Additions of Hydrogen Halides

Hydrogen bromide adds to alkynes at a shghtly slower rate than it adds to alkenes. This regioselective reaction is a Markovnikov addition. 

The reaction occurs when a proton adds to the triple bond to form an alkenyl carbocation, which the nucleophilic bromide ion subsequently captures. Of the two possible carbocations, the secondary is more stable than the primary because the secondary carbocation is more highly substituted. The more stable intermediate forms in the rate-determining step of the reaction because it has a lower energy barrier. 

Hydrogen bromide adds to alkynes more slowly than to alkenes. To understand why, consider the carbocations formed in the addition of a proton to 1-hexene and 1-hexyne. 

The positive carbon atoms of the alkyl and alkenyl carbocations are sp - and sp-hybridi2ed, respectively. The sp hybrid orbital is of lower energy than the sp hybrid orbital. If electrons were present in these orbitals, the ones in the sp hybrid orbital would be more strongly attracted to the carbon atom. Because the carbocation does not have electrons in the hybrid orbital, it requires more energy to form the alkenyl carbocation than the alkyl carbocation. That is, the alkenyl carbocation is less stable than an alkyl carbocation. For this reason, the rate-determining step for forming the alkenyl carbocation is somewhat slower than the step forming the alkyl carbocation. 

The addition product of one mole of HBr to an alkyne can be isolated because the electron-withdrawing bromine atom diminishes the reactivity of the product toward electrophiles. The electronegative bromine atom withdraws electron density from the n bond of the alkenyl bromide, thereby decreasing the availability of it electrons to electrophiles. Nevertheless, when the alkenyl bromide 

Hydrogen halides, (HCl, HBr) may be added easily to the double bond of alkenes. 

In this reaction the carbon atoms to which hydrogen and bromine atoms are attached are not important, there is only one possible product. 

Addition of hydrogen halides to unsymmetrical alkenes on the other hand, may form two possible products. However, only one of these products usually predominates. 

As it is seen in the above example, for the addition of HBr to an unsymmetrical alkene, hydrogen attaches to the carbon atom that has the greater number of hydrogen atoms. This is known as Markovnikov s rule. 

According to Markovnikov s rule addition of HCl to 1-butene gives 2-chlorobutane. 

CHAPTER 4 ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS 

A more complete discussion of the mechanism of ionic addition of hydrogen halides to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or not discrete carbocations are always involved is considered there. 

The term regioselective is used to describe addition reactions that proceed selectively in one direction with unsymmetrical alkenes. Markownikoff s rule describes a specific case of regioselectivity that is based on the stabilizing effect that alkyl and aryl substituents have on carbocations. 

Terminal and disubstituted intermal alkenes react very slowly with HCl. The rate is greatly accelerated in the presence of silica or alumina in noncoordinating solvents such as dichloromethane or chloroform. Preparatively convenient conditions have been developed in which HCl is generated in situ from SOCI2 or ClCOCOCl. These heterogeneous reaction systems give Markownikoff addition. The mechanism is thought to involve interaction of the silica or alumina surface with HCl. 

Another convenient procedure for hydrochlorination involves adding trimethylsilyl chloride to a mixture of an alkene and water. Good yields of HCl addition products (Markownikoff orientation) are obtained. These conditions presumably involve generation of HCl from the silyl chloride, but it is unclear if the silicon plays any further role in the reaction. 

In nucleophilic solvents, products that arise from reaction of the solvent with the intermediate may be encountered. For example, reaction of cyclohexene with hydrogen bromide in acetic acid gives cyclohexyl acetate as well as cyclohexyl bromide. This result is readily understood as resulting from acetic acid and bromide ion acting as competing nucleophiles. 

The stereochemistry of the addition of hydrogen halides to a variety of alkenes has been investigated. The stereochemistry is dependent on the alkene structure and also on the reaction solvent and temperature. The addition of hydrogen chloride to 1-methylcyclopentene, for example, is entirely anti in nitromethane at 25 C.  

Addition of hydrogen bromide to cyclohexene and cis- and rrans-2-butene also takes place by anti addition. 1,2-Dimethylcyclohexene is an example of an alkene for which the stereochemistry of hydrogen chloride addition is solvent and temperature dependent. At —78 C in dichloromethane 88% of the product is the result of syn addition, whereas at 0 C in ether 95% of the product results from anti addition. Syn addition is particularly common with olefins having a phenyl substituent. Table 4.1 lists examples of several olefins for which the stereochemistry of addition of hydrogen chloride or hydrogen bromide has been studied. 

The term regiospecific can be used when a single product is formed. The addition of hydrogen bromide to styrene, for example, is regiospecific because the phenyl group strongly stabilizes cationic character. 

A more complete discussion of the mechanism of addition of hydrogen halides to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or not discrete carbocations are involved is considered there. Even when a carbocation is not involved, the regioselectivity of electrophilic addition is the result of attack of the electrophile at the more electron-rich carbon of the double bond. Alkyl substituents increase the electron density of the terminal carbon by hyperconjugation (see Part A, Section 1.1.8). 

