Electrophilic Addition of Bromine to Ethylene

Electrophilic Addition of Bromine to Ethylene THE OVERALL REACTION  

Step 1 Reaction of ethylene and bromine to form a bromonium ion intermediate  

Step 2 Nucleophilic attack of bromide anion on the bromonium ion  

Step 2 of Mechanism 6.6 is a nucleophilic attack by Br at one of the carbons of the cyclic bromonium ion. For reasons that will be explained in Chapter 8, reactions of this type normally take place via a transition state in which the nucleophile approaches carbon from the side opposite the bond that is to be broken. Recalling that the vicinal dibromide formed from cyclopentene is exclusively the trans stereoisomer, we see that attack by Br from the side opposite the C—Br bond of the bromonium ion intermediate can give only fran5-l,2-dibromocyclopentane in accordance with the experimental observations. 

TABLE 6.3 Relative Rates of Reaction of Some Representative Alkenes with Bromine 

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

The transition state structure for the addition of molecular fluorine to ethylene VIIc was found by the 3-2IG calculations [17] to closely resemble that for the addition of HCl (IV). Of special interest are the computational results obtained on transition state structures Vlld, Vile for gas phase electrophilic addition reactions of molecular chlorine and bromine to ethylene. Whereas the four-centered structure VIIc for the fluorination of ethylene indicates concerted c/s-addition, which is in accord with experimental finding for this reaction [18], the transition state geometries for chlorination and bromination may be regarded as cyclic halonium ions backed by halogenide counter-ions. Noteworthy is that the calculations [17] predict the heterolysis to occur intrinsically without any assistance from polar solvents. The three-centered structures Vlld, Vile help to clarify the reason for trans-stereoselectivity of the chlorination and bromination reactions of ethylene [1, 19]. 

Electrophilic reactions are not usually observed, e.g., it was not possible to obtain a haloacetate from F5SCH=CH2 under the same conditions (acetic acid, bromine, mercury acetate) where perfluoroalkyl-substituted ethylenes are known to yield haloacetates. An exception is the rather easy (47) addition of SO3 to F SCH=CF2> although a significant amount of by-product is formed, one of which is formed in an intricate reaction that leads to the degradation of the SF5-group (48). 

Mixtures of bromine and chlorine add to olefins such as cyclohexene, styrene, ethylene, stilbene, and cinnamic acid to give the bromochlorides. The products isolated are those expected for the electrophilic addition of a bromine atom and a nucleophilic addition of a chlorine atom. 

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... 

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. 

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... 

It is concluded that the selectivities of electrophilic additions are not directly related to the reactivities but to the transition-state positions. Extensive comparison with similar data on the bromination and hydration of other ethylenic compounds bearing a conjugated group shows that this unexpected reactivity-selectivity behaviour can arise from an imbalance between polar and resonance effects (Ruasse, 1985). Increasing resonance in the ground state would make the transition state earlier and attenuate the kinetic selectivity more strongly than it enhances the reactivity. Hydration and halogenation probably respond differently to this imbalance. 

Electrons flow from the ir 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 in a bimolecular elementary step. 

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]. 

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