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What is Markownikoff’s/ Markovnikov’s Rule?

Alkenes undergo addition reactions where the substrate (alkene) and the reagent combine to form an adduct. One pie bond of the alkene is sacrificed to create two new stronger sigma bonds with the reagent in the reaction.

If the alkene is symmetrical with the same number of groups on either end, in that case, the alkene adds to the reagent to form one primary product. However, unsymmetrical alkenes with different groups across the double bond can have more than one way of attachment.

symmetrical_versus_unsymmetrical_alkene

The mystery of the attachment preference was solved by a Russian chemist Vladimir Markovnikov who in 1869 proposed a rule for the addition reactions between unsymmetrical alkenes/ alkynes and hydrogen halides.  According to him,

‘In the addition reaction between an unsymmetrical alkene or alkyne and hydrogen halide (HX) to form alkyl halide, the negative part of the reagent (X-) always attaches to the more substituted carbon.’

Across the double bond, a more substituted carbon always has fewer hydrogen atoms and a greater number of carbon atoms attached to it. The rule helps in predicting the regiochemistry of the reaction, that is, the decision on where the incoming groups will connect on the double/triple bond.

Markovnikov_rule

A mnemonic or shortcut to remember this is-

‘Markovnikov says - NO Member Shall Cheat.'

Where,

NO stands for Negative iOn

Member Shall Cheat for Most Substituted Carbon.

mnemonic_markovnikov_rule

Reactivity of Hydrogen Halides towards Markovnikov’s reaction

The hydrogen halides (HX) are polar due to the difference in electronegativity. The halogen is more electronegative than the Hydrogen and pulls the bond electrons towards itself. Therefore, the electronegative halogen carries a partial negative charge (δ-) and the Hydrogen a partial positive charge (δ+).

Amongst the hydrogen halides, the reactivity order is-

H-I>H-Br>H-Cl>H-F

The reactivity of HI is highest in the group as it is easy to break the HI bond, due to its long bond length. Post the breakage of the H-I bond, the conjugate ion I- formed is more stable (due to its larger size to charge distribution) and nucleophilic than conjugate ions of HBr, HCl, and HF. The reactivity is least for HF. 

Reactivity_hydrogen_halide_markovnikov_rule

Markovnikov Reaction Mechanism

The addition of hydrogen halide (HX) to the alkene double bond is a two-step process via the electrophilic addition mechanism.

The hydrogen halide reversibly exists as H+ and X ions in the reaction. In the first step, the pie bond of the nucleophile alkene picks up the positively charged proton (H+) to give a carbocation intermediate.

The carbocation formed is a sturdy 20 carbocation that is stabilized by the electron-donating positive inductive effect of the neighboring alkyl groups. Therefore, when a double bond picks up a proton, it forms a more stable 2o carbocation at the most substituted carbon than 1o carbocation at least substituted carbon.

markovnikov_reaction_mechanism


After the carbocation formation, in the second step of the reaction, the negatively charged halogen attacks the carbocation center to form an alkyl halide. The net result is an exothermic reaction with the loss of one π bond and the formation of two σ bonds, an energetically favorable process.

Equally substituted unsymmetrical alkene

In an unsymmetrical alkene, if the two carbon atoms of the double bond are equally substituted with the hydrogens, in that case, the halogen would attach to both ends in equal probability to form two products in equal ratios.

equally_substituted_unsymmetrical_alkene

Markovnikov Reaction in Alkynes

The addition reactions of alkynes and hydrogen halide give vinyl halides. In vinyl halides, the halogen is attached to the double bond. If an excess of Hydrogen halide is used, the second HX addition generates geminal dihalides where two halogens are connected to the same carbon atom. The mechanism proceeds like an alkene addition reaction with the formation of a stable carbocation followed by the attack of the bromide ion.

markovnikov_alkynes

Below are a few examples of alkenes following Markovnikov's Rule for the addition of HBr. Here the bromide will add to the most substituted Carbon, that is, the Carbon carrying fewer Hydrogens across the double bond.

Examples_markovnikov_rule

In 1-Propene addition of HBr gives 2-bromopropane.

2-methyl-prop-1-ene on the addition of HBr gives 2-bromo-2-methylpropane.

In example 3, the 2- butene is a symmetrical alkene. It does not follow Markovnikov’s rule for the addition of HBr across the double bond. The addition of Br- from either end would give the same product.

In example 4, the symmetrical nature of 2-butene is lost on substituting a methyl at the 3rd position to give 3-methyl-but-2-ene. It now obeys Markovnikov’s rule to form 2-bromo-2-methylbutane.

Few other reagents that follow Markovnikov’s rule for addition across the unsymmetrical alkene are- water (H2O), halogen (X2), and hypohalous acid (HOX), H2SO4, Iodine monochloride (ICl), etc. Reagent add to form corresponding products with various other functional groups as shown below.

functional_groups_markovnikov

HBr in the presence of peroxide adds across the double bond of unsymmetrical alkene opposite to Markovnikov’s rule, therefore called the Anti-Markovnikov’s Rule. The Bromine attaches to the less substituted carbon in the reaction, and the reaction mechanism proceeds by radical formation.

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