The anti-Markownikoff addition of hydrogen bromide to olefins was one of the earliest free-radical reactions to be put on a firm mechanistic basis. In the presence of a suitable initiator, such as peroxides, a radical chain mechanism becomes competitive with the ionic mechanism for addition of hydrogen bromide  

The stereochemistry of radical addition of hydrogen bromide to olefins has been studied with both acylic and cyclic olefins. Anti addition is favored, which is contrary to what would be expected if the -carbon formed in the intermediate radical were rapidly rotating with respect to the remainder of the molecule  

The stereospecificity can be explained in terms of a bridged structure similar to that involved in discussion of ionic bromination of olefins  

This stereochemistry is also explained in terms of bromine-bridged radicals. 

Product mixtures from radical chain addition of hydrogen chloride to olefins are much more complicated than is the case for hydrogen bromide. The problem is that the rate of abstraction of hydrogen from HCl is not large relative to addition of the alkyl radical to the olefin, and this results in the formation of short polymers called telomers  

Further evidence for a bromine-bridged radical comes from radical substitution of bptically active 2-bromobutane. Most of the 2,3-dibromobutane that is formed is racemic, indicating that the chiral center has been involved in the reaction. When the 3-deuterated reagent is used, it can be shown that the hydrogen (or deuterium) that is abstracted is replaced by bromine with retention of stereochemistry. These results are consistent with a bridged bromine radical. 

CH3CH2CHCH3 - CH3CHCHCH3 CH3CH-CHCH3 - CH3CHCHCH3 + CH3CHCHCH3 Br Br Br 

Further evidence for a bromine-bridged radical comes from radical substitution of optically active 2-bromobutane. Although the main product, as expected from 

Other mechanisms must also operate, however, to account for the fact that 5%-10% of the optical activity at the initial chiral center is maintained. Isotopic labels also demonstrate that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with the bromination reaction  

The addition of HX (X = C1, Br, I) to an alkene, to form alkyl halides, occurs in two steps. The first step involves the addition of a proton (i.e. the electrophile) to the double bond to make the most stable intermediate carbocation. The second step involves nucleophilic attack by the halide anion. This gives a racemic alkyl halide product because the carbocation is planar and hence can be attacked equally from either face. (These addition reactions are the reverse of alkyl halide elimination reactions.) 

The proton adds to the less substituted carbon atom 

R 1 R-C-H The tertiary rather than secondary carbocation is formed because this 

The regioselective addition of HX to alkenes produces the more substituted alkyl halide, which is known as the Markovnikov (Markovnikoff) product. Markovnikov s rule states that on addition of HX to an alkene, H attaches to the carbon with fewest alkyl groups and X attaches to the carbon with most alkyl groups . This can be explained by the formation of the most stable intermediate carbocation. 

Occasionally, the intermediate carbocations can also undergo structural rearrangements to form more stable carbocations. A hydrogen atom (with its pair of electrons) can migrate onto an adjacent carbon atom in a hydride shift. This will only occur if the resultant cation is more stable than the initial cation. 

Halosuccinic acids (chloro and bromo) can be prepared by the addition of the elements of hydrohalic acid to the double bond  

Chlorosuccinic acid 54 (X = Cl) is prepared by heating a mixture of fumaric acid and acetic acid saturated with hydrochloric acid. Bromosuccinic acid 54 (X = Br) can similarly be prepared by treating fumaric acid with hydrogen bromide. 

These haloacids have not found any major industrial use. As can be seen, a racemic mixture results from the synthesis. The primary use has been as an intermediate for synthesis. Those derivatives of MA that can not be easily produced by nucleophilic addition may be achieved by nucleophilic substitution on these halosuccinic acids. 

Free-Radical Addition Reactions 12.4.1. Addition of Hydrogen Halides 

Because the bromine adds to the less substituted carbon atom of the double bond, generating the more stable radical intermediate, the regioselectivity of radical-chain hydrobromination is opposite to that of ionic addition. The early work on the radical mechanism of addition of hydrogen bromide was undertaken to understand why Maikow-nikofF s rule was violated under certain circumstances. The cause was found to be conditions that initiated the radical-chain process, such as peroxide impurities or light. 

The stereoehemishy of radieal addition of hydrogen bromide to alkenes has been studied with both acyehe and eyehe alkenes. Anti addition is favored. This is 

In agreement with this mechanism is the fact that the addition has been demonstrated to be syn for several typical alkenes. The rate of diimide reductions has been shown to be affected by torsional and angle strain in the alkene/ More strained double bonds react at accelerated rates. For example, the more strained trans double bond is selectively reduced in cis, trans-l,5-cyclodecadiene.  

