Alkenes react with bromine

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 on the right shows what happens with the simplest alkene, ethylene (ethene). 

In order to understand this reaction, and the other similar ones you will meet in this chapter, you need to think back to Chapter 5, where we started talking about reactivity in terms of nucleophiles and electrophiles. As soon as you see a new reaction, you should immediately think to yourself, Which reagent is the nucleophile which reagent is the electrophile Evidently, neither the alkene nor bromine is charged, but Brj has a low-energy empty orbital (the Br-Br a ), and is therefore an electrophile. The Br-Br bond is exceptionally weak, and bromine reacts with many nucleophiles like this. 

In the reaction with ethylene, the alkene must be the nucleophile, and its HOMO is the C=C n bond. Other simple alkenes are similarly electron-rich and they typically act as nucleophiles and attack electrophiles. 

When it reacts with Bij, the alkene s filled jc orbital (the HOMO) will interact with the bromine s empty a orbital to give a product. But what will that product be Look at the orbitals involved. 

Very hindered alkenes form bromonium ions that are resistant to nucleophilic attack. In this very hindered case, the bromonium ion is sufficiently stable to be characterized by X-ray crystallography. 

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Give the structure of the product formed when each of the following alkenes reacts with bromine in water ... 

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. 

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 ... 

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. 

Test for an alkene. At the left is a solution of bromine in carbon tetrachloride. Addition of a few drops of an alkene causes the red color to disappear as the alkene reacts with the bromine. 

The metal catalyst is not absolutely required for the aziridination reaction, and other positive nitrogen sources may also be used. After some years of optimization of the reactions of alkenes with positive nitrogen sources in the presence of bromine equivalents, Sharpless et al. reported the utility of chloramine-T in alkene aziridinations [24]. Electron-rich or electron-neutral alkenes react with the anhydrous chloramines and phenyltrimethylammonium tribromide in acetonitrile at ambient temperature, with allylic alcohols being particularly good substrates for the reaction (Schemes 4.18 and 4.19). 

For example, when a solution of red-brown bromine is mixed with an alkene such as 1-hexene, CH2=CHCH,CH2CH2CH , the color due to bromine is lost as the Br2 molecules attack the double bond to produce CH2Br—CHBrCI I2CI12CI LCFT, (Fig. 2.9). However, benzene does not react with bromine. 

One can demonstrate the particular stability of aromatic compounds by their characteristic chemical reactions. For example, benzene reacts with bromine only with difficulty and gives bromobenzene, a substitution product (see Section 8.4). This leaves the aromatic ring intact. By contrast, a typical alkene reacts readily with bromine by an addition process... 

Problem 8.50 Heating C H,Br (A) with alcoholic KOH forms an alkene, C H (B), which reacts with bromine to give C HgBr2(C). (C) is transformed by KNHj to a gas, C4HJ (D), which forms a precipitate when passed through ammoniacal CuCl. Give the structures of compounds (A) through (D). M... 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

The epoxide 6 is naturally electrophilic, but where does the epoxide come from By far the most important method of epoxide synthesis is the treatment of alkenes 19 with peroxy acids RCO3H 21. Alkenes are naturally nucleophilic 2 they react with bromine to give dibromides 20 and with electrophilic peroxyacids 21 to give epoxides. Again, these reactions convert nucleophilic alkenes into electrophilic derivatives. A very popular reagent for epoxidation is mCPBA (meta-chloro-perbenzoic acid) 21 R = 3-chlorophenyl but many other compounds are used. 

The mechanism followed in this reaction is similar to that discussed for alkenes with HBr. However, the first stage of the mechanism involves the nucleophilic alkene reacting with an electrophilic centre, and yet there is no obvious electrophilic centre in bromine. The bond between the two bromine atoms is a covalent a bond with both electrons equally shared between the bromine atoms. 

An allyl radical can be brominated at both termini of the radical. This is why two allyl bromides can result from the Wohl-Ziegler bromination of an alkene if the allyl radical intermediate is unsymmetrical (examples see Figures 1.29-1.31). Even more than two allyl bromides may form. This happens if the substrate possesses constitutionally different allylic H atoms, and if, as a result thereof, several constitutionally isomeric allyl radicals form and react with bromine without selectivity. 

Sulfonic acids add to alkenes and alkynes. The reaction of an alkyne with ara-toluenesulfonic acid and treatment with silica gives the vinyl sulfonate (C=C—OS02To1) Cyclic sulfonates can be generated by the reaction of an allylic sulfonate salt (C=C—C—OSOs ) with silver nitrate in acetonitrile containing an excess of bromine and a catalytic amount of water. Sultones are formed when alkenes react with PhlO and two equivalents of Me2SiS03Cl. ... 

The it bond of alkenes reacts with electrophiles, e.g. bromine. 

B) Upon treatment with strong base (r-butoxide), X loses H and Br to give Y, C5Hg, which does react with bromine and KMn04, it must have an alkene and a ring. Only one isomer is formed. 

In the presence of a radical initiator, alkenes react with reactive molecules such as hydrogen bromide to give simple 1 1 adducts rather than a polymer. The initiator radical reacts rapidly with an HBr molecule to give a bromine atom (6.49), which starts the chain reaction. In the first propagation step, the bromine atom adds to the alkene 61 to give the adduct radical 62 (reaction 6.50). Since 62 abstracts a hydrogen atom from HBr by reaction (6.51) more rapidly than it would add to the alkene to form a polymer radical as in (6.43), the chain continues with reactions (6.50) and (6.51) as the propagating steps, and the product is the primary bromo compound 63. This anti-Markovniko addition is in the reverse direction to the polar addition discussed in Chapter 5. Since the radical chain reaction is faster than the polar reaction, the anti-Markovnikov product dominates if radicals are present. If the Markovnikov product is required, the reaction must be carried out in the dark, in the absence of free radical initiators, and preferably with a radical inhibitor present. 

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