Bromination of styrene

In the bromination of styrene, a po-+ plot is noticeably curved. If the extremes of the curves are taken to represent straight lines, the curve can be resolved into two Hammett relationships with p = —2.8 for electron-attracting substituents and p = —4.4 for electron-releasing substituents. When the corresponding -methylstyrenes are examined, a similarly curved ap plot is obtained. Furthermore, the stereospecificity of the reaction in the case of the -methylstyrenes varies with the aryl substituents. The reaction is a stereoespecific anti addition for strongly electron-attracting substituents but becomes only weakly stereoselective for electron-releasing substituents, e.g., 63% anti, 37% syn, forp-methoxy. Discuss the possible mechanistic basis for the Hammett plot curvature and its relationship to the stereochemical results. 

Nucleophilic solvents can compete with halide ion for the cationic intermediate. For example, the bromination of styrene in acetic acid leads to significant amounts of the acetoxybromo derivative. 

Table 6 Chemoselectivity (% solvent-incorporated products) in bromination of styrenes, XC6H4CH = CH2.

The kinetic effect of trans-fi-alkyl or heteropolar groups R on bromination of styrenes, PhCH=CHR, has also been investigated and compared with the same R-effects on ethylene (Bienvenue-Goetz and Dubois, 1975). A fairly linear relationship (26) between the two sets of data is observed. The... 

Transition-state shifts with reactivity and selectivities in hydration and bromination of styrenes ArCY=CH2... 

A selective bromination of styrene with bromine absorbed into a molecular sieve was 36 37... 

The effect of cyclodextrin complexation on the bromination of styrene, methyl cinnamate, phenylacetylene, and allylbenzene has been studied.27 The corresponding bromohydrin was obtained as a major product along with dibromide in the bromination... 

The values of k2 for ring-substituted phenylacetylenes are well correlated by the Hammett relationship, using a+ constants, and p is very large, — 5-2. The p value compares well with those obtained in the acid-catalysed hydration of arylacetylene derivatives (see Table 1) and is entirely consistent with the formation of a vinyl cation 45 in which the positive charge is at the carbon next to the phenyl ring and is largely shared by it. Interestingly, the p value (by use of a+) based on k2 rate coefficients for the bromination of styrene in acetic acid is — 4-5. 

Arylalkene bromination is a typical electrophilic addition to form an a-carbocation, but markedly non-linear structure-reactivity relationships were observed for brominations of styrene [32], trans-stilbene [37] and a-methylstilbene [38] (Ruasse and Dubois, 1972, 1974 Ruasse et al., 1978 Ruasse and Argile, 1983). Some of these curvatures could not be interpreted directly by the Y-T equation but some were related to a mechanistic changeover based on a multipathway scheme (Ruasse, 1990). Three pathways leading to the and carbocations and to the bromonium ion in the bromination of trans-stilbene (Ruasse and Dubois, 1972, 1974) are shown in Scheme 10. 

The rates of bromination of a number of alkynes have been measured under conditions that permit comparison with the corresponding alkenes. The rate of bromination of styrene exceeds that of phenylacetylene by about For dialkylacetylene-... 

Mechanism proposed for second-order bromination of styrenes. (Adapted from reference 87.)... 

Bromination of styrene Thiolene-based resin MSR device Cygan et al. [79]... 

The bromination of styrene in carbon tetrachloride was followed by U V spectrophotometry. The disappearance of styrene was exponential when a 10-fold excess of bromine was used. When a 15-fold excess was used, the styrene loss was still exponential, but the observed rate was 2.25 times higher. What conclusions can you make about the mechanism ... 

As shown in Table I, when styrene in the a-CD complex was brominated at 0°C for 2 h, the levorotatory dibromide (2) ([a] -47.0 ) and racemic bromohydrin (3) were isolated in 90% yield (2 3 = 96 4). Bromination of the olefin in the -CD complex gave no 3 but 2 ([a] f -5.5°) in 95% yield at the same reaction condition. The chiral induction for the reaction of the a-CD complex rose to 9 times that of the fi-CD complex. The same sign of the specific rotations of 2 shows that styrene forms complexes with a- and y5-CDs such that the access of bromine to the olefinic plane occurs into the same enantiotopic face in the two cases, and this face may be slightly less blocked by the inclined plane in both the asymmetric cavities of CDs. A detailed mechanism, however, cannot be described at the present time, because neither crystalline nor molecular structures were determined for the solid CD complexes. No bromination of styrene in the CD complexes occurred at a temperature of —10 °C or below because the vapor pressure of bromine is not enough to sustain the reaction bromine solidifies at -7.3 °C. 

Fig. 4. Changes of specific rotations of dibromide during the course of the gas-solid bromination of styrene in the a-CD complex at 0°C. O, Change of specific rotations of dibromide 2, measured in dichloromethane , change of conversions (%) of styrene 1.

The rates of bromination of a number of alkynes have been measured under conditions that permit comparison with the corresponding alkenes. The rate of bromination of styrene exceeds that of phenylacetylene by about 10. " For internal alkyne-disubstituted alkene comparisons, the ratios range form 10 to 10 , being greatest in the least nucleophilic solvents. Bromination of aUcyl-substituted alkynes shows rate enhancement by both alkyl substituents, and this indicates that the transition state has bridged character. ... 

Nitration of toluene, nitration of dialkyl-substituted thioureas [64] ViUermaux-Dushman-instantaneous neutralization and rapid redox reactions [65] Bromination of styrene [66]... 

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