An alkene is a hydrocarbon that contains at least one carbon–carbon double bond.
The general formula is \$CnH{2n}\$, e.g. \$C2H4\$ (ethene) or \$C3H6\$ (propene).
In an addition reaction, atoms or groups are added across the double bond, turning it into a single bond.
Because the double bond is the only reactive site, the reaction usually gives a single, predictable product.
When a simple reagent like \$HX\$ (where \$X\$ = F, Cl, Br, I) reacts with an alkene, the mechanism follows the Markovnikov rule:
Because the orientation is fixed by this rule, only one product can form.
Think of it like a puzzle piece that can only fit in one spot – there’s no alternative arrangement.
| Alkene | Reagent \$HX\$ | Product (Markovnikov) |
|---|---|---|
| \$C2H4\$ (ethene) | \$HBr\$ | \$CH3CH2Br\$ (1‑bromopropane) |
| \$C3H6\$ (propene) | \$HCl\$ | \$CH3CHClCH3\$ (2‑chloropropane) |
| \$C4H8\$ (but-1‑ene) | \$HBr\$ | \$CH3CH2CHBrCH_3\$ (2‑bromobutane) |
Imagine a row of dominoes (the double bond) that can only fall in one direction.
When you push the first domino (add \$HX\$), the rest follow in a single, predictable pattern.
Just like that, the addition of \$HX\$ to an alkene leads to one clear product.
In a simple addition reaction of an alkene with \$HX\$, the Markovnikov rule ensures that only one product is formed.
This makes it easier to predict the outcome and plan your experiments.