The major product is determined by the relative stability of the carbocations that could form. HBr normally adds through the pathway producing the more stable carbocation, which leads to the product predicted by Markovnikov's rule.
In this electrophilic addition reaction, the electron-rich carbon-carbon double bond acts as a nucleophile. Its electron pair forms a bond with , while the H bonded to Br bond breaks by heterolytic fission, producing .
For an unsymmetrical alkene, the proton can bond to either carbon of the double bond. These alternatives produce different carbocation intermediates:
| Protonation pathway | Carbocation formed | Consequence |
|---|---|---|
| H bonds to the carbon already carrying more H atoms | More substituted carbocation | Favoured pathway and major product |
| H bonds to the carbon carrying fewer H atoms | Less substituted carbocation | Less favoured pathway and minor product |
Carbocation stability generally follows:
Alkyl groups stabilize the positively charged carbon by donating electron density. Therefore, the pathway with the more substituted carbocation has a lower-energy intermediate and occurs more readily. The bromide ion then donates an electron pair to the carbocation, forming the C bonded to Br bond.
For example, propene reacts as follows:
Protonation at the terminal carbon produces a secondary carbocation at carbon 2, so 2-bromopropane is the major product. The alternative pathway produces a less stable primary carbocation and therefore only a minor amount of 1-bromopropane.
A common misconception is that Markovnikov's rule is the cause of the product distribution. It is actually a convenient description of the outcome; carbocation stability is the mechanistic explanation.
Exam technique: For an IB Chemistry HL question, draw both possible carbocations, classify each as primary, secondary, or tertiary, and use relative stability to justify the major product. Curly arrows must begin at an electron pair or bond and end where the new bond forms.
