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Inorganic chemistry,Markovnikov's rule orMarkownikoff's rule describes the outcome of someaddition reactions. The rule was formulated by Russian chemistVladimir Markovnikov in 1870.[1][2][3]
The rule states that with the addition of aprotic acid HX or other polar reagent to an asymmetricalkene, the acid hydrogen (H) or electropositive part gets attached to the carbon with more hydrogen substituents, and thehalide (X) group or electronegative part gets attached to the carbon with more alkyl substituents. This is in contrast to Markovnikov's original definition, in which the rule states that the X component is added to the carbon with the fewest hydrogen atoms while the hydrogen atom is added to the carbon with the greatest number of hydrogen atoms.[4]
The same is true when an alkene reacts with water in an additional reaction to form an alcohol that involves carbocation formation. Thehydroxyl group (OH) bonds to the carbon that has the greater number of carbon-carbon bonds, while the hydrogen bonds to the carbon on the other end of the double bond, that has more carbon–hydrogen bonds.
The chemical basis for Markovnikov's Rule is the formation of the most stablecarbocation during the addition process. Adding the hydrogen ion to one carbon atom in the alkene creates a positive charge on the other carbon, forming a carbocation intermediate. The more substituted the carbocation, the more stable it is, due toinduction andhyperconjugation. The major product of the addition reaction will be the one formed from the more stable intermediate. Therefore, the major product of the addition of HX (where X is some atom more electronegative than H) to an alkene has the hydrogen atom in the less substituted position and X in the more substituted position. But the other less substituted, less stable carbocation will still be formed at some concentration and will proceed to be the minor product with the opposite, conjugate attachment of X.
Also calledKharasch effect (named afterMorris S. Kharasch), these reactions that do not involve acarbocation intermediate may react through other mechanisms that haveregioselectivities not dictated by Markovnikov's rule, such asfree radical addition. Such reactions are said to beanti-Markovnikov, since the halogen adds to the less substituted carbon, the opposite of a Markovnikov reaction.
The anti-Markovnikov rule can be illustrated using the addition ofhydrogen bromide to isobutylene in the presence of benzoyl peroxide or hydrogen peroxide. The reaction of HBr with substituted alkenes was prototypical in the study of free-radical additions. Early chemists discovered that the reason for the variability in the ratio of Markovnikov to anti-Markovnikov reaction products was due to the unexpected presence of free radical ionizing substances such as peroxides. The explanation is that the O-O bond in peroxides is relatively weak. With the aid of light, heat, or sometimes even just acting on its own, the O-O bond can split to form 2radicals. The radical groups can then interact with HBr to produce a Br radical, which then reacts with the double bond. Since the bromine atom is relatively large, it is more likely to encounter and react with the least substituted carbon since this interaction produces less static interactions between the carbon and the bromine radical. Furthermore, similar to a positive charged species, the radical species is most stable when the unpaired electron is in the more substituted position. The radical intermediate is stabilized byhyperconjugation. In the more substituted position, more carbon-hydrogen bonds are aligned with the radical's electron deficient molecular orbital. This means that there are greater hyperconjugation effects, so that position is more favorable.[5] In this case, the terminal carbon is a reactant that produces a primary addition product instead of a secondary addition product.
A new method of anti-Markovnikov addition has been described by Hamilton and Nicewicz, who utilize aromatic molecules and light energy from a low-energy diode to turn the alkene into a cation radical.[6][7]
Anti-Markovnikov behaviour extends to more chemical reactions than additions to alkenes. Anti-Markovnikov behaviour is observed in thehydration ofphenylacetylene by auric catalysis, which givesacetophenone; although with a specialruthenium catalyst[8] it provides the otherregioisomer2-phenylacetaldehyde:[9]
Anti-Markovnikov behavior can also manifest itself in certainrearrangement reactions. In atitanium(IV) chloride-catalyzed formalnucleophilic substitution atenantiopure1 in the scheme below, two products are formed –2a and2b Due to the two chiral centers in the target molecule, the carbon carrying chlorine and the carbon carrying the methyl and acetoxyethyl group, four different compounds are to be formed: 1R,2R- (drawn as 2b) 1R,2S- 1S,2R- (drawn as 2a) and 1S,2S- . Therefore, both of the depicted structures will exist in a D- and an L-form. :[10]
This product distribution can be rationalized by assuming that loss of thehydroxy group in1 gives the tertiarycarbocationA, which rearranges to the seemingly less stable secondary carbocationB. Chlorine can approach this center from two faces leading to the observed mixture of isomers.
Another notable example of anti-Markovnikov addition ishydroboration.
