Radical Addition of HBr (Anti-Markovnikov)
Dissolve a trace of benzoyl peroxide in a flask of 2-methylpropene. Bubble in HBr gas. Something backwards happens: the bromine lands on the carbon that "should" have gotten the hydrogen. Ordinary ionic HBr addition to this alkene gives tert-butyl bromide, the Markovnikov product. This occurs because the reaction proceeds through the more stable tertiary carbocation. But with peroxides present, the major product is isobutyl bromide — Br on the primary, less-substituted carbon. Same reagents, same alkene, opposite regiochemistry. This is not a new rule bolted onto Markovnikov's rule. It is a completely different mechanism, radical chain addition. It obeys its own, equally rigorous, logic of intermediate stability. The puzzle matters because it is the classic demonstration that regiochemistry is not a property of a molecule. Instead, it is a property of a mechanism. The alkene doesn't "know" whether to be anti-Markovnikov. The mechanism selects which carbon gets attacked first. That choice is dictated by which radical intermediate is lower in energy. This is exactly how the ionic pathway is dictated by which carbocation is lower in energy. Both pathways are governed by the same underlying principle — build the more stable reactive intermediate. This principle is applied to two different intermediates (carbocation vs. carbon radical). These intermediates happen to have the same relative stability order (3° > 2° > 1°). This is precisely why the two mechanisms give opposite products from the same starting alkene. This reaction was also historically pivotal. Morris Kharasch's discovery in 1933 that "peroxide effects" reversed HBr's regiochemistry was significant. It was one of the first clean pieces of evidence for radical chain mechanisms in organic synthesis. This came decades before ESR spectroscopy could directly detect radicals. It taught chemists that reaction "anomalies" are not failures of a rule. Instead, they are signatures of a hidden mechanistic switch. Here, an adventitious trace of O–O bond homolysis turned an ionic reaction into a radical one. This article builds the chain mechanism bond by bond. First comes initiation (peroxide homolysis, then Br• generation). Next comes propagation (the two alternating steps that determine regiochemistry and carry the chain). Finally comes termination. We derive why Br adds first (not H•), and why it adds to the terminal carbon. We quantify the thermodynamics with real bond dissociation energies. This way, the regiochemistry is not asserted but calculated from bond energies.
- Anti-Markovnikov HBr addition only happens with HBr, only with radical initiators (peroxides, O₂ traces, or light/heat), and only via a chain mechanism — not with HCl or HI.
- The regiochemistry-determining step is Br• addition to the alkene, which adds to the carbon that generates the more stable carbon radical (3° > 2° > 1°).
- This puts Br on the less substituted carbon — the opposite regiochemical outcome from the Markovnikov (ionic) pathway, even though both mechanisms favor the same intermediate stability order.
- The chain has two propagation steps that must alternate: Br• adds to alkene (C–C π bond broken, new C–Br σ bond, C-radical formed), then the C-radical abstracts H from H–Br (regenerating Br•).
- Both propagation steps are exothermic for HBr; this is why only HBr undergoes clean radical addition — HCl's first step is endothermic and HI's second step is endothermic, breaking the chain.
- Peroxides act catalytically: they only need to generate the first Br•; after that, the chain is self-sustaining through hundreds of propagation cycles.
- Key idea: Regiochemistry in radical HBr addition is decided by which carbon radical is more stable, exactly as Markovnikov selectivity is decided by which carbocation is more stable — same principle, different intermediate, opposite carbon.
