Bromination of E -Stilbene

Purpose To determine the stereochemistry of the electrophilic addition of bromine to an alkene. 

Bromine is a hazardous chemical that may cause serious chemical bums. Do not breathe its vapors or allow it to come into contact with the skin. Perform all operations involving the transfer of the pure liquid or its solutions at a hood and wear latex gloves. If you get bromine on your skin, wash the area immediately with soap and warm water and soak the affected area in 0.6 M sodium thiosulfate solution, for up to 3 h if the burn is particularly serious. 

Dispense the 1 M bromine in dichloromethane solution from burets or similar devices fitted with Teflon stopcocks and located in hoods. 

Bromine reacts with acetone to produce the powerful lachrymator a-bromoacetone, BrCH2COCH3. Do not rinse glassware containing residual bromine with acetone  

All parts of this experiment should be conducted in a hood if possible. 

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Apply the procedure given for bromination of (E)-stilbene to study this reaction with (Z)-stilbene. Develop a protocol whereby you could demonstrate the stereochemical outcome of the dibromide(s) produced. Consult with your instructor before undertaking any experimental procedures. 

Purpose. You will synthesize the second intermediate in the b series of Sequential Reactions by carrying out the bromination of (E)-stilbene to obtain meso-stHbene dibromide. This product is the precursor to diphenylacetylene, the next synthetic intermediate in the b series. A further purpose of this experiment is to demonstrate the stereospecific addition of bromine to alkenes. 

In the present reaction, bromination of (E)-stilbene yields meso-stilhene dibromide. Thus, this reaction is classed as stereospecific because the other possible diastereomers are not formed. 

Stilbene adds bromine relatively slowly, trans-Addition of bromine, which is usual with olefins, occurs with cw-stilbene in cold CS2 in the dark, affording 83% of (db)-l, 2-dibromo-1,2-diphenylethane (meso-isomer, m.p. 236°. The former product is converted into the latter by, e.g., bromine or iodine in CC14 in diffuse daylight.39 The meso-compound arises as main product on bromination of trans-stilbene in CS240 or ether.41 ( + )-m, .p. 93-94°, and meso- 1,2-di-chloro-l,2-diphenylethane (a.a -dichlorobibenzyl), m.p. 191-193°, are both formed in the reaction of trans-stilbene with a saturated solution of chlorine in anhydrous ether under the influence of ultraviolet or sun-light.39,41... 

The reaction of bromine with (E)-stilbene (47) to give meso-stilbene dibromide (48) as the major product (Eq. 10.21) is another example of an electrophilic addition reaction of alkenes. The addition of bromine to many alkenes is a stereospecific reaction that proceeds by anti addition to the double bond. However, the addition of bromine to 47 is not stereospecific because small amounts of dl-stilbene dibromide (49) are also formed in this reaction. The formation of wreso-stilbene dibromide presumably occurs via the nucleophilic attack of bromide on the intermediate cyclic bromonium ion, 50. The possible interconversion of 50 and the acyclic carbocation 51 (Eq. 10.22) is one possible way to account for the presence of dl-stilbene dibromide in the product. 

Using suitable stereochemical structures, write the mechanism for the addition of bromine to (E)-stilbene to give nxeso-stilbene dibromide via the intermediate cyclic bromonium ion 50. Use curved arrows to symbolize the flow of... 

The reaction of bromine with cyclopentene according to Equation 10.18 is stereospecific and proceeds by anti addition. On the other hand, the addition of bromine to (E)-stilbene gives both mcso-stilbene dibromide (48) and dl-stilbene dibromide (49). Rationalize this difference by comparing the relative stabilities of the carbocations resulting from ring-opening of the cyclic bromonium ions 38 and 50. 

It is important to realize that if two different groups are present on one or both of the sp carbon atoms of the alkene linkage, chiral carbon centers are generated when bromine is added, though if a chiral product were formed from achiral reagents, one would expect it to be racemic. In the case of (E)-stilbene, two chiral centers are generated. However, due to the symmetry of the reactants and the stereoselectivity of the reachon, only the meso diastereomer is formed ... 