Hydrogen chloride and hydrogen bromide add to olefins to give addition products. Many years ago, it was noted that additions usually take place to give the product in which the halogen atom is attached to the more substituted end of the olefin. This type of behavior was sufficiently general that the name Markownikoff s 

Because of the involvement of carbonium ion intermediates, rearrangement is a possibility. Reaction of t-butylethylene with hydrogen chloride in acetic acid gives both rearranged and unrearranged product. The rearranged acetate may also be 

The stereochemistry of the addition of hydrogen halides to a variety of alkenes has been investigated. The addition of hydrogen chloride to 1-methylcyclopentene is 

Alkynes undergo hydrohalogenation, the addition of hydrogen halides, HX (X = Cl, Br. 1). 

Two equivalents of HX are usually used addition of one mole forms a vinyl halide, which then reacts with a second mole of HX to form a geminal dihalide. 

Addition of HX to an alkyne is another example of electrophilic addition, because the electrophilic (H) end of the reagent is attracted to the electron-rich triple bond. 

With only one equivalent of HX, the reaction stops with formation of the vinyl halide. 

One currently accepted mechanism for the addition of two equivalents of HX to an alkyne involves two steps for each addition of HX addition of H (from HX) to form a carboca-tion, followed by nucleophilic attack of X . Mechanism 11.1 illustrates the addition of HBr to 

All of the halogen acids, HF, HC1, HBr, and HI, add to alkenes to give alkyl halides, as shown in the following example in which hydrogen chloride adds to 2-butene  

In this example the electrophile is a proton and the nucleophile is a chloride anion. The mechanism is just as described in the previous section first the electrophilic proton adds to produce a carbocation intermediate, and then the chloride nucleophile bonds to the carbocation. 

Because 2-butene is a symmetrical alkene, it does not matter which carbon initially bonds to the proton. Only one product, 2-chlorobutane, is possible. In the case of an un-symmetrical alkene—that is, one with different substituents on the two carbons of the double bond—two products could be formed, depending on which carbon bonds to the 

When this reaction is run in the laboratory, the only product formed is 2-chloropropane. No 1-chloropropane is observed. A reaction such as this one that produces only one of two possible orientations of addition is termed a regiospecific reaction. (A reaction that produces predominantly one possible orientation but does form some of the product with the other orientation is termed a regioselective reaction.) 

In 1869 Vladimir Markovnikov studied the regiochemistry of a large number of these addition reactions. On the basis of his observations, he postulated an empirical rule that can be used to predict the orientation of additions to alkenes  

The issue of the role of bridged radicals in the stereochemistry of halogenation has recently been examined computationally and a new interpretation offered. The structure, rotational barriers, and for halogen atom abstraction for P-haloethyl radicals were studied. For the reactions where X=C1 or Br, the halogen atom abstraction reaction shows a preference for a trans TS. 

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MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Our belief that carbocations are intermediates m the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements sometimes occur For example the reaction of hydrogen chloride with 3 methyl 1 butene is expected to produce 2 chloro 3 methylbutane Instead a mixture of 2 chloro 3 methylbutane and 2 chloro 2 methylbutane results... 

Markovmkov s rule is obeyed because the mechanism of sulfuric acid addition to alkenes illustrated for the case of propene m Figure 6 8 is analogous to that described earlier for the electrophilic addition of hydrogen halides... 

Furthermore kinetic studies reveal that electrophilic addition of hydrogen halides to alkynes follows a rate law that is third order overall and second order in hydrogen halide... 

The addition of hydrogen halides to alkenes has been studied from a mechanistic point of view over a period of many years. One of the first aspects of the mechanism to be established was its regioselectivity, that is, the direction of addition. A reaction is described as regioselective if an unsymmetrical alkene gives a predominance of one of the two possible addition products the term regiospecific is used if one product is formed... 

The addition of hydrogen halides to dienes can result in either 1,2- or 1,4-addition. The extra stability of the allylic cation formed by proton transfer to a diene makes the ion-... 

Addition of hydrogen halides to simple allenes initially gives the vinyl halide, and if the second double bond reacts, a geminal dihalide is formed. " ... 

Addition of hydrogen halide across fluoroalkenes and fluoroalkylalkenes is an important route to halogen-containing fluoroorganics Both lontc and free radical... 

It is worth remembering that a theory can never be proven correct. It can only be proven incorrect, incomplete, or inadequate. Thus, theories are always being tested and refined. As important as anything else in the scientific method is the testabie hypothesis. Once a theory is proposed, experiments are designed to test its validity. If the results are consistent with the theory, our belief in its soundness is strengthened. If the results conflict with it, the theory is flawed and must be modified. Section 6.7 describes some observations that support the theory that car-bocations are intermediates in the addition of hydrogen halides to alkenes. 

Addition of hydrogen halides (Sections 6.4-6.7) A proton and a halogen add to the double bond of an alkene to yield an alkyl halide. Addition proceeds in accordance with Markovnikov s rule hydrogen adds to the carbon that has the greater number of hydrogens, halide to the carbon that has the fewer hydrogens. 

Evidence from a variety of sources, however, indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations, and their- involvement in these additions has been questioned. Eor example, although electrophilic addition of hydrogen halides to alkynes occurs more slowly than the conesponding additions... 

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