Electrophilic addition hydrogen bromide

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

Both resonance forms of the allylic carbocation from 1 3 cyclopentadiene are equivalent and so attack at either of the carbons that share the positive charge gives the same product 3 chlorocyclopentene This is not the case with 1 3 butadiene and so hydrogen halides add to 1 3 butadiene to give a mixture of two regioisomeric allylic halides For the case of electrophilic addition of hydrogen bromide at -80°C... 

When the major product of a reaction is the one that is formed at the fastest rate we say that the reaction is governed by kinetic control Most organic reactions fall into this category and the electrophilic addition of hydrogen bromide to 1 3 butadiene at low temperature is a kmetically controlled reaction... 

Isoprene has sometimes been used as a starting matenal in the laboratory synthesis of ter penes In one such synthesis the first step is the electrophilic addition of 2 moles of hydrogen bromide to isoprene to give 1 3 dibromo 3 methylbutane... 

The high reactivity of pyrroles to electrophiles is similar to that of arylamines and is a reflection of the mesomeric release of electrons from nitrogen to ring carbons. Reactions with electrophilic reagents may result in addition rather than substitution. Thus furan reacts with acetyl nitrate to give a 2,5-adduct (33) and in a similar fashion an adduct (34) is obtained from the reaction of ethyl vinyl ether with hydrogen bromide. 

Khaiasch proposed that hydrogen bromide can add to alkenes by two different mechanisms, both of which aie regiospecific. The first mechanism is electrophilic addition and follows Maikovnikov s rule. 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

Electrophilic addition to 9-vinylcarbazole occurs in the Markovnikov sense, thus hydrogen chloride,hydrogen bromide,chlorine, and bromine in carbon tetrachloride, and iodine chloride in pyridine are recorded as adding with initial electrophilic attack at the methylene. Mercuric acetate in methanol gave 9-(2-acetoxymercuri-l-methoxyethyl)carbazole. Although 9-vinylcarbazole gave an iodohydrin, comparable reaction with methanolic sodium hypochlorite led to 9-(2-chlorovinyl)carbazole. Catalytic reduction of the latter produced 9-(2-chloroethyl)carbazole. Tri-phenyltin hydride gave 96. ... 

Although apparently undergoing electrophilic substitution, the reaction of pyran-2-one with bromine involves addition to the double bond and subsequent elimination of hydrogen bromide (Scheme 31) (69JOC2239). In general electrophilic substitution of pyran-2-ones is a difficult process and forcing conditions are usually required. 

To understand the reason for the pronounced selectivity in the orientation of addition of electrophiles, it will help to consider one example, hydrogen bromide addition to 2-methylpropene. Two different carbocation intermediates could be formed by attachment of a proton to one or the other of the doublebond carbons ... 

The electrophilic addition of hydrogen-bearing boranes to alkenes (hydroboration), now recognized as a vital component of organic synthesis, is covered in Chapter 1.7 of this volume. The boron compounds that do fall under the realm of this chapter are boron bromides. Suzuki and coworkers have demonstrated... 

Higher alkynes can be synthesised from alkenes through a two-step process which involves the electrophilic addition of bromine to form a vicinal dibromide then dehydrohalogenation with strong base(Following fig.). The second stage involves the loss of two molecules of hydrogen bromide and so two equivalents of base are required. 

The first step of electrophilic addition is the one in which alkene acts as a nucleophile and uses its two 71 electrons to form a new bond to the hydrogen of HBr. As a new bond is formed, the H-Br bond breaks since hydrogen can form one bond. Both electrons in that bond end up on the bromine atom to produce a bromide ion. Since the electrons form the ji bond have been used for the formation of a new o bond, the ji bond is no longer present. Because of this, the left hand carbon is left with only three bonds and becomes positively charged. This is called a carbocation since the positive charge is on a carbon atom. 

The mechanism of 1, 4-addition starts off in the same way as a normal electrophilic addition. We shall consider the reaction of a conjugated diene with hydrogen bromide as an example (Following fig.). One of the alkene units of the diene uses its n electrons to form a bond to the electrophilic hydrogen of HBr. The H-Br bond breaks at the same time to produce a bromide ion. The intermediate carbocation produced has a double bond next to the carbocation centre and is called an allylic carbocation. 

In Chapter 11 the electrophilic addition of hydrogen bromide to alkenes, which proceeds by an ionic mechanism, was discussed. Recall that this reaction follows Markovnikov s rule and the bromine adds to the more highly substituted carbon, as illustrated in the following example ... 

This is illustrated by the products of addition of hydrogen bromide to vinyl acetylene (177) (Traynard, 1962) (equation 35). Only in the case of halogen addition have products formally arising from electrophilic attack to the double bond been isolated in substantial amounts (Petrov et al., 1960). 

Electrophilic addition to alkenes, such as the addition of hydrogen bromide under ionic conditions, follows the Markownikoff rule which states that in the addition of HX to an alkene, the hydrogen atom (the electrophile) becomes attached to the less-substituted carbon atom. The addition of halogens such as bromine proceeds via a halonium ion, and takes place with an overall trans stereochemistry. 

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