Bromination bromine addition, olefin

For chemical processes, some examples are the elimination of aromatics by sulfonation, the elimination of olefins by bromine addition on the double bond (bromine number), the elimination of conjugated diolefins as in the case of the maleic anhydride value (MAV), and the extraction of bases or acids by contact with aqueous acidic or basic solutions. 

Since bromine addition to olefins leads to brominated compounds of synthetic interest, there are many studies of bromination products in the literature. Typical examples have been reviewed by Schmid and Garratt (1977). However, there are few systematic product analysis studies related to the role of the structure and the solvent in determining bromination selectivities. 

A cyclic complex derivable by addition of positive bromine to a double bond has already been discussed as an example of the neighboring group effect. The same intermediate, except perhaps for the nature of the bonds to bromine, is formed in the addition of bromine to olefins and is responsible for the stereochemistry of the addition reaction and the nature of the by-products.232... 

It is noteworthy that the nature of the ionic intermediate formed in bromine addition to olefins and the solvent properties also govern the competition between nucleophilic trapping and elimination. Thus 1,1-diphenylethylene, 11, gives the corresponding dibromide 13 (or solvent incorporated products, 14) and vinyl bromide, 12, in a ratio changing from 99 1 to 5 95 depending on solvent and on bromine concentration.(20) (see Table III results)... 

Lifetimes of the ionic intermediates of nucleophilic substitution are generally correlated to the pathways followed under given reaction conditions. Information on the lifetimes of ionic intermediates formed by bromine addition to olefins in methanol, as determined by the azide clock method, do not allow the different reaction pathways to be distin-... 

One example of selective photochemistry with lasers is the gasphase photochemical addition of bromine to olefin molecules induced by the monochromatic light near 6940 A from a pulsed, tunable ruby laser, as studied by Tiffany... 

The other bromine atom comes from another bromine-containing molecule or ion. If the concentration is sufficiently low, there is a low probability that the proper species will be in the vicinity once the intermediate forms. The intermediate in either case reverts to the initial species and the allylic substitution competes successfully. If this is true, it should be possible to brominate an olefin in the allylic position without competition from addition, even in the absence of NBS or a similar compound, if a very low concentration of bromine is used and if the HBr is removed as it is formed so that it is not available to complete the addition step. This has indeed been demonstrated.126... 

The bromonium ion concept does rationalize the stereochemistry of bromine additions to double bonds very satisfactorily if it is recognized that olefins that can form highly stabilized carbocations need not form such a structure. As al-ready noted, nonconjugated olefins give predominantly anti addition. Cornu-gated olefins, in which the intermediate carbocation would be stabilized by Resonance- however, give a mixture of svn and anti adducts. 

A typical example of a stereospecific olefin addition reaction is the addition of bromine to olefins. If d.v-2-pentene is used as the substrate, only the 2R,3R and 2S,3S pair will be produced (they are enantiomers). 

In the above discussion of stereoselectivity the mechanisms of various reactions have been used to rationalize why some are stereoselective and some are not. Thus the bromination of olefins proceeds via a bridged bromonium ion intermediate and gives only trails addition across the double bond [reactions (6.2) and (6.3)]. In contrast, the addition of HBr across a double bond gives a carboca-tion intermediate that does not maintain the facial integrity of the olefin and is thus much less stereoselective [reaction (6.1)]. In these examples the mechanism of the reaction is used to explain and understand the diastereoselectivity that is observed. There are many other examples (usually in textbooks) where the mechanism of a reaction is used to rationalize the stereoselectivity of the process. To do this requires that the mechanism be known with certainty. 

This process is quite common for carbon-centered free radicals because die carbon-carbon a bond which is formed is stronger by about 30 kcal dian die n bond which is broken. Other radical species, however, are well known to undergo olefin additions as well. The addition of bromine to olefins is die key step in the anti-Markovnikov addition of HBr to olefins. 

It is clear that the mechanism in Scheme 25 parallels (at least from the qualitative point of view) the mechanism of the addition of bromine to olefins shown in Scheme 11. Kinetic investigations indicate that the oxymercuration reaction involves a series of fast equilibria until the mercuronium ion (53) is formed. The subsequent nucleophilic attack of the solvent is probably the rate-limiting step, as indicated by steric requirements in bulky alkenes111. In the bromine addition, the formation of the bromonium ion is the rate-limiting step (or the rate-limiting equilibrium). However, the olefin reactivities in both reactions (bromination and oxymercuration) are identical when steric effects in the TS of the two addition reactions are taken into account110. 