Where this lives today
Anti-Markovnikov HBr addition remains a genuinely used regiocontrol tool: when a synthetic route needs a primary alkyl bromide from a terminal alkene (for a subsequent displacement, Grignard formation, or Wittig-type chemistry where a linear chain is needed), chemists deliberately add a radical initiator (AIBN or peroxides) or simply do not rigorously exclude air, since adventitious peroxides in stored alkenes and ethers already bias many reactions toward the radical pathway unless explicitly inhibited (BHT and other radical inhibitors are added to stabilize alkene-containing reagents in storage for exactly this reason). Mechanistically, this reaction is the gateway example for an enormous family of industrially important radical chain processes: thiol-ene "click" additions (RSH across alkenes, used in polymer crosslinking and bioconjugation), atom-transfer radical addition/polymerization (ATRA/ATRP, used to make well-defined polymers), and radical hydrosilylation. All of these share the identical logical skeleton — initiation generates a chain-carrying radical, a first propagation step decides regiochemistry by picking the more stable carbon radical, a second propagation step regenerates the chain carrier — with HBr addition serving as the pedagogical prototype because its thermodynamics (via BDEs) can be worked out by hand. Understanding this mechanism is also essential context for Radical Polymerization, where the same propagation logic (radical adds to the less hindered/more stabilizing carbon) determines "head-to-tail" monomer connectivity in polymer chains, and for Halogenation of Alkanes, which uses an analogous radical chain (initiation/propagation/termination) but for C–H substitution rather than alkene addition. It is frequently taught side by side with Markovnikov's Rule and Regioselectivity specifically to demonstrate that regiochemical rules are mechanism-dependent, not substrate-dependent.
Ask the tutor: Why does the same "build the more stable intermediate" principle give opposite regiochemistry for radicals versus carbocations, when the stability order (3° > 2° > 1°) is identical for both?
The physical picture — homolysis, chains, and radical stability
Everything here starts from homolytic bond cleavage rather than heterolytic cleavage. In ionic HBr addition, the alkene's π electrons attack H⁺ (both electrons go to one atom, a heterolytic process building a carbocation), and then Br⁻ delivers its electron pair to the cation. In the radical pathway, bonds break so that each fragment keeps one electron — homolysis — represented by single-barbed ("fishhook") arrows instead of the full double-barbed arrows of ionic mechanisms. This single change in electron bookkeeping is the entire reason the regiochemistry flips. The reaction is a radical chain: a short initiation phase creates one radical, and then two propagation steps repeat over and over (hundreds to thousands of times per initiation event), each one consuming one alkene and one H–Br and regenerating the chain-carrying radical Br•. Radical stability enters at the first propagation step: Br• can add to either carbon of the alkene, and it "chooses" whichever addition produces the more stable carbon radical, because that transition state is lower in energy (Hammond postulate: for these exothermic radical additions, the TS resembles the product-like radical, so radical stability directly controls the barrier).
The chain-carrying species is always Br•, never H•, and that is not arbitrary — it is a direct consequence of bond energies, derived below. Because Br• is the attacking species in propagation step 1, and Br• is an electrophilic, sterically small radical, it adds to the carbon that leaves behind the most stabilized radical center — the more substituted carbon becomes the radical, and Br sits on the less substituted carbon.
The core ideas — bond energetics govern the chain
Radical chain propagation steps must each be exothermic (or at least not too endothermic) for the chain to be efficient; the overall driving force is expressed through bond dissociation energies (BDEs). For a generic propagation step breaking bond X–Y and forming bond X–Z (radical abstraction/addition), the enthalpy change is
where: BDE(bond broken) is the energy required to homolyze the bond consumed in that step, and BDE(bond formed) is the energy released forming the new bond; a negative means the step is exothermic. For the overall reaction of an alkene with HBr, summing both propagation steps recovers the known overall exothermicity of HBr addition:
where: step 1 is Br• addition to the C=C π bond (π bond broken, C–Br σ bond formed) and step 2 is H-atom transfer from H–Br to the carbon radical (H–Br bond broken, new C–H bond formed). Reading these relations: a chain mechanism is only kinetically viable when both propagation steps are exothermic or mildly so — one very endothermic step creates a bottleneck with a large activation barrier that kills the chain length, which is exactly the reason HCl and HI fail here while HBr succeeds.
