Ethylene bromine addition

Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated (ref. 1). This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity, which is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. 

In fact, the analogy between the mechanisms of heterolytic nucleophilic substitutions and electrophilic bromine additions, shown by the similarity of kinetic substituent and solvent effects (Ruasse and Motallebi, 1991), tends to support Brown s conclusion. If cationic intermediates are formed reversibly in solvolysis, analogous bromocations obtained from bromine and an ethylenic compound could also be formed reversibly. Nevertheless, return is a priori less favourable in bromination than in solvolysis because of the charge distribution in the bromocations. Return in bromination implies that the counter-ion, a bromide ion in protic solvents, attacks the bromine atom of the bromonium ion rather than a carbon atom (see [27]). Now, it is known (Galland et al, 1990) that the charge on this bromine atom is very small in bridged intermediates and obviously nil in /f-bromocarbocations [28]. 

Phenylthioacetylene has been prepared by elimination of thiophenol and dehydrobromination of cis-1,2-bis(phenylthio)ethylene5 and cis-1-bromo-2-phenylthioethylene,2 7 respectively. The latter was obtained by addition of thiophenol to propiolic acid in ethanol and subsequent one-pot bromine addition, decarboxylative dehalogenation, and careful distillation to remove the trans isomer.2.7 on the other hand, cis-1,2-bis(phenylthio)ethylene was prepared by double addition of thiophenol to cis-1,2-dichloroethylene.5a d Although these procedures can provide useful amounts of phenylthioacetylene, they were found to be somewhat less satisfactory in our hands as far as operation and/or overall yields are concerned. Furthermore, we have encountered problems with regard to the reproducibility of one-pot dehydrobrominations of phenylthio-1,2-dibromoethane.6 However, the stepwise execution of the double dehydrobromination, as described in the modified procedure reported here, provides preparatively useful quantities of phenylthioacetylene in a practical manner. 

By similar treatments, the effects of many substituents and combinations of substituents were evaluated, and found to give a reasonably linear correlation with reaction rates when plotted by the Hammett-Taft method. The value of —3.3, determined graphically, is close to the value of — 3.49 reported for solvolysis of substituted cyclohexyl tosylates. Although the general validity of the multiple-path treatment has still to be demonstrated, it may be noted that similar calculations, using an attenuation factor of 0.51, produced a reasonable correlation of substituent effects at C(3> and C(i7) with rates of bromine addition to the 5,6-ethylenic bond [17]. 

A project for the synthesis of eupomatenoid-1 involved dehydrogenation of ( )-eupomatenoid-8 (7.1b), available via dehydrodiisoeugenol (7.1a) (see Section VI. 1). It turned out, however, that the double bond in the side chain has an adverse effect on this reaction attempted under a variety of conditions (70d), and the preparation of 5-propenyl-benzofurans by this sequence is satisfactory only if the conjugated ethylenic function is protected, e.g. by bromine addition (6). 

Most of the chemical reactions as well as experimental structure determinations are performed in solutions. On the contrary, usual quantum chemical computations usually deal with isolated chemical species. This may lead to erroneous conclusions. For instance, the addition of bromine to an ethylenic hydrocarbon is known for having a different mechanism in the gas phase and in solution. In this example, the velocity constant vary by a factor of 10 when going from carbon tetrachloride to water as a solvent (Reichardt 1979), although the mechanism is the same These features are confirmed by appropriate quantum chemical computations which show that the transition state of ethylene-bromine would be dissymmetric and 55kcal/mol above the van der Waals complex in the case of the isolated species (Yamabe et al. 1988), while with a simple simulation of the solvent effect one finds a symmetric transition state lying 30.79 kcal/mol above the van der Waals complex in a non dipolar solvent and 0.02 kcal/mol in water (Assfeld 1994). 

Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine... 

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

FIGURE 6 12 Mechanism of electrophilic addition of bromine to ethylene... 

Ethylene bromohydrin has been prepared by the reaction between ethylene glycol and hydrobromic acid and phosphorus tribromide. It has also been prepared by the direct addition of hypobromous acid to ethylene, and by the reaction between ethylene and dilute bromine water. With ethylene oxide now available at a reasonable price, the method described is probably the best because of the high yields and the convenience of reaction. 

Alkyl fluorides have been prepared by reaction between elementary fluorine and the paraffins, by the addition of hydrogen fluoride to olefins, by the reaction of alkyl halides with mercurous fluoride, with mercuric fluoride, with silver fluoride, or with potassium fluoride under pressure. The procedure used is based on that of Hoffmann involving interaction at atmospheric pressure of anhydrous potassium fluoride with an alkyl halide in the presence of ethylene glycol as a solvent for the inorganic fluoride a small amount of olefin accompanies the alkyl fluoride produced and is readily removed by treatment with bromine-potassium bromide solution. Methods for the preparation of alkyl monofluorides have been reviewed. ... 

Bromine and chlorine add rapidly to alkenes to yield 1,2-dihalides, a process called lialogenatioii. For example, approximately 6 million tons per year of 1,2-dichloroethane (ethylene dichloride) are synthesized industrially by addition... 

Benzene shows neither the typical reactivity nor the usual addition reaction of ethylene. Benzene does react with bromine, Brs, but in a different type of reaction ... 

Proper selection of plastic matrix fire retardants and property enhancers offers acceptable combinations of impact properties and heat-distortion temperature (HDT) values for fire retardant plastics. This can be demonstrated by fire retardant styrenics. Fire retardant enhancers have special interest as property enhancers for example the addition of a highly flammable material such as ethylene propylene diene terpolyer (EPDM), dramatically improves the bromine efficiency of octabromodiphenyl oxide in ABS by increasing char-forming without changing the Sb-Br reaction. 

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