Bromination of phenylacetylene

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

Electrophilic additions of the halogens to alkynes have not been much studied. In acetic acid a given olefin reacts with bromine 103 to 105 times more rapidly than the corresponding acetylene. However, the relative rates are enormously solvent-dependent and decrease with solvent polarity. Thus bromina-tion of styrene is 2590 times faster than bromination of phenylacetylene in acetic acid, but only 0.67 times as fast in HzO. Solvation of the transition state must be... 

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

Thermodynamics and kinetics need not go hand in hand. Consider all possible products resulting from addition of one equivalent of bromine to phenylacetylene (phenylacetylene+Br2) and to styrene (styrene+Br2). Calculate the heat of reaction for each addition. (The energy of Br2 is given at right.) Is addition to the alkyne or to the alkene more favorable ... 

Figure 2. General ton-pair scheme for the addition of bromine to phenylacetylene in acetic acid (33).

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. 

Also from this study the difference in the rate of bromination of alkenes and alkynes is quite evident. Thus the bimolecular coefficient (k2) for styrene is 2 x 103 times that for phenylacetylene and k2 for 3-hexene is 1-4 x 105 times the value for 3-hexyne. Since the first two compounds are believed to react via carbonium ions and the latter two via bromonium ions there seems to be an extra factor of 102 in the stability of bromonium ions from alkenes relative to those from alkynes. 

Phenylacetylene adds 1 mole of bromine in strongly cooled chloroform, yielding 78% of a -dibromostyrene.111 If bromine is added to the ice-cooled chloroform solution until a permanent bromine coloration is obtained, then there results a 92% yield of (l,l,2,2-tetrabromoethyl)benzene, m.p. 74-74.5°.112 The silver salt of phenylacetylene is readily converted by iodine in potassium iodide solution into triiodostyrene.113... 

Similarly, the addition of bromine to phenylacetylene in acetic acid was found to give a mixture of (E)- to (Z)-l,2-dibromo-l-phenylethene that varied from 70 30 without added bromide to 97 3 with 0.1 M LiBr. Konig, J. Wolf, V. Tetrahedron Lett. 1970,1629. 

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. ... 

Bromination usually follows a two-step mechanism, the rate-limiting step involving formation of an adduct with Br. Calculate energies for Br addition to phenylacetylene and styrene, leading to phenylacetylene+Br+ and styrene+Br+, respectively. (The energy of Br+ is given at right.) Which reaction is more favorable Is this the same preference as seen for Br2 addition ... 

Bromine, reaction with phenylacetylene and sodium hydroxide to give phenylbromoethyne, 46, 86 N-Bromoacetamide, as reagent for bromofluorination of 1-heptene, 46,10... 

To a 2-1. bottle equipped with a rubber stopper and immersed in a mixture of ice and water (slush) there is added a cold (about 0°) solution containing 300 g. (7.5 moles) of sodium hydroxide (Note 1) and 800 ml. of water. The mixture is swirled or stirred while 160 g. (2 moles) of bromine is added. Phenylacetylene (84 g,., 0.82 mole) (Note 2) is then added to the yellow solution, and the resulting mixture is stoppered and shaken. The rubber stopper is wired down, the bottle is covered with opaque cloth or paper, and the bottle is then placed in a mechanical shaker for 60 hours at room temperature (Note 3). 

The system A -bromoacetamide-hydrogen fluoride adds bromine monofluoride to acetylene derivatives.32 Hex-1-yne. hex-3-yne, 1.4-dichlorobut-2-yne, and phenylacetylene give the corresponding bromofiuoroalkenes, in no case is the addition of a second molecule of bromine monofluoride observed. Hex-1 -yne (3) produces 95% of the (E)- and 5% of the (Z)-isomcr 4. 

---