Goal: show quantitatively, from bond dissociation energies, why HBr undergoes a clean radical chain addition while HCl and HI do not, and why the chain always carries Br• (never H•).
Step 1 — assemble the relevant BDEs. Use standard homolytic bond dissociation energies (kcal/mol, gas phase, 298 K):
These are the bonds broken/formed in propagation for propene + HX, using the 2° radical pathway (Markovnikov-type addition of X• to terminal carbon gives the more stable 2° radical).
Step 2 — propagation step 1 (X• adds to alkene): breaks π bond, forms C–X σ bond.
For X = Br:
Mildly exothermic — a small, easily surmountable barrier (radical additions are typically fast even when only slightly exothermic, since the transition state is early and the intrinsic barrier is low).
Step 3 — propagation step 2 (C-radical abstracts H from H–X): breaks H–X bond, forms C–H bond.
For X = Br:
Strongly exothermic. Both steps favorable ⇒ overall chain works cleanly for HBr:
Step 4 — repeat for HCl (X = Cl).
Step 2 is now endothermic: abstracting H from H–Cl is uphill because the H–Cl bond (103 kcal/mol) is stronger than the C–H bond being formed (98 kcal/mol). This step has a large barrier, the chain-carrying radical is regenerated too slowly, and radical addition of HCl is not synthetically useful — HCl adds only by the ionic (Markovnikov) pathway.
Step 5 — repeat for HI (X = I).
Now step 1, the addition of I• to the alkene, is endothermic (the C–I bond, only ≈53 kcal/mol, is too weak to compensate for breaking the π bond). I• addition is reversible and unproductive; the chain cannot get started efficiently, so HI does not undergo useful radical addition either.
Step 6 — conclusion. Only for X = Br are both propagation steps exothermic:
This is the entire chemical reason the "peroxide effect" is observed for HBr and essentially never (synthetically) for HCl or HI. The asymmetry between the two propagation steps (mildly exothermic addition, strongly exothermic abstraction) also explains why Br• — not H• — is the persistent chain carrier: H• is never generated at all in this mechanism; only Br• and the carbon radical alternate.
Go deeper · Derivation B · 2 minRegiochemistry: why Br• adds to the terminal (less substituted) carbon
Goal: show, by comparing the two possible modes of Br• addition to propene, that addition to the terminal carbon is favored because it generates the more stable secondary radical, and quantify the selectivity using radical stabilization energies.
Step 1 — write both possible additions. Br• can add to C1 (terminal CH₂) or to C2 (internal CH) of propene, CH₃–CH=CH₂:
More precisely: Path B places Br on C2 and the radical on C1, giving a primary radical, •CH₂–CHBr–CH₃.
Step 2 — invoke radical stability order (established independently from hyperconjugation and BDE data: tertiary > secondary > primary > methyl radical, mirroring but numerically distinct from carbocation stability):
A lower C–H BDE at a given carbon means the radical formed there (after removing that H) is more stabilized — hyperconjugative donation from adjacent C–H σ bonds into the singly-occupied p orbital lowers the radical's energy, and more substituted carbons have more such adjacent C–H (or C–C) bonds available.
Step 3 — apply Hammond-postulate reasoning to the exothermic addition step. Because for Br• addition is only mildly exothermic (Step 2 of the derivation above, to kcal/mol depending on substitution), the transition state lies relatively late along the reaction coordinate and resembles the product radical fairly closely. Consequently, the activation energy difference between the two addition modes tracks the radical stability difference:
Path A's transition state is lower in energy because it develops partial secondary-radical character, while Path B's transition state develops partial (less stable) primary-radical character.
Step 4 — translate the energy difference into a rate/selectivity ratio. Using the Arrhenius-type relation for the ratio of rates of two competing, parallel first-order-in-alkene steps:
Taking a representative (a typical secondary-vs-primary radical stability difference reflected in the transition state) at , with :
So Path A (Br on terminal carbon, secondary radical) is predicted to dominate by roughly two orders of magnitude over Path B under purely kinetic (radical-stability) control — consistent with the essentially single-product outcome (anti-Markovnikov selectivity) observed experimentally for terminal alkenes with only 2° vs 1° radical choices; the selectivity becomes even more lopsided (and essentially exclusive) when the choice is between a tertiary and a primary radical, as in 2-methylpropene.