The result of the halogenation of frans-cinnamic acid, as in the case of (E)-stilbene in Experiment [A2b], is an anti addition of molecular bromine. The enantiomeric products are shown here ... 

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. ... 

Experiments have shown that the bromination of cis- and frans-phenylpropenes in CC14 is non-selective.134 For stilbene, trims addition occurs in solvents having low dielectric constants, but the reaction loses stereoselectivity if e rises above 35.135... 

E-Stilbene reacts with bromine predominantly by the usual process of /ram-addition and affords the optically inactive, nonresolvable meso-di-bromide the much lower melting J/-dibromide (Chapter 60) is a very minor product of the reaction. 

For the procedure in which a solution of bromine is used, why does solid not separate immediately after you begin adding the solution to that containing (E)-stilbene ... 

In the present reaction, both the bromine and the (E)-stilbene are achiral. However, the bromonium ion that is produced is chiral. In this ion, the bromine atom bridges both carbon atoms of the original carbon-carbon double bond to form a three-membered ring intermediate. The generation of a cyclic species has a profound effect on the stereochemistry of the second step of the bromine addition. 

In a similar way bromination of powdered (E)-o-stilbene carboxylic acid with bromine vapour or with powdered pyridine. HBr.Brj complex in solid state at room temperature gave selectively erythro-1,2-dibromo-l, 2-dihydro-o-stilbene carboxylic acid. However, bromination with bromine in solution gives 4-bromo-3-phenyl-3,4-dihydroisocoumarin as the major product (Scheme 3). 

It is essential to apply both tests, since some symmetrically substituted ethylenic compounds (e.g., stilbene C HjCH=CHC8Hj) react slowly under the conditions of the bromine test. With dilute permanganate solution the double bond is readily attacked, probably through the intermediate formation of a cis diol ... 

The possible formation of a delocalised benzyl type carbocation (16) results in much lower (70%) ANTI stereoselectivity than with trans 2-butene (5 =100% ANTI stereoselectivity, p. 180), where no such delocalisation is possible. It is also found that increasing the polarity, and ion-solvating ability, of the solvent also stabilises the carbocation, relative to the bromium ion, intermediate with consequent decrease in ANTI stereoselectivity. Thus addition of bromine to 1,2-diphenylethene (stilbene) was found to proceed 90-100% ANTI in solvents of low dielectric constant, but =50% ANTI only in a solvent with e = 35. 

Judged by the magnitude of , these eliminations have product-like transition states. With respect to stilbene chemistry the pattern of Fig. 27 is rather general. That is, the relative reactivity of the two stilbenes is often small, e.g. within a factor of two in some (not all) additions of bromine (Buckles et al., 1967), 2,4-dinitrosulfenyl chloride (Slobodkin and Kharasch, 1960) or trichloromethyl (Cadogan and Inward, 1962). 

The mechanism of bromination is discussed more fully in Section 5.3, but the fundamental cause of the stereospecificity is the involvement of the positively charged bromonium ion intermediate. The bromonium ion is opened by an anti approach of the bromide, leading to net anti addition. Entry 1 in Scheme 2.7 illustrates this behavior. Stereoisomeric products are obtained from the E- and Z-isomers, both as the result of anti addition. Stereospecificity is diminished or lost when the bromonium ion is not the only intermediate in the reaction. Entry 2 in Scheme 2.7 shows this behavior for cw-stilbene in nitromethane, where most of the product is the result of syn addition. The addition is anti in less polar solvents such as cyclohexane or carbon tetrachloride. The loss of anti stereospecificity is the result of a change in mechanism. The polar solvent permits formation of a carbocation intermediate. If the bromonium ion can open to a carbocation, a mixture of syn and anti products is formed. In the stilbene case, the more stable anti product is formed. Some loss of stereospecificity is also observed with 1-phenylpropene, where the phenyl group provides stabilization of an open carbocation intermediate. Part of the product from both isomers is the result of syn addition. 

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