Addition of Hydrogen Halides to Alkenes

Rationale for Markovnikov regiochemistry in terms of carbocation stability. 

The spelling currently popular is Markovnikov, although different spellings were used in the first publications in German and French (a) Markownikoff, W. Liebigs Ann. Chem. 1870,153,228 (particularly the discussion beginning on p. 256) (b) Markovnikoff, V. Compt. Rend. 1875,81,668. 

A density functional theory analysis supported this model for Markovnikov orientation Aizman, A. Contreras, R. Galvan, M. Cedillo, A. Santos, J. C. Chamorro, E. /. Phys. Chem. A, 2002,106, 7844. 

Addition of HI to propene gave only 2-iodopropane, whether antioxidants or peroxides were added. Addition of HI to allyl bromide produced only l-bromo-2-iodopropane, and addition of HI to allyl chloride gave only l-chloro-2-iodopropane. The reason for the Markovnikov orientation may be that HI reacted with peroxides to give iodine, thus stopping initiation of radical chain reaction. Kharasch, M. S. Norton, J. A. Mayo, F. R. /. Am. Chem. Soc. 1940, 62, 81. 

Addition of HF to alkenes has been shown to follow Markovnikov s rule (Sharts, C. M. Sheppard, W. A. Org. React. 1974,21,125 and references therein). Direct addition of HF is seldom carried out because of the danger of working with that reagent and because of the development of newer synthetic reagents. Moreover, it is both safer and more convenient to use KF to synthesize a 2-fluoroalkane from the corresponding 2-chloroalkane than to add HF to a 1-alkene. 

The addition of a hydrogen halide, e.g. HCl, to an alkene is a simple example of an electrophilic addition. 

This type of synthetic reaction requires the use of gaseous hydrogen halide. If aqueous acid were employed, then, although water is not such a good 

Since HCl will be completely dissociated in water, the electrophile in this case will be the hydronium ion, although the same carbocation will be produced. The reaction is completed by nucleophilic attack of water, followed by loss of a proton, thus regenerating the acid catalyst. The overall conversion thus becomes hydration of the alkene. This is an important industrial process, typically employing sulfuric acid, but it 

Attack of the nucleophile onto a planar carbocation may take place from either face with equal probability, so that it is easy to see that, when a new chiral centre results, a racemic product will be formed, a similarity with SnI processes (see Section 6.2). 

since the same carbocation intermediate will be formed, it can be deduced that the nature of the product will not be dependent upon the configuration of the double bond. 

The simple mechanism shown for addition of HBr to 2-butene applies to a large number of electrophilic additions. We can use this mechanism to predict the outcome of some fairly complicated reactions. For example, the addition of HBr to 2-methyl-2-butene could lead to either of two products, yet only one is observed. 

The first step is protonation of the double bond. If the proton adds to the secondaiy carbon, the product will be different from the one formed if the proton adds to the tertiary carbon. 

PROBLEM SOLVING M When the proton adds to the secondary carbon, a tertiary carbocation results. 

The second half of the mechanism produces the final product of the addition of HBr to 2-methyl-2-butene. 

Note that pirotonation of one carbon atom of a double bond gives a carbocation on the carbon atom that was nat protonated. Therefore, the proton adds to the end of the double bond that is less substituted to give the more substituted carbocation (the more stable carbocation). 

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 

As will be indicated when the mechanism is discussed in more detail, discrete carbocations may not be formed in all cases. An unsymmetrical alkene will nevertheless follow Markownikoff s rule, because the partial positive charge that develops will be located preferentially at the carbon that is better able to accommodate the electron deficiency, that is, the more substituted one. 

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors.  

The order of reactivity of the hydrogen halides is HI HBr HCl, and reactions of simple alkenes with HCl are quite slow. The studies that have been applied to determining mechanistic details of hydrogen halide addition to alkenes have focused on the kinetics and stereochemistry of the reaction and on the effect of added nucleophiles. The kinetic studies often reveal complex rate expressions which demonstrate that more than one process contributes to the overall reaction rate. For addition of hydrogen bromide or Itydrogen 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second  

In the addition of hydrogen halides to alkenes, it is usually found that the nucleophilic halide ion becomes attached to the more-substituted carbon atom. This general observation is called Markovnikov s rule. The basis for this regioselectivity lies in the relative ability of the carbon atoms to accept positive charge. The addition of hydrogen halide is initiated by protonation of the alkene. The new C—H bond is formed from the 7T electrons of the carbon-carbon double bond. It is easy to see that if a carbo-cation is formed as an intermediate, the halide will be added to the more-substituted carbon, since protonation at the less-substituted carbon atom provides the more stable carbocation intermediate. 

The reaction probably involves interaction of a complex formed from the alkene and hydrogen halide with a second hydrogen halide molecule, since productive ter-molecular collisions are rare  

The addition of hydrogen halides to alkenes is catalysed by proton-exchanged zeolite A. Markovnikov addition occurs with a pure zeolite but 

In Section 6.1, we saw that HBr adds to ethene to give bromoethane. Next, let s consider the addition reaction of HBr to a symmetrical alkene such as m-2-butene. Only one product is possible because the two carbon atoms of the double bond are equivalent. 

The order of reactivity of hydrogen halides with aUtenes is HI HBr HCl, which corresponds to the order of decreasing acidity. The reaction is an electrophilic addition, in which the proton is the electrophile. The alkene, a Lewis base, accepts a proton from the acid and forms a carbocation. Then, the nucleophilic halide ion reacts with the carbocation to form the addition product. 

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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... 

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 to alkenes is not stereospecific. In contrast, addition of Br2 proceeds exclusively with anti stereochemistry. 

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 stereochemistry of addition of hydrogen halides to alkenes depends on the structure of the alkene and also on the reaction conditions. Addition of hydrogen bromide to cyclohexene and to E- and Z-2-butene is anti.6 The addition of hydrogen chloride to 1 -methylcyclopentene is entirely anti when carried out at 25° C in nitromethane.7... 

Table 4.1. Stereochemistry of Addition of Hydrogen Halides to Alkenes...

The addition of hydrogen halide to alkene is another classical electrophilic addition of alkene. Although normally such reactions are carried out under anhydrous conditions, occasionally aqueous conditions have been used.25 However, some difference in regioselectivity (Markovnikov and anti-Markovnikov addition) was observed. The addition product formed in an organic solvent with dry HBr gives exclusively the 1-Br derivative whereas with aq. HBr, 2-Br derivative is formed. The difference in the products formed by the two methods is believed to be due primarily to the difference in the solvents and not to the presence of any peroxide in the olefin.26... 

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. 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base.150 The best yields are obtained with sodium methoxide in methanol.151 If less basic conditions are desirable, the use of iodine monochloride and sodium acetate gives good yields.152 As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct ionic addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

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