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Radical Addition
HBr with Peroxides

Understand how peroxides flip the regioselectivity of HBr addition to alkenes via a radical chain mechanism, producing the anti-Markovnikov product.

Organic Chemistry 1

What is Radical Addition?

In a typical alkene addition, HBr adds across the double bond via an ionic (polar) mechanism. However, when peroxides \(\text{ROOR}\) are present, the reaction proceeds through a completely different pathway: a free-radical chain mechanism.

This results in the anti-Markovnikov product: the bromine atom ends up on the less substituted carbon of the alkene, which is the opposite of normal ionic HBr addition.

Key Vocabulary

Radical
An atom or molecule with one or more unpaired electrons, denoted by a dot: \(\text{Br}^{\boldsymbol{\cdot}}\)
Peroxide
A compound with an \(\text{O-O}\) bond (e.g., \(\text{ROOR}\)) that breaks homolytically to start the chain
Anti-Markovnikov
The halogen adds to the less substituted carbon, opposite to Markovnikov's rule

Why Only HBr Works

Of all the hydrogen halides (HF, HCl, HBr, HI), only HBr undergoes radical addition with peroxides. This is due to the energetics of each propagation step.

HF / HCl
The \(\text{H-X}\) bond is too strong: the radical abstraction step is endothermic and too slow to sustain a chain
HBr
Both propagation steps are exothermic: bond strengths are perfectly balanced for a sustained chain
HI
The \(\text{H-I}\) bond is too weak: the addition step to the alkene is endothermic

The Radical Chain: Three Steps

1
Initiation: The peroxide \(\text{O-O}\) bond undergoes homolysis (heat or light), generating alkoxy radicals \(\text{RO}^{\boldsymbol{\cdot}}\). The alkoxy radical then abstracts H from HBr to form \(\text{Br}^{\boldsymbol{\cdot}}\).
2
Propagation (Step A): The bromine radical \(\text{Br}^{\boldsymbol{\cdot}}\) adds to the less substituted carbon of the alkene, forming the more stable carbon radical.
3
Propagation (Step B): The carbon radical abstracts H from another HBr molecule, yielding the anti-Markovnikov product and regenerating \(\text{Br}^{\boldsymbol{\cdot}}\).

Initiation Step

Step 1:
\(\text{RO-OR}\)
 heat/hv
\(2 \text{ RO}^{\boldsymbol{\cdot}}\)
 Step 2:
\(\text{RO}^{\boldsymbol{\cdot}}\)
 +
\(\text{H-Br}\)
\(\text{ROH}\)
 +
\(\text{Br}^{\boldsymbol{\cdot}}\)
 The Br· radical is the reactive species that enters propagation

Propagation Steps

Propagation A:
\(\text{Br}^{\boldsymbol{\cdot}}\)
 +
\(\text{CH}_2\text{=CHR}\)
\(\text{BrCH}_2\text{—}\cdot\text{CHR}\)
 Br adds to less substituted C; radical on more substituted C Propagation B:
\(\text{BrCH}_2\text{—}\cdot\text{CHR}\)
 +
\(\text{H-Br}\)
\(\text{BrCH}_2\text{—CH}_2\text{R}\)
 +
\(\text{Br}^{\boldsymbol{\cdot}}\)
 Br· is regenerated and continues the chain reaction Net result: Anti-Markovnikov addition of HBr

Why Anti-Markovnikov?

The regioselectivity is determined by radical stability in Propagation Step A. The bromine radical adds to the less substituted carbon so that the resulting carbon radical is on the more substituted carbon.