Step 5 — sanity/limiting case. If the alkene were symmetric (e.g., 2-butene, CH₃CH=CHCH₂CH₃ is not symmetric — use exactly symmetric 2,3-dimethyl-2-butene analog or simplest symmetric case cis-2-butene, CH₃–CH=CH–CH�3): both carbons are equally substituted, both possible radicals are secondary, , and — no regiochemical preference exists because there is no radical-stability difference to exploit. This correctly predicts that radical HBr addition to internally symmetric alkenes gives a single product not because of "special" anti-Markovnikov selectivity but because there is only one possible product in the first place — the derivation's dependence on genuine substitution asymmetry is confirmed by this limit.
Worked examples
Example 1 (foundational): 1-hexene + HBr, peroxide-initiated. 1-Hexene is CH₂=CH–CH₂CH₂CH₂CH₃. Under peroxide (ROOR) initiation: Initiation. RO–OR undergoes homolysis (Δ or hν) to 2 RO•; RO• abstracts H from HBr (RO• + H–Br → ROH + Br•), generating the chain-carrying Br•. Propagation step 1. Br• adds to C1 (terminal CH₂), not C2, because addition to C1 places the unpaired electron on C2 — a secondary carbon — generating CH₃CH₂CH₂CH₂–•CH–CH₂Br, a secondary radical. Addition to C2 instead would place the radical on C1 (a primary carbon, •CH₂–), which is less stable. By the stability argument above (Step 2–3 of Derivation 2), the pathway through the secondary radical dominates. Propagation step 2. The secondary radical abstracts H from H–Br: CH₃CH₂CH₂CH₂–•CH–CH₂Br + H–Br → CH₃CH₂CH₂CH₂–CH₂–CH₂Br + Br•, regenerating Br• to carry the chain forward. Product: 1-bromohexane, BrCH₂CH₂CH₂CH₂CH₂CH₃ — Br on the terminal (originally sp², less-substituted) carbon. This is opposite to the ionic product (2-bromohexane, from H⁺ addition to C1 giving the more stable 2° cation at C2, then Br⁻ attack at C2).
Example 2 (applied): distinguishing peroxide vs. no-peroxide conditions synthetically — a route to a primary bromide from an easily available terminal alkene. Suppose a synthesis calls for 1-bromo-3-phenylpropane, PhCH₂CH₂CH₂Br, starting from allylbenzene, PhCH₂–CH=CH₂ (3-phenyl-1-propene). Simple ionic HBr addition (no peroxide) would protonate C1 or C2; the more stable cation determines the outcome. Protonation at C1 gives a secondary benzylic-adjacent... actually check: protonating the terminal CH₂ (C3, using IUPAC numbering PhCH₂–CH=CH₂ with C1 attached to Ph) forms a cation at C2 (secondary, plain alkyl, not benzylic) — while protonating C2 gives a cation at C1, which is benzylic (stabilized by conjugation with the aromatic ring) and adjacent to the CH₂Ph. The benzylic cation is far more stable, so ionic HBr addition gives PhCH(Br)–CH₂–CH₃ (Markovnikov, benzylic bromide) — not the linear product. To reach the linear primary bromide PhCH₂CH₂CH₂Br, radical (peroxide) conditions are required: Propagation step 1. Br• adds to the terminal carbon (C3, away from the ring) to generate the radical at C2... but wait — check which radical is more stable. Addition of Br• to C3 places the radical at C2 (a simple secondary radical, not stabilized by the ring since it's one carbon removed). Addition of Br• to C2 (internal carbon) would place the radical at C1 — directly adjacent to the phenyl ring, i.e., a benzylic radical, which is dramatically more stable than an ordinary secondary radical (resonance delocalization into the ring, extra stabilization on the order of 12–15 kcal/mol relative to a simple 2° radical). So Br• actually adds preferentially to C2 (not the terminal carbon) here, because the resulting benzylic radical at C1 is far more stable than the