degree of symmetry.9 In view of the low adhesion typical of fluorocarbons, this spherical molecule is expected to have extraordinary lubricant properties. Curiously, bromination of Ceo is reversible on heating otherwise, the reactions of fullerenes resemble those of alkenes or arenes (aromatic hydrocarbons). 

The addition reaction between bromine dissolved in an organic solvent, or water, and alkenes is used as a chemical test for the presence of a double bond between two carbon atoms. When a few drops of this bromine solution are shaken with the hydrocarbon, if it is an alkene, such as ethene, a reaction takes place in which bromine joins to the alkene double bond. This results in the bromine solution losing its red/brown colour. If an alkane, such as hexane, is shaken with a bromine solution of this type, no colour change takes place (Figure 14.13). This is because there are no double bonds between the carbon atoms of alkanes. 

Also, according to Equation 1.9, the overall reaction radical chlorination takes place on a given substrate considerably faster than the overall reaction radical bromination. If we consider this and the observation from Section 1.7.3, which states that radical chlorinations on a given substrate proceed with considerably lower regioselectivity than radical brominations, we have a good example of the so-called reactivity/selectivity principle. This states that more reactive reagents and reactants are less selective than less reactive ones. So selectivity becomes a measure of reactivity and vice versa. However, the selectivity-determining step of radical chlorination reactions of hydrocarbons takes place near the diffusion-controlled limit. Bromination is considerably slower. Read on. 

The difficulty in obtaining this monomer in the pure state arises from the fact that the known methods of preparation involve the simultaneous formation of considerable amounts of the isomers of spiropentane which are difficult to remove. The method adopted as giving the most satisfactory yields is that of Applequist, Fanta, and Henrickson (I, 2). The spiropentane is prepared by reaction of zinc dust with pentaerythrityl tetrabromide in alcohol in presence of the sodium salt of ethylenediamine-tetraacetic acid as complexing agent. The yield of hydrocarbon (spiropentane plus various ethylenic compounds) is of the order of 84%. The spiropentane is obtained in the pure state by treating the mixture with bromine in dibromomethane. The yield of pure spiropentane was found to be 62%. 

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