Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

When a hydrogen halide adds to an alkene, hydrogen adds to the carbon of the alkene that has the greater number of hydrogens attached to it, and the halogen to the carbon that has the fewer hydrogens. 

The first of these statements is close to the way Vladimir Markovnikov expressed it in 1870 the second is the way we usually phrase it now. These two statements differ in an important way—a way that is related to the scientific method. 

Adherence to the scientific method is what defines science. The scientific method has four major elements observation, law, theory, and hypothesis. 

Most observations in chemistry come from experiments. If we do enough experiments we may see a pattern running through our observations. A law is a mathematical (the law of gravity) or verbal (the law of diminishing returns) description of that pattern. Establishing a law can lead to the framing of a rule that lets us predict the results of future experiments. This is what the 1870 version of Markovnikov s rule is a statement based on experimental observations that has predictive value. 

As we have just seen, Markovnikov s rule can be expressed in two ways  

When a hydrr n halide adds to an alkene, protonation of the double bond occurs in the direction that gives the more stable carbocation. 

The product of the reaction is derived from the more stable carbocation—in this case, it is a tertiary carbocation that is formed more rapidly than a secondary one. 

In general, alkyl substituents increase the reactivity of a double bond toward electrophilic addition. Alkyl groups are electron-releasing, and the more electron-rich a double bond, the better it can share its tr electrons with an electrophile. Along with the observed regioselectivity of addition, this supports the idea that carbocation formation, rather than carbocation capture, is rate-determining. 

7 CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES 

Our belief that carbocations are intermediates in the addition of hydrogen halides to alkenes is strengthened by the observation 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. 

Addition begins in the usual way, by protonation of the double bond to give, in this case, a secondary carbocation. This carbocation can be captured by chloride to give 2-chloro-3-methylbutane (40%) or it can rearrange by way of a hydride shift to give a tertiary carbocation. The tertiary carbocation reacts with chloride ion to give 2-chloro-2-methylbutane (60%). 

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Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes... 

Although a carbocation is not the only possible intermediate in the addition of hydrogen halides to alkenes, the observation of characteristic carbocation rearrangement products strongly supports the role of a carboca-tion-like species in this reaction under these surface-mediated conditions. Addition of HCI to neat 3,3-dimethyl-l-butene (7) produced 60% of the rearranged product (2-chloro-2,3-dimethylbutane, 33) and about 40% of the imrearranged 2-chloro-33-dimethylbutane (34). Fahey and... 

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