Electrophilic addition: Markovnikov and the rearrangements

An alkene is a two-electron donor with the electrons parked in a π-bond that sits on the outside of the molecule, easy for anything electron-hungry to grab. Every reaction in this guide starts the same way: an electrophile takes those two electrons, breaks the π-bond, and leaves a carbocation. What happens next — where the nucleophile adds, whether the cation rearranges, whether stereochemistry is preserved — is what separates the mechanisms below. Once you have completed SN1 vs SN2 and E1 vs E2, electrophilic addition is the mirror image of elimination: instead of losing H and X across a σ-bond to make a π-bond, you use a π-bond to acquire two new σ-bonds.

The general mechanism in three steps

For most electrophilic additions:

1. π-bond attacks electrophile. The alkene’s electrons form a bond to whichever end of the electrophile is δ+. The other end of the electrophile (usually a halide or an oxygen) leaves as an anion. You now have a carbocation on one alkene carbon and a bond to the electrophile on the other.
2. Nucleophile captures the carbocation. The anion from step 1 (or the solvent, or something else in solution) attacks the cation, forming the second new σ-bond.
3. (Optional) Rearrangement between steps 1 and 2. If the initial carbocation can shift a hydrogen or a methyl group to give a more stable cation, it will. This step is silent in the product — you only see it as a shifted regiochemistry.

Two reactions in this family skip steps 2 and 3 by using a bridged intermediate instead of an open cation: bromination and hydroboration. Those we will handle separately.

Markovnikov’s rule, restated

Markovnikov’s 1870 observation: for HX addition to an unsymmetrical alkene, the hydrogen adds to the carbon that already has more hydrogens; the halogen adds to the more substituted carbon.

The modern explanation is not about the “richer getting richer” — it is about the intermediate. When HX protonates the alkene, the H can end up on either carbon, generating one of two possible carbocations. The reaction goes through the more stable cation. More substituted cations are more stable (tertiary > secondary > primary > methyl), so H adds to the carbon that leaves the more-substituted cation, and X ends up there. Markovnikov’s rule is a memory device; the real rule is “the mechanism goes through the more stable cation.”

This restatement matters because it predicts when Markovnikov fails. Any change that alters the cation-stability ranking flips the regiochemistry.

Regiochemistry map for common electrophiles

• HCl, HBr, HI → Markovnikov (halide on more substituted carbon).
• H₂O with H₂SO₄ catalysis → Markovnikov (OH on more substituted carbon). This is acid-catalysed hydration.
• HBr in the presence of peroxides (ROOR) → anti-Markovnikov (Br on less substituted carbon). The peroxide generates a bromine radical, and the reaction switches from cationic to radical addition; radicals have opposite selectivity because the more stable radical carbon is where the carbon radical sits, not where the halide sits. HCl and HI do not switch — only HBr, because Br-H and Br-Br bond strengths are the ones that enable the radical chain (McMurry §7-8).
• Hydroboration-oxidation (BH₃, then H₂O₂/OH⁻) → anti-Markovnikov (OH on less substituted carbon), and syn addition. Goes through a four-centered concerted transition state, not a cation, so Markovnikov’s logic does not apply. Sterics put the boron on the less substituted carbon; oxidation replaces B with OH stereospecifically.
• Br₂, Cl₂ in inert solvent → anti-addition of two halides. Goes through a bridged halonium ion, which the second halide opens by backside attack, giving trans-1,2-dihalides.
• Br₂ / H₂O (or Cl₂ / H₂O) → halohydrin, anti addition, Markovnikov placement of OH (OH on more substituted carbon). The bridged halonium is opened by water, which attacks the more substituted (more cation-like) carbon.

The one takeaway that survives all of these: whenever the mechanism goes through an open carbocation, Markovnikov applies. When it goes through a bridged intermediate or a concerted addition, the rule can flip.

Carbocation rearrangements

The single biggest source of unexpected products in electrophilic addition is a hydride shift or methyl shift between steps 1 and 2. If the initial cation is secondary but a 1,2-hydride shift would produce a tertiary cation, the shift happens faster than nucleophile capture, and the product reflects the rearranged position.

Example: HCl addition to 3-methyl-1-butene. Protonation of the alkene gives a secondary cation at C2. A 1,2-hydride shift from C3 (which sits between the cation and a tertiary carbon) yields a tertiary cation at C3. Cl⁻ captures the tertiary cation, giving 2-chloro-2-methylbutane, not 2-chloro-3-methylbutane as naive Markovnikov would predict. Clayden Ch. 19 has the arrow-pushing diagram; the shift is the diagnostic sign that a mechanism goes through an open cation.

Rules for spotting a rearrangement risk:

• Initial cation is secondary and there is an adjacent tertiary center → hydride shift almost certain.
• Initial cation is secondary and there is an adjacent quaternary center bearing a methyl → methyl shift possible.
• Initial cation is tertiary → no rearrangement typically needed (already stable).
• Initial cation is primary → does not form under solution conditions; a different mechanism must be operating.

If you see an addition product that does not match Markovnikov and you know there are no peroxides, rearrangement is the second thing to check.

Worked example — HBr addition to 2-methylpropene

Stereochemistry: syn, anti, and racemic

Electrophilic addition mechanisms fall into three stereochemical categories:

• Cationic (HCl, HBr, HI, H₂O/H⁺): open cation is planar, sp² hybridized. Nucleophile can attack from either face. Product is racemic if the cation carbon is a new stereocenter.
• Bridged halonium (Br₂, Cl₂, Br₂/H₂O): the halonium ion blocks one face. Nucleophile attacks from the opposite face — anti-addition, stereospecific.
• Concerted (hydroboration, ozonolysis, hydrogenation): both new bonds form on the same face — syn-addition, stereospecific.

If a problem tells you the product’s stereochemistry, you can often back out which mechanism was operating. Anti addition of two Br across a cyclopentene → halonium. Syn addition of B and H across a bicyclic alkene → hydroboration. Racemic mixture with H and Br → open cation from HBr.

When alkenes fail to react

Not every alkene is a good electrophilic-addition substrate. The reaction slows dramatically when:

• The alkene is electron-poor — an α,β-unsaturated carbonyl (enone) has a carbonyl group pulling electron density out of the π-bond. Enones do the opposite: they undergo nucleophilic addition (Michael addition), not electrophilic.
• The alkene is sterically crowded — tetrasubstituted alkenes react slowly with bulky electrophiles because there is no room for a bridged intermediate to form.
• The alkene is inside an aromatic ring — benzene does not do electrophilic addition under normal conditions; it does electrophilic aromatic substitution to preserve the aromaticity. This is the qualitative reason arenes are treated as a separate reaction family.

Practice problems

Where to go next

• The degrees of unsaturation calculator will confirm your addition product carries one fewer π-bond than the starting alkene — a quick check against ring closures or over-addition.
• The next mechanism family to master is electrophilic aromatic substitution (nitration, sulfonation, halogenation of benzene) — arenes protect their π-electrons by substituting rather than adding, and the substitution version has its own regiochemistry rules (ortho/para directors vs meta directors).
• If a problem hands you an alkene with an unusual substituent, functional groups cheatsheet is a quick classifier.

The core discipline: identify the electrophile, identify the more stable cation (or the bridged intermediate), and only then draw the product. Skipping straight to “H goes to less-substituted, X goes to more-substituted” fails as soon as peroxides or rearrangements enter the picture.