Markovnikov’s rule was proposed by Russian chemist Vladimir Markovnikov in the year 1869. It is the condition that applies to the unsymmetrical alkene or alkyne predicting the regiochemistry of the addition of hydrogen halide to give alkyl halide. As per the Markovnikov rule,
‘For an unsymmetrical alkene or alkyne, when treated with hydrogen halide HX, the negative part of the reagent attaches to carbon having less number of hydrogen across the double bond giving alkyl halide.’
In short, the negative ion attacks the most substituted carbon with less hydrogens across the double bond. A mnemonic or shortcut to remember this is-
‘Markovnikov says - NO Member Shall Cheat.'
NO stands for Negative iOn
Member Shall Cheat for Most Substituted Carbon.
The negative ion comes from the hydrogen halide (HX) that is polar in nature. The electronegative halogen carries a partial negative charge and the hydrogen a partial positive charge. Amongst the hydrogen halides, the reactivity order is-
The acidity of HI is highest in the group as it is easy to break the HI bond. The conjugate base I- is more stable and nucleophilic than HBr, HCl and least is for HF.
The addition of haloacid (HX) to the double bond is a two step process and goes via the electrophilic addition mechanism.
In the first step, the π bond of the nucleophilic alkene first picks up the positively charged proton to give a carbocation intermediate. As the 2o carbocations are stabilized by hyperconjugation and induction it is more stable than 1o hence when a double bond picks up a proton; it tends to form more stable 2o carbocation than 1o.
After the formation of the carbocation at most substituted carbon, the attack of the negatively charged halogen takes place giving the alkyl halide. The loss of one π bond and formation of two σ bonds makes the reaction exothermic and energetically favorable.
If two carbon atoms of the double bond are equally substituted by hydrogen, then two products would be formed in equal ratios.
When hydrogen halide adds to the alkyne, it gives vinyl halide. In the presence of an excess of Hydrogen halide, a second addition of HX results in geminal dihalides. The mechanism is similar to the alkene addition forming a stable carbocation followed by the attack of the bromide ion.
Let’s look at few examples of alkenes following Markovnikov's Rule for the addition of HBr wherein the bromide will add to the most substituted carbon or the carbon carrying less hydrogens across the double bond-
From 1-Propene after Markovnikov’s addition of HBr, we get 2-bromopropane.
2-methyl-prop-1-ene on the addition of HBr according to the rule gives 2-bromo-2-methyl-propane.
2- butene is a symmetrical alkene (equally substituted) and it need not follow a rule for the addition of HBr across the double bond. Addition from either side would give the same product.
The symmetrical nature of 2-butene is lost on substituting a methyl at the 3rd position to give 3-methyl-but-2-ene and it now it obeys Markovnikov’s rule to give 2-bromo-2-methylbutane.
Remember, the formation of most stable carbocation is preferred by Markovnikov’s rule.
The Markovnikov reaction rule for addition to the unsymmetrical alkene is not only followed by the hydrogen halide but also by other reagents such as water, halogen, and water, Water and H2SO4, Iodine monochloride, with the more electronegative atom carrying the negative charge.
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