Hydrogen chloride to alkenes

Hydrochlorination. The addition of hydrogen chloride to alkenes in the absence of peroxides takes place by an electrophilic substitution mechanism. The orientation is in accord -with Markovnikov s mle in -which the hydrogen atom adds to the side of the double bond that -will result in the... 

Product mixtures from radical-chain addition of hydrogen chloride to alkenes are much more complicated than is the case for addition of hydrogen bromide. The problem is that the rate of abstraction of hydrogen from hydrogen chloride is not fast relative to the rate of addition of the alkyl radical to the alkene. This results in the formation of low-... 

Exercise 10-24 Write two different radical mechanisms for peroxide-initiated addition of hydrogen chloride to alkenes and consider the energetic feasibility for each. 

Studies aimed at 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. Kinetic studies often reveal rate expressions that indicate that more than one process contributes to the overall reaction rate. For addition of hydrogen bromide or hydrogen chloride to alkenes, an important contribution to the overall rate is often made by a third-order term. 

The kinds of molecules we know how to make will increase sharply as a result of material in Chapters 9 and 10. Addition of hydrogen bromide and hydrogen chloride to alkenes provides access to alkyl halides. We already have the SnI and Sn2 reactions available for transforming the alkyl halides into other compounds (Fig. 7.94, p. 312). 

In Section 9.3 (p. 366), the addition of hydrogen chloride to alkenes was presented with the reaction always leading to Markovnikov addition. Chapter 9 stressed the initial formation of the more stable carbocation followed by addition of the residual halide ion. The product of Markovnikov addition must be produced in this reaction (Fig. 11.23). Of course, HBr reacts the same way as HCl. 

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... 

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

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 ... 

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... 

A more complex cumulenyl carbenoid 80 may be generated in situ from 1,4-dihalobut-2-ynes and two equivalents of base (Scheme 3.21). Insertion into organozirconocene chlorides gives allenyl zirconium species 81, which are regioselectively protonated to afford enyne products 82 [38], The stereochemistry of the alkene in 82 stems from the initial elimination of hydrogen chloride to form 80. 

Exercise 10-18 Predict the predominant product from addition of hydrogen chloride to each of the following alkenes. Give your reasoning. 

Addition of hydrogen chloride and hydrogen iodide to alkenes follows Markovnikov s rule. Until 1933 the situation with respect to hydrogen bromide Was exceedingly confused. It had been reported by some workers that addition of hydrogen bromide to a particular alkene yields a product in agreement with Markovnikov s rule by others, a product in contradiction to Markovnikov s rule and by still others, a mixture of both products. It had been variously reported that the product obtained depended upon the presence or absence of water, or of light, or of certain metallic halides it had been reported that the product obtained depended upon the solvent used, or upon the nature of the surface of the reaction vessel. 

Addition of hydrogen chloride to three typical alkenes is outlined below, with the two steps of the mechanism shown. In accord with Markovnikov s rule, propylene yields isopropyl chloride, isobutylene yields icrr-butyl chloride, and 2-methyl-2-butene yields rer/-pentyl chloride. 

The stereochemistry of the second step of an addition initiated by a nonbridging electrophile like a proton will be controlled by which surface of the intermediate cation 5.57 is more easily attacked by the nucleophile. The addition of hydrogen chloride to an alkene is not stereospecifically anti, because the chloride does not necessarily attack the cation either specifically anti or syn to the proton,478 in contrast to addition initiated by bridging electrophiles like bromine, or metallic electrophiles like the mercuric ion, described below. The stereochemistry will depend instead on ion pairing or on the substituents in the cation 5.57, and how they influence the conformation at the time the nucleophile attacks. 

The stereochemistry of addition of hydrogen halides to unconjugated alkenes is usually anti. This is true for addition of HBr to l,2-dimethylcyclohexene, cyclohexene, 1,2-dimethylcyclopentene, cyclopentene, Z- and -2-butene, and 3-hexene, among others. Anti stereochemistry is also dominant for addition of hydrogen chloride to 1,2-dimethylcyclohexene and l-methylcyclopentene. Temperature and solvent can modify the stereochemistry, however. For example, although the addition of HCl to 1,2-dimethylcyclohexene is anti near room temperature, syn addition dominates at —78°C. °... 

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... 

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