Anti Markovnikov Hbr Roor

Alkene Reactions: Anti-Markovnikov Bromination

Anti-Markovnikov Hydrobromination of Alkenes (HBr/Peroxide Radical Chain)

Hydrobromination in the presence of peroxides (often abbreviated “HBr/ROOR”) inverts the usual Markovnikov outcome: bromine installs on the less substituted alkene carbon while hydrogen appears on the more substituted carbon. The pathway is a radical chain, not an ionic addition. Peroxide homolysis (triggered by heat or light) produces alkoxy radicals that intercept HBr to generate bromine radicals (Br·). Bromine radicals add to the alkene so the carbon-centered radical now sits at the more substituted position, then that radical abstracts H· from HBr to furnish the alkyl bromide and regenerate Br·. No carbocations, no rearrangements, and the peroxide effect is synthetically useful only for HBr under standard instructional conditions. Use the Reaction Solver to contrast anti-Markovnikov vs. Markovnikov outcomes for your substrate, and export step-by-step radical artwork from the Mechanism Solver when you need custom SVGs.


Introduction

Peroxide-initiated hydrohalogenation channels alkenes through a radical chain. Heat or light cleaves the O–O bond of the peroxide, forming alkoxy radicals that convert HBr into Br·. Bromine radicals add to the less substituted alkene carbon so that the more substituted carbon bears the radical. A second HBr delivers H· to that radical, yielding the anti-Markovnikov alkyl bromide and regenerating Br·. Because the key steps involve radicals rather than carbocations, no rearrangements or Br⁻ additions occur.


Quick Summary

  • Reagents: HBr with a radical initiator (e.g., benzoyl peroxide, tert-butyl hydroperoxide, di-tert-butyl peroxide) plus heat or light to initiate homolysis.
  • Outcome: Anti-Markovnikov addition — Br attaches to the less substituted alkene carbon; H attaches to the more substituted carbon.
  • Mechanism: Radical chain. Initiation: ROOR → 2 RO·; RO· + HBr → ROH (for BPO: PhCO₂H) + Br·. Propagation: Br· adds to the alkene, then abstracts H· from HBr to regenerate Br·.
  • Rearrangements: No carbocations; radical 1,2-shifts are not a feature of this chain and are rarely observed.
  • Selectivity: Only HBr displays the peroxide effect; for HCl the H· abstraction is endothermic, and for HI the I· addition is slow/reversible, so the chain stalls.
  • Common pitfalls: Forgetting to show fishhook arrows, omitting a clear initiation, drawing Br⁻ addition, or proposing carbocation rearrangements.
  • Related guides: Compare with ionic HX additions and bromonium anti additions.

Mechanism (Radical Chain)

Step 1: homolytic cleavage of benzoyl peroxide forming two alkoxy radicals.
Step 1 — Benzoyl peroxide homolysis generates two alkoxy radicals.

Initiation 1. Heat or light cleaves the peroxide O–O bond (e.g., in benzoyl peroxide). Two fishhook arrows describe homolytic bond breaking to produce a pair of alkoxy radicals.


Step 2: benzoyloxyl radical abstracts a hydrogen atom from HBr to form Br radical.
Step 2 — Alkoxy radical abstracts H· from HBr to produce Br·.

Initiation 2. An alkoxy radical performs hydrogen-atom transfer (HAT) with HBr: RO· + HBr → ROH + Br· (for benzoyl peroxide, the product is PhCO₂H + Br·). This step generates the chain carrier Br·.


Step 3: bromine radical adds to the less substituted alkene carbon to place the radical on the more substituted carbon.
Step 3 — Br· adds to the alkene; the more substituted carbon carries the radical.

Propagation 1. Bromine radical adds to the less substituted alkene carbon so the more substituted carbon bears the radical. Radical stability and polar effects both favor this regiochemistry.


Step 4: carbon-centered radical abstracts hydrogen from HBr, forming the alkyl bromide and regenerating Br radical.
Step 4 — Carbon radical abstracts H· from HBr; Br· is regenerated.

Propagation 2. The carbon-centered radical removes H· from a second HBr molecule. The product alkyl bromide forms and Br· is regenerated to continue the chain.