alternative secondary radical — giving, after H-abstraction from HBr at C1: PhCH₂–CHBr–CH₃ initially seems plausible, but let's be careful with connectivity: addition of Br to C2 and radical at C1 (benzylic) gives Ph–•CH–CH(Br)–CH₃ after H-transfer becomes Ph–CH₂–CH(Br)–CH₃, i.e., 1-bromo-1-phenylpropane analog — this shows the radical mechanism here actually still favors a branched, benzylic-stabilized product, not the linear terminal bromide, precisely because "anti-Markovnikov" really means "onto the carbon giving the more stable radical," and when a benzylic position is available, that overrides simple 2°-vs-1° reasoning. To make PhCH₂CH₂CH₂Br cleanly, a synthesis would instead use a non-conjugated terminal alkene (e.g., 4-phenyl-1-butene, PhCH₂CH₂–CH=CH₂) where the "terminal vs. internal" radical choice is a clean secondary-vs-primary comparison with no competing benzylic stabilization, and peroxide-initiated HBr addition then cleanly delivers the terminal primary bromide PhCH₂CH₂CH₂CH₂Br. This example is included specifically to correct a common oversimplification: "anti-Markovnikov" is not literally "always put Br on the terminal CH₂" — it is "put Br wherever leaves the more stable radical," and with a benzylic (or allylic) position available, that can still be an internal carbon.
Example 3 (exam/competition level): predicting the major product and estimating selectivity for a trisubstituted alkene, with quantitative comparison of tertiary vs. secondary radical pathways. Consider 2-methyl-2-pentene, (CH₃)₂C=CH–CH₂CH₃, treated with HBr and a peroxide initiator. Label the alkene carbons: C2 = (CH₃)₂C=, C3 = =CH–CH₂CH₃. Two possible Br• addition modes:
- Mode A: Br• adds to C3 → radical forms at C2. C2 bears two methyl groups and one C3 substituent — this radical is tertiary.
- Mode B: Br• adds to C2 → radical forms at C3. C3 bears one H, one CH₂CH₃, and one C2 substituent — this radical is secondary.
Mode A is strongly favored (tertiary radical vs. secondary radical). Using representative BDE-derived radical stabilization energies (3° radical stabilized by ≈ 5–6 kcal/mol relative to 2°, based on BDE(3° C–H) ≈ 96.5 vs. BDE(2° C–H) ≈ 98.5 kcal/mol), and assuming (via Hammond postulate, since these are exothermic additions with early-ish transition states, taking roughly half that difference reflected in , ≈ 2.5–3 kcal/mol) the selectivity at 298 K:
So the tertiary-radical pathway (Mode A) should outcompete the secondary-radical pathway by roughly two orders of magnitude. Propagation step 2 for Mode A: the tertiary radical (CH₃)₂•C–CH(Br)–CH₂CH₃... careful with connectivity — Br adds to C3, radical at C2: (CH₃)₂•C–CHBr–CH₂CH₃. This tertiary radical abstracts H from H–Br:
Major product: 3-bromo-2-methylpentane, (CH₃)₂CH–CHBr–CH₂CH₃, with Br on C3 (originally the more substituted alkene carbon in terms of H-count is C3, but the key point is Br sits on the carbon that does not become the tertiary radical). Cross-checking against the ionic (Markovnikov) product for the same alkene: ionic addition would protonate C3 (less substituted, giving the more stable tertiary cation at C2), then Br⁻ attacks C2, giving 2-bromo-2-methylpentane, (CH₃)₂CBr–CH₂CH₂CH₃ — a genuinely different constitutional isomer from the radical product. This example demonstrates that even for more complex, unsymmetrically trisubstituted alkenes, the radical mechanism's regiochemical outcome is fully predictable once the two possible radical intermediates are drawn and compared for substitution level — no memorization of "Br goes here" is needed, only the radical stability hierarchy.