We referred above to the large substituent effect on the heterolytic addition of bromine to olefins, which is a polar process actually involving intermediate ionic species. By contrast, the addition of alkyl radicals to olefins involves... 

Instead of a bromonium ion, in certain cases an isomeric acyclic and sufficiently stable cation can occur as an intermediate of the bromine addition to olefins. This holds true for bromine-containing benzyl cations. Therefore, the bromine addition to (3-methyl styrene shown in Figures 3.5 and 3.6 takes place without stereocontrol. 

An important question now arises here. If bromine is the reacting species why does it not react with the double bond either by ionic or free radical mechanism The answer is that its concentration is too low and this slows the rate of addition and in the circumstances only allylic substitution successfully takes place. This means that if there is regular, slow and steady supply of bromine and if somehow HBr formed, be removed to check addition, then it should be possible to brominate an olefine in allylic position even in absence of NBS and this has been demonstrated by Tedder et al. 

A comparable study of bromine addition to steroidal A -derivatives with varied substitution at C(3> and/or C(x ) also revealed rate retardation by electronegative substituents [103], and an analysis of data by the Taft method [104] yielded a reaction constant in the range —2.0 to — 2.7f. Although the Hammett and Taft reaction constants ( and q ) are not strictly comparable, these values indicate a similarity of reaction mechanism between the two types of olefin. Since chlorination in saturated aliphatic systems is believed to involve chloronium ions rather than classical carbonium ions, we would expect chlorine addition to A -steroids to produce a -value not greatly different from bromination. This does not appear to have been studied. 

Addition of fluorine to a C=C bond is different from that of the other halogens in one important respect the heat of reaction exceeds the dissociation energy of an aliphatic C-C bond (80 kcl/mole) quite appreciably. Addition and substitution reactions of fluorine are so exothermic that most organic substances burn or explode if brought into direct contact without special arrangements.15 Addition of F2 to olefins has no preparative importance 48 in the aromatic series addition and substitution result simultaneously. In electrophilic addition, i.e., in most cases, chlorine reacts faster than bromine with olefins, but in nucleophilic addition bromine reacts faster than chlorine.16... 

For addition of bromine a gaseous unsaturated hydrocarbon can be led into cooled bromine that is the process for, e.g., synthesis of 1,2-dibromoethane in the laboratory33 and the preparation of butadiene tetrabromide.34 Usually, however, a solution of bromine in a suitable solvent (see page 104) is dropped into one of the olefin in the same solvent, with stirring and ice-cooling, until the bromine is no longer decolorized. Sensitive unsaturated compounds are preferably treated with the bromine addition product of a tertiary amine hydrobromide or of dioxane (see pages 112, 113). 

Here the reaction is assisted by conjugative and hyperconjugative effects, and is retarded by the inductive effect which withdraws electrons from the reaction site. A similar effect is found in the addition of bromine to olefins (RCH=CH2) in acetic acid at 25°, viz-... 

However, a more complex kinetic expression occurs when the addition of the less reactive halogens is studied in solvents of low dielectric constant. The reaction of chlorine with olefins in most solvents shows second-order kinetics, although in carbon tetrachloride there seems to be a number of significant mechanisms which are effected in poorly defined kinetic orders". In acetic acid, however, the simple kinetic equation (4.2) is observed for chlorine and for bromine additions at low concentrations. At higher concentrations of bromine, a term showing second-order dependence upon the halogen concentrations is found, viz. 

Bromine addition in [BMIM][PF6] and [BMIM][BF4] is a stereospecific anti-addition process with dialkyl substituted alkenes, alkyl substituted alkynes and trans-stilbenes, whereas ds-stilbenes and aryl alkynes give mixtures of syn- and anti-addition products, although in the case of dx-diaryl substituted olefins the anti-stereoselectivity is generally higher than in chlorinated solvents. In the case of diaryl substituted olefins, such as stilbenes, it has been shown that stereoselectivity in molecular solvents depends primarily on two factors (i) the nature of the intermediates and (ii) the lifetime of the ionic intermediates [53]. Bridged bromiranium... 

Transition metal-catalyzed regio- and sto-eoselective amino-bromination of olefins with TsNH2 and NBS as nitrogen and bromine sources. 4 Studies have appeared on the use of NBS in additions to alkenes and in the isomerization of alkenes. " ... 

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