Electrophilic Addition to Alkenes

A representative example — structures drawn live.

The π bond is the nucleophile

A carbon–carbon double bond is a region of high electron density: the two π electrons sit above and below the plane of the σ framework and are loosely held. That makes the alkene a nucleophile. In electrophilic addition, the π electrons reach out to attack an electrophile (an electron-poor species), breaking the π bond and forming a new σ bond. This first step generates a positively charged intermediate — a carbocation or a bridged ion — which is then captured by a nucleophile in the second step. Overall, two new groups add across the former double bond, converting the alkene into a saturated product.

Addition of HX: Markovnikov and carbocations

When an alkene reacts with a hydrogen halide (HCl, HBr, HI), the π bond grabs the H (the electrophilic end), forming a carbocation and releasing X^–. The proton adds to one alkene carbon; the positive charge lands on the other. The molecule chooses the pathway that produces the more stable carbocation — tertiary > secondary > primary, thanks to hyperconjugation and induction. Then the halide adds to that cationic carbon.

The net regiochemical outcome is Markovnikov's rule: the hydrogen adds to the carbon that already has more hydrogens, and X adds to the more substituted carbon. A modern way to state it: "the electrophile adds so as to give the more stable cation." For example, HBr + 2-methylpropene gives 2-bromo-2-methylpropane, because protonating the terminal CH₂ gives a tertiary cation.

Watch for rearrangements. Because a discrete carbocation forms, it can shift a hydride or alkyl group to reach an even more stable cation. Whenever a 1,2-hydride or methyl shift would upgrade a secondary cation to tertiary, expect a rearranged product.

Acid-catalyzed hydration: adding H–OH

Treating an alkene with water and a catalytic strong acid (typically dilute H₂SO₄) adds H and OH across the double bond to give an alcohol. The mechanism mirrors HX addition: (1) the π bond is protonated to form the more stable carbocation, (2) water attacks the cation, and (3) the resulting oxocarbenium/protonated alcohol loses a proton to regenerate the acid catalyst. The regiochemistry is Markovnikov — the OH ends up on the more substituted carbon. Because it, too, runs through a free carbocation, hydration is also prone to rearrangements.

Halogenation: anti addition through a halonium ion

Alkenes react with Br₂ or Cl₂ to give vicinal dihalides (a halogen on each of the two adjacent carbons). The mechanism is different in an important way. As X₂ approaches, the π electrons attack one halogen and displace the other; instead of an open carbocation, a three-membered cyclic halonium ion (bromonium or chloronium) forms, bridging both carbons. The released X^– then attacks from the opposite face, opening the ring.

Because the two halogens add to opposite faces, halogenation is a stereospecific anti addition. The bridged intermediate also blocks the rearrangements you see with open carbocations, so halogenation is "cleaner" in that respect.

Halohydrin formation: halogen plus water

If the halogenation is run in water (or another nucleophilic solvent) instead of an inert one, the halonium ion is intercepted by water rather than by halide. The product is a halohydrin — a molecule with –X on one carbon and –OH on the adjacent carbon. Two selectivity rules follow directly from the mechanism:

• Anti addition: water opens the halonium from the back face, so X and OH end up anti, just as in dihalide formation.
• Markovnikov-like regiochemistry: water attacks the more substituted carbon of the halonium ion, because that carbon carries more partial positive charge. So the OH goes on the more substituted carbon and the halogen on the less substituted one.

The unifying pattern

Every reaction here is the same idea wearing different clothes: the nucleophilic π bond attacks an electrophile, forming a cationic intermediate, and a nucleophile then adds to whichever carbon best stabilizes the positive charge. When that intermediate is an open carbocation (HX, hydration), you get Markovnikov regiochemistry with possible rearrangements and non-stereospecific addition. When it is a bridged halonium ion (halogenation, halohydrins), you get anti stereochemistry, no rearrangement, and nucleophile attack at the more substituted carbon. Learn the intermediate and the outcome follows.