Limiting cases, scope & common traps
| Scenario | What happens | Why |
|---|---|---|
| HBr + peroxide (ROOR) or O₂ trace, terminal alkene | Anti-Markovnikov: Br on less-substituted carbon | Br• adds to give the more stable (more substituted) radical |
| HBr, rigorously peroxide-free, dark, inert atmosphere | Markovnikov: Br on more-substituted carbon | Ionic mechanism operates; H⁺ adds to give more stable carbocation |
| HCl + peroxide | Still Markovnikov (peroxide effect absent) | Propagation step 2 (H-abstraction from HCl) is endothermic; chain doesn't propagate efficiently |
| HI + peroxide | Still Markovnikov (peroxide effect absent) | Propagation step 1 (I• addition to alkene) is endothermic; weak C–I bond can't compensate for lost π bond |
| Symmetric alkene (e.g., cis-2-butene) | Single product regardless of mechanism | Both alkene carbons equivalent; no regiochemical question exists |
| Allylic/benzylic system available | Radical adds to give the resonance-stabilized (allylic/benzylic) radical, which may be an internal carbon | "Anti-Markovnikov" is really "radical-stability-Markovnikov"; benzylic/allylic stabilization can dominate over simple substitution counting |
| HBr/peroxide with internal (non-terminal), unsymmetrical alkene | Br still goes to the carbon that avoids forming the less stable radical | The rule is about relative radical stability of the two possible outcomes, not literally "terminal vs. internal" |
The chemist behind it
Origins · 1 minMorris S. Kharasch and how this was discovered
Morris Kharasch emigrated from Ukraine to the United States as a teenager and built one of the most productive physical-organic laboratories of the mid-twentieth century at the University of Chicago. He was fascinated by why seemingly simple addition reactions sometimes gave inconsistent regiochemical results across different laboratories — a "reproducibility crisis" of sorts for HBr addition to alkenes, where some groups reported clean Markovnikov product and others reported the opposite. Rather than dismissing the discrepancy as sloppy technique, Kharasch and his student Frank Mayo systematically varied the purity of reagents and identified peroxide contamination (from aerial oxidation of the alkene or solvent) as the hidden variable controlling which product formed. Their 1933 paper proposing a radical chain mechanism to explain the "peroxide effect" was a landmark piece of physical organic reasoning: it inferred an invisible chain mechanism purely from reaction outcomes, rigorously excluding alternative explanations, years before direct spectroscopic radical detection existed. Kharasch went on to a long career studying free-radical reactions broadly, including radical additions, halogenations, and organometallic radical chemistry, training many students who became leaders in physical organic chemistry.
Does radical HBr addition still follow "Markovnikov's rule" in some hidden sense?
Not in the textbook sense of "H to the carbon with more H's already," but yes in the deeper sense that both mechanisms build the more stable reactive intermediate. Ionic addition builds the more stable carbocation (Markovnikov); radical addition builds the more stable carbon radical, and because Br• (not H•) is the species that attacks the alkene first, the more-substituted carbon ends up hosting the radical rather than the bromine. The underlying logic — go through the lower-energy intermediate — is identical; only the identity of the attacking species and the intermediate (cation vs. radical) differ, which is why the outcome looks reversed.
Why is a peroxide needed at all if HBr can react with alkenes ionically on its own?
Peroxides are not needed to make the reaction happen at all — they are needed to make the radical pathway happen instead of the ionic pathway. Without any initiator, and with rigorously purified, peroxide-free HBr and alkene, the ionic (Markovnikov) mechanism dominates because there is no radical chain carrier available. A small amount of peroxide undergoes O–O homolysis (a weak bond, BDE roughly 35–40 kcal/mol) even at mild temperatures, generating alkoxy radicals that abstract H from HBr to make the first Br•; from that point, the radical chain outcompetes the ionic pathway because chain propagation is fast and requires no ionization energy (no need to form a bare carbocation in a possibly non-ionizing solvent).
