Markovnikov’s rule

REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION MARKOVNIKOV S RULE... 

An article in the December 1988 issue of the Journal of Chemical Education traces the historical development of Markovnikov s rule In that article Markovnikov s name is spelled Markow nikoff which is the way it appeared in his original pa per written in German... 

Markovnikov s rule like Zaitsev s organizes experimental observations in a form suitable for predicting the major product of a reaction The reasons why it works will appear when we examine the mechanism of electrophilic addition m more detail... 

Let s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCH=CH2 (a) according to Markovnikov s rule and (b) opposite to Markovnikov s rule (a) Addition according to Markovnikov s rule... 

Most observations in chemistry come from ex periments If we do enough experiments we may see a pattern running through our observations A /aw 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... 

A theory is our best present interpretation of why things happen the way they do The modern ver Sion of Markovnikov s rule which is based on mecha nistic reasoning and carbocation stability recasts the rule in terms of theoretical ideas Mechanisms and explanations grounded in them belong to the theory part of the scientific method... 

Addition opposite to Markovnikov s rule is some times termed anti Markovnikov addition... 

Acid catalyzed hydration converts alkenes to alcohols with regioselectivity according to Markovnikov s rule Frequently however one needs an alcohol having a structure that corresponds to hydration of an alkene with a regioselectivity opposite to that of Markovnikov s rule The conversion of 1 decene to 1 decanol is an example of such a transformation... 

A proton and a halogen add to the double bond of an alkene to yield an alkyl halide Addition proceeds in ac cordance with Markovnikov s rule hy drogen adds to the carbon that has the greater number of hydrogens halide to the carbon that has the fewer hydro gens... 

Alkenes react with sulfuric acid to form alkyl hydrogen sulfates A proton and a hydrogen sulfate ion add to the double bond in accordance with Markovnikov s rule Alkenes that yield tertiary carboca tions on protonation tend to polymerize in concentrated sulfuric acid (Section 6 21)... 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Hydrogen halides add to alkynes in accordance with Markovnikov s rule to give alkenyl halides In the presence of 2 moles of hydrogen halide a second addition occurs to give a geminal dihalide... 

Acid catalyzed hydration of alkenes (Section 6 10) Water adds to the double bond in accordance with Markovnikov s rule... 

Hydroboration-oxidation of alkenes (Section 6 11) H and OF add to the double bond with a regioselectivity opposite to that of Markovnikov s rule This is a very good synthetic method addition is syn and no rearrangements are observed... 

Addition in accordance with Markovnikov s rule gives 1 2 dibromopropane Addition oppo site to Markovnikov s rule gives 1 3 dibromopropane... 

Anti Markovnikov addition (Sections 6 8 6 11) Addition re action for which the regioselectivity is opposite to that pre dieted on the basis of Markovnikov s rule... 

Electrophilic Addition. In the following example, an a-olefin reacts with a Lewis acid to form the most stable intermediate carbocation. This species, in turn, reacts with the conjugate base to produce the final product. Thus electrophilic addition follows Markovnikov s rule. 

Addition of chlorine or bromine in the presence of water can yield compounds containing haUde and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

Regioselectivity of Hydrogen Halide Addition Markovnikov s Rule... 

Under free-radical conditions in the presence of peroxides, addition takes place with a regioselectivity opposite to that of Markovnikov s rule. 

Unlike the addition of concentrated sulfuric acid to fonn alkyl hydrogen sulfates, this reaction is canied out in a dilute acid medium. A 50% water/sulfuric acid solution is often used, yielding the alcohol directly without the necessity of a separate hydrolysis step. Markovnikov s rule is followed ... 

There is a pronounced tendency for boron to become bonded to the less substituted carbon of the double bond. Thus, the hydrogen atoms of diborane add to C-2 of 1-decene, and boron to C-1. This is believed to be mainly a steric effect, but the regioselectivity of addition does conespond to Markovnikov s rule in the sense that hydrogen is the negatively polarized atom in a B—H bond and boron the positively polarized one. 

Addition of water to the double bond of an alkene takes place in aqueous acid. Addition occurs according to Markovnikov s rule. A carbocation is an intermediate and is captured by a molecule of water acting as a nucleophile. 

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