Markovnikov / anti-Markovnikov — Alkenes & Alkynes

When HBr adds to propene, the bromine could land on either carbon of the double bond. It lands overwhelmingly on one of them. Explaining why — and then learning how to make it land on the other one instead — is one of the most practically useful things in this chapter, because it turns a reaction that gives a product into a pair of reactions that give whichever product you want.

First, the word: regiochemistry

Regiochemistry is the question of which atom a reagent ends up attached to when more than one position was available. It is not about three-dimensional arrangement — that is stereochemistry — but purely about position. Asking which carbon of an alkene takes the bromine is a regiochemistry question.

Keeping the two words separate matters, because a reaction can be highly regioselective and not stereoselective at all, or the reverse. Hydrohalogenation is the first; bromination is the second.

Markovnikov's rule, restated mechanistically

The classic statement is "the hydrogen adds to the carbon that already has more hydrogens." That is a correct summary and a poor explanation, and it fails on the variants below.

The real content is one deeper fact: the carbocation forms wherever it is most stable. Protonating the less substituted carbon leaves the positive charge on the more substituted one, which is the better cation — 3° > 2° > 1° > methyl, by hyperconjugation and inductive donation from the alkyl groups. The nucleophile then attacks there, so the halogen or hydroxyl ends up on the more substituted carbon.

Once you hold it this way, the rule generalizes. A cation stabilized by resonance beats one stabilized by alkyl groups, so adding HBr to styrene, PhCH=CH₂, puts the bromine on the benzylic carbon even though that carbon is formally secondary. "More hydrogens" would not have told you that; "more stable cation" does.

Markovnikov selectivity is a statement about the transition state leading to the cation, not about the product. Because the rate-determining step is protonation, the pathway with the lower barrier wins — and the barrier is lower for the route to the more stable cation. This is Hammond's postulate in action: for an endothermic step, the transition state resembles the product of that step, so anything stabilizing the cation also stabilizes the transition state leading to it.

Hydroboration–oxidation: a genuinely different mechanism

Same alkene, same element added, opposite ends — and the difference is entirely mechanistic. Acid-catalysed hydration goes through a carbocation, so the proton adds wherever leaves the better cation behind and the OH is stuck with the other carbon. Hydroboration has no cation at all: boron and hydrogen add across the double bond in one concerted step, with boron taking the less hindered carbon. Markovnikov is a consequence of a mechanism, not a law, and changing the mechanism changes the answer.Markovnikov is not a rule about hydrogens being rich or poor. It is a statement about which carbocation formed — and a mechanism with no carbocation in it is free to ignore it entirely. That is exactly what hydroboration does.

BH₃, usually as its THF or dimethyl sulfide complex, reacts with an alkene in a single concerted step: boron and hydrogen add across the double bond simultaneously, from the same face, with no intermediate at all.

Because there is no carbocation, regiochemistry cannot be set by cation stability. It is set by sterics and by the polarity of the B–H bond: boron is less electronegative than hydrogen, so boron is the electrophilic end, and the bulky boron preferentially bonds to the less hindered, less substituted carbon. The hydrogen goes to the more substituted one.

A subsequent oxidation with H₂O₂ and NaOH replaces the boron with OH, with complete retention of both position and configuration. The net result is OH on the less substituted carbon, added syn with the hydrogen — the opposite regiochemistry from acid-catalyzed hydration, hence "anti-Markovnikov."

Because hydroboration–oxidation has no carbocation, it never rearranges. That makes it valuable twice over: when you need the anti-Markovnikov alcohol, and when you need any alcohol from a substrate whose cation would shift. Its syn stereochemistry is a third benefit — both new groups arrive on the same face, so on a ring the product is a single, predictable diastereomer.

Worked example — one alkene, two alcohols

Start with 1-methylcyclohexene and ask for each hydration route.

H₃O⁺: protonation gives the tertiary cation at C1; water attacks there. Product: 1-methylcyclohexan-1-ol, the Markovnikov alcohol, with OH on the more substituted carbon.

BH₃ then H₂O₂/NaOH: boron adds to the less substituted C2 and hydrogen to C1, syn. Product: trans-2-methylcyclohexan-1-ol — anti-Markovnikov, and with defined relative stereochemistry because the syn addition fixed both new centers at once.

Same starting material, two different regiochemistries and two different stereochemical outcomes, chosen by reagent.

Radical HBr addition: anti-Markovnikov by another route

In the presence of peroxides (ROOR), HBr addition abandons the ionic pathway for a radical chain. A peroxide fragments to give radicals, which abstract H from HBr to give a bromine radical — and it is the bromine radical, not H⁺, that adds first.

The bromine radical adds to whichever carbon leaves the more stable radical behind, and radical stability follows the same 3° > 2° > 1° ordering as carbocations, for the same hyperconjugation reason. So bromine adds to the less substituted carbon, leaving the radical on the more substituted one. That radical then abstracts H from another HBr, propagating the chain.

Net result: Br on the less substituted carbon, H on the more substituted one — precisely the reverse of ionic HBr addition. The product is racemic, since the radical is close to planar.

The peroxide effect works for HBr only. With HCl the propagation step that would form the C–Cl bond is too endothermic, and with HI the hydrogen abstraction step is. Only HBr has both propagation steps energetically feasible, so HCl and HI add via the normal ionic Markovnikov pathway whether or not peroxides are present. This is a favourite exam trap.

The three routes side by side

Conditions	Intermediate	Regiochemistry	Stereochemistry
HBr	carbocation	Markovnikov	racemic
HBr, ROOR	radical	anti-Markovnikov	racemic
H₃O⁺	carbocation	Markovnikov (OH)	racemic
BH₃ / H₂O₂	none (concerted)	anti-Markovnikov (OH)	syn

What carries forward

The pattern established here — that changing the mechanism changes the regiochemistry — recurs throughout the rest of the course. It is why kinetic and thermodynamic enolates give different products in Module 11, why 1,2- and 1,4-addition compete in conjugate addition, and why ortho/para and meta directors exist in Module 13. Whenever two reagents give opposite products from the same substrate, the explanation is that they are running different mechanisms.