Bromine, reaction with alkenes

The reaction with bromine is very rapid and is easily carried out at room temperature, although the reaction is reversible under some conditions. In the case of bromine, an alkene-Br2 complex has been detected in at least one case. Bromine is often used as a test, qualitative or quantitative, for unsaturation. The vast majority of double bonds can be successfully brominated. Even when aldehyde, ketone, amine, so on functions are present in the molecule, they do not interfere, since the reaction with double bonds is faster. 

Acetylene undergoes electrophilic addition reaction with bromine in the dark. Bromine adds successively to each of the two n bonds of the aUcyne. In the first stage of the reaction, acetylene is converted to an alkene, 1,2-dibromoethene. In the final stage, another molecule of bromine is added to the n bond of this alkene, and produces 1,1,2,2-tetrabromoethane. 

A-14. Compound A, on reaction with bromine in the presence of light, gave as the major product compound B (C9H19Br). Reaction of B with sodium ethoxide in ethanol gave 3-ethyl-4,4-dimethyl-2-pentene as the only alkene. Identify compounds A and B. 

From the structural formula of the desired product, we see that it is a vicinal bromohydrin. Vicinal bromohydrins are made from alkenes by reaction with bromine in water. 

Epoxides give rise to many 1,2-difunctionalised compounds such as 48 with control over stereochemistry. Reactions of the epoxide 49 from 44 give the anti stereochemistry in 48 in contrast to the syn stereochemistry in 43. Other compounds made from alkenes include 1,2-bromides and bromohydrins from reaction with bromine alone or bromine and water. 

The pattern you saw for epoxidation with peroxy-acids (more substituted alkenes react faster) is followed by bromination reactions too. The bromonium ion is a reactive intermediate, so the rate-determining step of the brominations is the bromination reaction itself. The chart shows the effect on the rate of reaction with bromine in methanol of increasing the number of alkyl substituents from none (ethylene) to four. Each additional alkene substituent produces an enormous increase in rate. The degree of branching (Me versus n-Bu versus t-Bu) within the substituents has a much smaller, negative effect (probably of steric origin) as does the geometry (E versus Z) and substitution pattern (1.1- S... 

Electrophilic attack on alkenes by bromine often goes through three-membered ring cyclic bromoni-um ions and we can sometimes tell that this is so by studying the stereochemistry. Here are two reactions of styrenes that look very similar—a reaction with bromine and one with PhSCl. With no further information, we might be tempted to assume that they both go by the same mechanism. However, the Hammett p values for the two reactions are rather different. 

Although aromatic hydrocarbons are unsaturated, they have very different chemical properties to alkenes and allies. For example, benzene doesn t undergo an addition reaction with bromine despite having a double bond. 

Bromine (Br2) is brown, and one of the classic tests for alkenes is that they turn a brown aqueous solution of bromine colourless. Alkenes decolourize bromine water alkenes react with bromine. The product of the reaction is a dibromoalkane, and the reaction below shows what happens with the simplest alkene, ethylene (ethene). 

Alkenes react with bromine to give the products of 1,2-addition. The reaction is classified as an electrophilic addition of bromine, and the two bromine atoms in the product, 1,2-dibromoalkane, are mutually trans. Therefore from the addition of bromine to trans-but-2-ene (2) the product is m o-2,3-dibromobutane (37). This result is explained as follows the initial step is nucleophilic attack by the double bond of the alkene on one bromine, with displacement of the other as bromide ion. The organic intermediate is a bromonium ion 36, whose formation is rationalized in Scheme 4.7. The bromide that was expelled in formation of 36 now becomes a nucleophile and attacks 36 with equal probability at C(2) or C(3). In each case the reaction occurs with inversion of configuration to produce 37. 

Dilithio-1-alkenes 81 on the other hand show a clean reaction with bromine even at —90 °C interestingly without addition to the double bond taking place e-g. 

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a... 

When cyclohexene reacts with bromine, the product is trans-isomers, but only frans-l,2-dibromocyclopentane (38) is formed in the reaction. (See Chapter 9, Section 9.5, to review diastereomers.) Only the ra. s-diastereomer formed, so this reaction is diastereospecific. This stereochemical preference has been confirmed in the reactions of many alkenes, over many years. If cyclohexene is viewed from the side, as in 41, it is clear that the initial reaction with bromine must deliver the Br of the bromonirun ion to one side of the ring or the other. It can be either on the top or the bottom because there is no facial bias in the C=C unit of cyclohexene. 

