# What is "Markownikow's rule"?

Nov 20, 2017

Well, $\text{Markownikow's rule}$ was an old observation that described the outcome of addition reactions to olefins....

#### Explanation:

Consider an unsymmetrical olefin, say propylene....$H B r$ will add to give EITHER $\text{1-propyl bromide}$, or $\text{2-propyl bromide}$, viz.

${H}_{2} C = C H C {H}_{3} + H B r \rightarrow {\underbrace{B r {H}_{2} C - C {H}_{2} C {H}_{3}}}_{\text{minor product}}$ $+ {\underbrace{{H}_{3} C - C B r H C {H}_{3}}}_{\text{major product}}$

The product distribution reflects the stability of the carbocation intermediate. The olefin is a nucleophile; it seeks an electrophile, and the protium ion of $H B r$ is the likely target.....now it could react at $\stackrel{2}{C}$ of the olefin to give a primary carbocation....

${H}_{2} \stackrel{+}{C} - C {H}_{2} C {H}_{3}$

....or it could react at $\stackrel{1}{C}$ of the olefin to give a secondary carbocation....

${H}_{3} C - \stackrel{+}{C} H C {H}_{3}$

$B {r}^{-}$ COULD react with EITHER of these intermediates.....

In terms of carbocation stability, ${\underbrace{{3}^{\circ} > {2}^{\circ} > {1}^{\circ} > {H}_{3} {C}^{+}}}_{\rightarrow \text{decreasing stability} \rightarrow}$, and thus the most likely reaction pathway is thru the most stable intermediate, and in the given example this is via the ${2}^{\circ}$ carbocation. And thus $\text{2-bromopropane}$ should be the species that dominates in the product mix. And this is the experimental observation.

$\text{TO HIM THAT HATH HYDROGEN SHALL BE GIVEN}$ $\text{HYDROGEN}$

And thus the observed $\text{Markownikow's rule}$ reflects carbocation stability. Sometimes reagents can be added that give $\text{anti-Markownikow additions}$, for instance peroxides....I urge you to read the relevant chapter in your text.