Step 5: anti-Markovnikov alkyl bromide product with Br on the terminal carbon.
Step 5 — Anti-Markovnikov alkyl bromide product.

Termination (unproductive). Radical coupling steps (Br· + Br· → Br₂, R· + Br· → R–Br, R· + R· → R–R) shut down the chain. Br₂ formation is a termination event, not a productive step, and is minimized by keeping radical concentrations low.

Mechanism Solver reminder: Recreate these radical-chain panels in the Mechanism Solver by requesting mechanismKey=hbr_roor_MechanismFunction with render=svg and engine=rdkit for clean, fishhook-arrow output.


Mechanistic Checklist (Exam Focus)

  • Draw single-electron arrows (•) for radical steps; double-headed arrows do not appear.
  • Initiation begins with peroxide homolysis, followed by HAT that generates Br·.
  • Propagation shows Br· adding to the alkene to place the radical on the more substituted carbon.
  • The carbon radical abstracts H· from HBr to yield the product and regenerate Br·.
  • Do not invoke carbocations, carbanions, or Br⁻ attacks — the mechanism is entirely radical.
  • Termination steps (e.g., Br· + Br· → Br₂) can be mentioned but are not the productive pathway.

Worked Examples

Example A — Radical Addition to an Exocyclic Alkene

  • Substrate: Exocyclic isopropenylcyclohexane.
  • Reagents: HBr with benzoyl peroxide initiation (heat or light to start the chain).
  • Key feature: Br· adds to the less substituted carbon of the exocyclic double bond; the more substituted carbon abstracts H· to deliver the anti-Markovnikov bromide.
  • Outcome: Bromine appears on the terminal exocyclic carbon; hydrogen adds to the tertiary ring carbon.
Exocyclic isopropenylcyclohexane substrate
Substrate — exocyclic isopropenylcyclohexane
+
HBr with ROOR initiation
Reagents — HBr / ROOR
→
Anti-Markovnikov bromide product
Product — anti-Markovnikov bromide

When Multiple Alkenes Are Present

Br· adds to the alkene that generates the more stable carbon radical (more substituted, resonance-stabilized, or benzylic). If two alkenes give radicals of similar stability, a mixture of products may form. Use the Reaction Solver to compare candidates and support your choice in written solutions.


Practical Tips & Pitfalls

  • Only HBr matters: The peroxide effect is reliable for HBr; analogous chains with HCl or HI are disfavored.
  • Single-electron arrows: Initiation and propagation steps must use fishhook (single-electron) arrows; double-headed arrows imply ionic chemistry.
  • Chain control: Termination events (Br· + Br· → Br₂, etc.) reduce yield; keep radical concentrations low.
  • Initiators/conditions: Common initiators include BPO, LPO, TBHP, DTBP, or azo initiators such as AIBN, each activated by heat or light.
  • Atmosphere: Exclude oxygen and radical inhibitors (e.g., BHT); an inert atmosphere improves chain length.
  • Safety: Peroxides/hydroperoxides are energetic and HBr is corrosive; follow institutional safety and handling guidelines.

Exam-Style Summary

  • Regiochemistry: Anti-Markovnikov — Br on the less substituted carbon, H on the more substituted carbon.
  • Mechanism: Radical chain (initiation → Br· formation; propagation → Br· addition, H· abstraction).
  • Carbocation/carbanion? None — intermediates are radicals.
  • Stereochemistry: Typically forms racemates when new stereocenters arise; addition is not stereospecific.
  • Unique to HBr: Peroxide effect is thermodynamically feasible only for HBr.

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FAQ / Exam Notes

Why does only HBr show the peroxide effect? For HBr both propagation steps are favorable: Br· adds to alkenes at useful rates and H· abstraction from HBr is exothermic. For HCl the H· abstraction step is endothermic, and for HI the I· addition to C=C is slow and often reversible, so the radical chain stalls.

Do I need to draw ROOR explicitly? Show peroxide homolysis in the initiation step, then it can be omitted in later propagation steps. After the first HAT, Br· carries the chain.

What about stereochemistry? Because attack occurs through planar radicals, new stereocenters are formed as racemic mixtures.