Dihalogenation of alkynes gives a dihalogenated alkene, which is also susceptible to reaction with bromine, chlorine, or iodine. Tetrahalo derivatives are available from dihalogenated alkenes (vinyl dihalides). When 1-pentyne reacts with one molar equivalent of diatomic bromine. 111 is the product. Because alkenes are also subject to reaction with halogens. 111 can react with a second molar equivalent of bromine to give 1,1,2,2-tetrabromopentane, 112. 

Another common reaction of alkenes uses diatomic halogens such as bromine (Br2) to form 1,2-dibromides (see Chapter 10, Section 10.4.1). In this reaction, the alkene reacts as a Lewis base with the bromine atom to form a bromonium ion. When 1,3-butadiene (3) reacts with bromine, both 1,2 and 1,4 addition products are formed, just as with the HBr reaction. The products are 3,4-dibromo-l-butene (32) and a mixture otE- and Z-l,4-dibromo-2-butene (33 and 34). Initial reaction with bromine gives bromonium ion 29 however, when this reacts with bromide ion, there are two sites for reaction. If bromide attacks the less stericaUy hindered carbon atom, the product is 32, but the bromine ion may also attack the C=C unit to give products 33 + 34. Nucleophilic attack of this type is called an Sj reaction (nudeophilic substitution at an allylic carbon with displacement of the leaving group). 

If we take the addition of hydrogen bromide to an alkene as a model for the alkene halogenation reaction, we can build a mechanism quite quickly. Just as the alkene reacts with hydrogen bromide to produce a carbocation and bromide (Br ), so reaction with bromine-bromine might give a brominated cation and bromide (Fig. 10.9).The n bond of the alkene acts as a nucleophile, displacing Br from bromine. Addition of bromide to the carbocation would give the dibromide product. 

Unlike alkanes, alkenes react with bromine at room temperature, and even in the dark. The reaction takes place either with bromine water or with a solution of bromine in an inert organic solvent such as hexane. For example, the reaction of ethene with bromine in hexane is ... 

Other alkenes undergo similar reactions with bromine in solution in hexane. For example, propene reacts to give 1,2-dibromopropane, and but-2-ene to form 2,3-dibromobutane ... 

Figure 21.16 Addition reaction of bromine with alkenes. Chemists used to test for the presence of double bonds by reaction with bromine. Since bromine easily undergoes an addition reaction with alkenes, the disappearance of the red-brown color of bromine indicates that it has reacted. These photographs show that the bromine color disappears in the presence of bacon. We conclude that this is evidence of alkenes in bacon. 

Comparison of the structure-reactivity relationships for a series of styrene and alkene addition reactions with bromine and with arylsulfenyl halides also supports the idea that a bromonium ion is formed in the rate-determining step. From other evidence the sulfenyl halides are known to add through a bridged intermediate and the similarity between the two reaction series points to a closely similar transition state. 

The reaction of cyclohexene with bromine is potentially rather complicated. We know that alkenes react with bromine by an electrophilic addition mechanism. Might not this reaction occur in competition with the aUylic bromination reaction at low concentrations of bromine The answer is no because the free-radical chain reaction is much faster than the addition reaction if the concentration of bromine is low. The free-radical chain reaction for reaction of cyclohexene with bromine has the foUowing steps. 

Cyclooctatetraene, which has eight 71 electrons, seems to fit the category of antiaromatic polyenes since it has 4 ti electrons ( = 2). Nevertheless, cyclooctatetraene is a stable molecule, and reacts like an alkene. For example, it undergoes addition reactions with bromine and is easily hydrogenated. Cyclooctatetraene is not planar. It is not antiaromatic because it exists in a tub conformation, so its 71 orbitals cannot overlap to form a continuous 7t system, which for 871 electrons would be very unstable. Therefore, cyclooctatetraene does not exhibit the general characteristics of either aromatic or antiaromatic compounds. It is nonaromatic. 

Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine... 

Relative Rates of Reaction of Some Representative Alkenes with Bromine... 

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