More Stable (\(3^\circ\) or \(2^\circ\))
 Radical on more substituted C Stabilized by hyperconjugation
Less Stable (\(1^\circ\))
 Radical on less substituted C Not formed (disfavored)
\(\text{Radical stability: } 3^\circ > 2^\circ > 1^\circ > \text{methyl}\)
(Same trend as carbocation stability, but less dramatic)

Markovnikov vs. Anti-Markovnikov

Ionic (No Peroxides) Radical (With Peroxides)
Reagent HX (e.g., HBr) HBr + ROOR
Mechanism Electrophilic addition Radical chain
Intermediate Carbocation Carbon radical
Br goes to More substituted C Less substituted C
Regiochemistry Markovnikov Anti-Markovnikov
Works with HF, HCl, HBr, HI HBr only

How to Apply the Mechanism

1
Check for peroxides: Look for \(\text{ROOR}\), \(\text{H}_2\text{O}_2\), or the label "peroxides" in the reagent. If absent, use ionic (Markovnikov) addition.
2
Confirm HBr: Only HBr undergoes radical addition. If the reagent is HCl, HI, or HF with peroxides, Markovnikov still applies.
3
Identify the alkene carbons: Determine which carbon is more substituted and which is less substituted.
4
Place Br on the less substituted carbon: H goes to the more substituted carbon. Draw the anti-Markovnikov product.
5
Verify: The product should have Br on the terminal (or less substituted) position.

Example 1: Propene

Reactant
\(\text{CH}_2\)
\(\text{CH}\)
\(\text{CH}_3\)
 less sub. more sub.
\(\text{HBr, ROOR}\)
 Product
\(\text{BrCH}_2\)
\(\text{CH}_2\text{CH}_3\)
 Anti-Markovnikov: Br on less substituted C (terminal carbon) Product: 1-bromopropane (not 2-bromopropane) Without peroxides, the product would be 2-bromopropane (Markovnikov)

Example 2: 2-Methylpropene

2-Methylpropene (isobutylene)
\(\text{CH}_3\)
\(\text{C}\)
\(\text{CH}_3\)
\(\text{CH}_2\)
more sub. (forms \(3^\circ\) radical)
 less sub.
\(\text{HBr, ROOR}\)
 Product
\(\text{CH}_3\)
\(\text{CH}\)
\(\text{CH}_3\)
\(\text{CH}_2\text{Br}\)
Br adds to terminal \(\text{CH}_2\) (anti-Markovnikov)
Product: 1-bromo-2-methylpropane (a \(1^\circ\) alkyl bromide)

Example 3: 1-Methylcyclohexene

1-Methylcyclohexene
\(\text{CH}_3\)
 C1 (more sub.) C2 (less sub.)
\(\text{HBr, ROOR}\)
 Product
\(\text{CH}_3\)
\(\text{Br}\)
\(\text{H}\)
 Br on C2 (less substituted), H on C1 (more substituted) Product: 1-bromo-2-methylcyclohexane (anti-Markovnikov)

Energy Profile of Propagation

Reaction Progress Energy Br· + Alkene TS 1 C radical + HBr TS 2 Product + Br· Overall Exothermic

Common Mistakes

✕ Applying to HCl or HI
Only HBr undergoes radical addition with peroxides. HCl, HI, and HF always follow Markovnikov, even with peroxides present.
✕ Forgetting Peroxides
Without peroxides (or another radical initiator), HBr follows normal Markovnikov addition via an ionic mechanism.
✕ Wrong Carbon Gets Br
In anti-Markovnikov, Br goes to the less substituted carbon. Students often mix this up and place Br on the more substituted C.

Summary Table

Feature Detail
Reagents Required HBr + peroxides (ROOR)
Mechanism Type Free-radical chain
Steps Initiation → Propagation (×2) → Termination
Regiochemistry Anti-Markovnikov
Br Placement Less substituted carbon
Key Intermediate Carbon radical (more stable = more substituted)
Works With HBr only (not HCl, HI, or HF)

Question 1 of 5

What is the major product when 1-butene reacts with HBr in the presence of peroxides?

Question 2 of 5

Which hydrogen halide undergoes radical addition to alkenes in the presence of peroxides?

Question 3 of 5

In the radical addition mechanism, what is the reactive species generated during the initiation step that attacks the alkene?

Question 4 of 5

Why does the bromine radical add to the less substituted carbon of the alkene?

Question 5 of 5

What is the product of treating 2-methylpropene with HBr without peroxides?