Organic Reactions & Mechanisms (CHEM 220) · Module 8 of 9
Hydroboration-Oxidation & Radical Addition
The two ways chemists flip the regiochemistry of alkene addition: a concerted mechanism and a radical one.
Hydroboration-Oxidation Mechanism
- Hydroboration-oxidation adds H and OH across a double bond via a two-step sequence: hydroboration first adds H and boron across the double bond in a single concerted step, with no carbocation forming and boron and hydrogen adding to the same face of the alkene (syn addition); a separate oxidation step then converts C–B to C–OH.
- Boron, being electron-poor, ends up on the less hindered, less substituted carbon (anti-Markovnikov), since this concerted transition state is both sterically and electronically favored that way.
Why does hydroboration add boron to the less substituted carbon of an alkene (anti-Markovnikov)?
Solution. The concerted four-centered transition state is both sterically favored, since boron is a relatively bulky group that prefers the less hindered carbon, and electronically favored, since the partial positive charge on the more substituted carbon in the transition state is better stabilized — both directing boron to the less substituted position.
Syn Addition & No Rearrangement
- Because hydroboration proceeds through a single concerted transition state without forming a discrete carbocation, no rearrangements occur, and both new bonds (C–H and C–B) form on the same face of the former double bond.
Why does hydroboration not produce rearranged products, unlike many carbocation-based additions?
Solution. Since there's no discrete carbocation intermediate — the mechanism is a single concerted step — there's no opportunity for a hydride or alkyl shift to occur, so no rearrangement happens.
Oxidation Step
- The oxidation step, using H₂O₂/NaOH, replaces the C–B bond with a C–OH bond, with retention of configuration at that carbon, converting the organoborane into the final anti-Markovnikov alcohol.
What happens stereochemically at the carbon-boron bond during the oxidation step of hydroboration-oxidation?
Solution. The C–OH bond forms with retention of configuration at that carbon, meaning the spatial arrangement does not change during the boron-to-oxygen swap.
Radical Addition (Anti-Markovnikov HBr)
- In the presence of peroxides, HBr adds to an alkene via a radical chain mechanism: a bromine radical (not H⁺) adds first, forming the more stable radical intermediate, reversing the regiochemistry to anti-Markovnikov.
- This mechanism only applies to HBr among the hydrogen halides — not HCl or HI — because of the specific thermodynamics of the propagation steps involved.
Why does the presence of peroxides reverse the regiochemistry of HBr addition to an alkene, compared to addition without peroxides?
Solution. Peroxides initiate a radical chain mechanism where a bromine radical, rather than a proton, adds first, forming the more stable carbon radical intermediate; this reverses which carbon ends up with the bromine compared to the ionic, carbocation-based mechanism that occurs without peroxides.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What is the overall regiochemistry of hydroboration-oxidation?
2. What is the stereochemistry of hydroboration addition?