Bromination of Ethylenes

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

STEREO-, REGIO- AND CHEMOSELECTIVITY OF BROMINATION OF ETHYLENIC COMPOUNDS... 

Apart from information on stereochemistry, bromine bridging does not provide a priori any rule regarding regio- and chemoselectivity. Therefore, we systematically investigated (ref. 3) these two selectivities in the bromination of ethylenic compounds substituted by a variety of more or less branched alkyl groups (Scheme 4). 

The electrophilic bromination of ethylenic compounds, a reaction familiar to all chemists, is part of the basic knowledge of organic chemistry and is therefore included in every chemical textbook. It is still nowadays presented as a simple two-step, trans-addition involving the famous bromonium ion as the key intermediate. T]nis mechanism was postulated as early as the 1930s by Bartlett and Tarbell (1936) from the kinetics of bromination of trans-stilbene in methanol and by Roberts and Kimball (1937) from stereochemical results on cis- and trans-2-butene bromination. According to their scheme (Scheme 1), bromo-derivatives useful as intermediates in organic synthesis... 

The bromination of ethylenic compounds is in most cases a very fast reaction. Half-lives of typical olefins are given in Table 1. Most of them are very short. In order to obtain extended and meaningful kinetic data, it has been necessary to find suitable reaction conditions and to design specific kinetic techniques. This was not done until 1960-1970. As a consequence, kinetic approaches to the bromination mechanism are relatively recent as compared with those to solvolytic reactions, for example. 

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 deuterium kinetic isotope effect (DKIE) for the electrophilic bromination of ethylene-/z4 and ethylene- 4 in methanol and dichloroethane at 25 °C has been... 

Bromination of ethylene oxide vu reported as early as 1862 by Wurtz18 7 to give a red solid of unknown constitution. Maas and Boomer1081 obtained the same red solid on warming to room temperature oxonium complexes of ethylene oxide and bromine prepared at —80°. Freezing-point results indicated for these complexes the empirical formulas CsH40 Bt and CxH 0-2Br. No explosion occurs on warming to room temperature, however. 

Discrete and continuum models for the solvent involvement have been employed to steady equilibrium and non-equilibrium solvation effects on bromination of ethylene. Two mechanisms were identified that lead to transition states of different symmetry. One mechanism operates in the gas phase and non-polar solvents. The second one, that leads to the typical C2V transition state, holds in medium-to-very polar solvents. In water, the solvent molecules participate actively and non-equilibration solvations effects proved to be substantial and larger than those previously reported for the >SN2 reaction.23... 

Bromination of ethylene ketals. Eaton" found that the most satisfactory route to the doubly unsaturated diketone from the corresponding saturated diketone was reaction of the bis-elhyleneketal (1) with pyridinium hydrobromide perbromide to give the dibromide (2) and dehydrohulogenution with potassium /-butoxide in dimethyl lulfoxide. 

The beneficial effects of solid surfaces on the rates of bromination reactions were observed as eai ly as 1923 [4-6], The polarity of the glass surface was found to enhance the rates of bromination of ethylene. Halogenation reactions over zeolite catalysts have been reviewed [1]. A summary of halogenations over non-zeolite (until mid-1999) and zeolite (1995-mid 1999) catalysts is given in Table 1. 

Ethylene dibromide (EDB or dibromoethane) is a volatile liquid produced by the bromination of ethylene. EDB is used as a lead scavenger in leaded gasoline and as a pesticide and fumigant in soil and on grain, fruits, and vegetables. Because EDB has been classified as a suspected human carcinogen and is a male reproductive toxin, its use as a pesticide has been restricted since 1984. 

Koemer T, Brown RS, Gainsforth JL, Klobulowski M (1998) Electrophilic bromination of ethylene and ethylene-d4 a combined experimental and theoretical study. J Am Chem Soc 120 5628-5636... 

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