Markovnikov's rule

The Markovnikov rule (after Vladimir Vasilyevich Markovnikov ) describes in organic chemistry, the products of electrophilic addition of hydrogen halides to carbon-carbon double bonds in unsymmetrical alkenes.

The rule states that in the addition of hydrogen halides to asymmetric alkenes the hydrogen atom is always bound to the already hydrogen -rich carbon atom. The halogen atom is accordingly bound to the hydrogen- poor, ie more highly substituted carbon atom. During the addition of water, the same effect is observed: the H atom is attached to the lower -substituted carbon atom, the OH group at the more highly substituted. So you can say generalizes that δElektrophil - Nucleophilδ - follow this rule ( exceptions: see below). As a mnemonic is: " He who has, more will be given ".

The cause of this course is that a positively charged and as well as possible stabilized carbenium ion is formed as an intermediate in an electrophilic addition. Is the positive charge on a tertiary carbon atom, it is stabilized by three I- effects of the adjacent alkyl groups. A secondary carbon atom is stabilized by only two I- effects and a primary by a. Besides I- effects also hyperconjugation causes a stabilization of the carbenium ion. Therefore, tertiary carbenium ions are less energetic and thus more stable than secondary, and these in turn more stable than primary carbenium ions. It formed almost exclusively tertiary carbenium ions.

If the stability of the two possible transition state carbenium ions but not determined exclusively by I effect of alkyl substituents, and anti -Markovnikov products may occur. Thus we obtain for the addition of HCl to acrylic acid ( CH2 = CH- COOH) for example, only 3-chloro propionic acid, because the more stable carbenium ion during the reaction not pulling in the immediate vicinity of the electron carboxy group (-I effect) arises.

In the radical addition and hydroboration the Markownikowregel not apply. Again, anti-Markovnikov products are obtained in which the hetero atom is bonded to the lower substituted carbon atom.

Emergence of alkenes from the Markovnikov product of the primary addition, can the direction ( regioselectivity ) be the elimination described by the terms Saytzeff rule and Hofmann product.

Example

Example of a reaction according to Markovnikov:

It is seen that the hydrogen atom added to the carbon atom (position 1), which carries more hydrogen atoms and the halogen ( here chlorine) at the more substituted carbon ( 2-position ) is added. Since the formation of the product of Markovnikov rule followed, it is called " Markovnikov product."

Product formation that would not follow the Markovnikov rule:

The anti -Markovnikov product forms, inter alia, to under the above conditions.

Criticism and alternatives

The application of the historically important Markovnikov rule leads to incorrect predictions about the expected addition products to unsymmetrical alkenes, when can form resonance-stabilized carbenium ions as intermediates. The mesomeric effects on play ( overcompensate ) the inductive effects. It always occurs that as an intermediate carbonium ion, which is stable. This automatically results in the addition direction.