Why does the mechanism specifically require HBr and not, say, molecular Br₂?
Br₂ addition to alkenes is a completely different, non-radical mechanism: the alkene π bond attacks Br₂ to form a bromonium ion (three-membered ring cation), which is then opened by bromide attack from the opposite face, giving anti-dibromide addition products with no C–H bond involved at all. The radical anti-Markovnikov chemistry discussed here specifically requires an H–X bond (H–Br) because propagation step 2 needs a hydrogen atom to abstract in order to both finish the product and regenerate the chain-carrying halogen radical. Without an abstractable H (as in Br₂), there is no comparable propagation step available.
Can radical addition occur with internal, fully substituted alkenes where both carbons are equally substituted?
Yes, and in that special case there is no regiochemical question to answer at all, because both possible carbon radicals are equally stable (or, for a truly symmetric alkene, the product is identical regardless of which carbon Br adds to). The radical-stability argument only produces a strong regiochemical preference when the two alkene carbons differ in substitution; for something like 2,3-dimethyl-2-butene, radical HBr addition still proceeds efficiently (both propagation steps are still exothermic, since that depends only on the C–Br and C–H bond strengths of the resulting tertiary radical/product, not on any left-right asymmetry), it simply gives a single, unambiguous product.
How was it experimentally proven that a radical chain, rather than some alternative ionic mechanism, is responsible for the reversed regiochemistry?
Several converging lines of evidence were used historically and are still used pedagogically: (1) the effect is abolished by radical inhibitors (like hydroquinone or O₂-scavengers under some conditions, or more dramatically enhanced by radical initiators like benzoyl peroxide or AIBN), consistent with a radical chain that inhibitors can quench; (2) the effect is specific to HBr among the hydrogen halides, matching the bond-energy argument that only HBr's two propagation steps are both exothermic; (3) light or heat alone (without added peroxide, but sufficient to homolyze trace peroxide contaminants or initiate directly) can also trigger the reversed regiochemistry, consistent with radical initiation rather than any special ionic pathway; (4) kinetic chain lengths measured by radical-trapping experiments are large (many propagation cycles per initiation event), consistent with an efficient chain rather than a stepwise ionic process.
Does the radical mechanism ever compete with, or partially mix with, the ionic mechanism in the same flask?
In practice, yes — reaction outcomes can be mixtures if peroxide concentration is low or variable, which is exactly why Kharasch and Mayo's original experiments showed inconsistent regiochemistry across different labs before the peroxide variable was identified and controlled. Rigorously peroxide-free HBr with careful exclusion of air and light gives clean Markovnikov product; deliberately added peroxide (or AIBN, or UV light with trace HBr homolysis) gives clean anti-Markovnikov product; in between, product ratios reflect the relative rates of the two competing mechanisms, which is why modern syntheses that need one specific regiochemistry deliberately add a stoichiometric or catalytic radical initiator (for anti-Markovnikov) or rigorously purify/inhibit (for Markovnikov) rather than leaving the outcome to chance.
Why is this reaction categorized under "Alkenes & Alkynes" rather than under "Radical Reactions" generically?
Because the regiochemical question being answered — where does the new substituent end up on the alkene's two former sp² carbons — is fundamentally a question about alkene reactivity and substitution pattern, directly paralleling Markovnikov's Rule and Regioselectivity. The radical chain machinery (initiation/propagation/termination) is a generically reusable mechanistic toolkit that also appears in alkane halogenation and polymerization contexts, but the specific pedagogical point of this reaction is what it teaches about alkene regiochemistry and the broader principle of Radical stability controlling addition direction, which is why it belongs alongside other alkene addition reactions.