Free-Radical Addition¶
An elementary chemical step in which a radical bonds to an unsaturated substrate and leaves radical character on the resulting adduct.
Core Idea¶
Free-radical addition is the local bond-forming step in which a radical attaches to an unsaturated site, such as an alkene double bond, leaving radical character on the adduct. In a schematic \(R^{\bullet}+C=C\rightarrow R-C-C^{\bullet}\), the new bond and remaining radical are the defining relation; the formula does not predict which carbon is attacked in every substrate. Original thiyl- and carbon-radical studies support the role map; IUPAC's analogous schematic is specifically an oxidative-condition example, not a universal definition. The following hydrogen transfer, capture or other product-forming event is a separate step.[ref-c2b9dbcddeff][ref-75561e500d2c][^ref-cb6ce0edf13e]
The seed's familiar self-sustaining chain is one possible pathway context, not this identity. Peroxide or light initiation, radical-radical termination and anti-Markovnikov orientation cannot be required of every radical addition. Historically, oxidants altered HBr-addition orientation in some terminal olefins, but a separate Harris–Smith study found no corresponding change in the tested nonterminal isoundecenoic acid.[ref-2973581e2521][ref-2657f5ee0772][^ref-75561e500d2c]
Scope of Application¶
The structure is used in radical chemistry when a radical attacks a specified multiple bond and a new radical-bearing adduct is mechanistically supported. HBr addition to certain terminal olefins and thiyl addition to alkenes have different radical sources and final products. In Tyson, Ament and Yoon's original photoredox study, thiyl radicals are generated and anti-Markovnikov hydrothiolation is reported for the tested olefins. Horvat and colleagues independently report photo-decarboxylative carbon-radical addition to electron-deficient alkenes, but no analogous addition to their tested electron-rich alkenes. The intermediate sequences are proposed mechanisms, not direct observation of every fleeting species.[ref-40c74901b5c5][ref-2973581e2521][ref-75561e500d2c][ref-cb6ce0edf13e]
The original 1933 Kharasch–Mayo article is represented here only by publisher title/metadata because its paid text was not inspected. The original 1935 Smith–Harris Nature abstract supports the prior terminal-olefin orientation history; a separate Harris–Smith Journal of the Chemical Society report supports the nonterminal isoundecenoic-acid comparison. These evidence levels should not be collapsed into a claim that a particular Br-radical intermediate was directly observed in those experiments.[ref-40c74901b5c5][ref-2973581e2521][^ref-2657f5ee0772]
Clarity¶
A final addition product is weaker evidence than a supported mechanism. IUPAC's general addition-reaction definition describes a completed reaction, not the isolated radical attack. An ionic pathway can also add to an alkene, while a radical taking H from a donor bond performs hydrogen-atom abstraction without adding to a multiple bond in that elementary move. A complete reaction mechanism represents its sequence, but is not identical to the individual bond-forming event. The checked live prime Addition is a typed sum operation and shares only a word with this chemistry.[ref-5b799f614c4f][ref-c2b9dbcddeff][^ref-75561e500d2c]
Manages Complexity¶
Isolating the radical attack allows comparison across very different reactions: bromine-radical addition in a proposed HBr chain and thiyl-radical addition in photoredox thiol–ene chemistry share radical, unsaturated bond and radical-adduct roles. Initiation, H-transfer, termination, reaction rate and product orientation can then be analyzed separately. This compact role map supports comparison without making the two whole mechanisms or their selectivity rules identical.[ref-2973581e2521][ref-75561e500d2c]
Abstract Reasoning¶
Identify the radical \(R^{\bullet}\) and the multiple-bond site. If attack makes a new bond while the adduct still carries radical character, it is the addition step; subsequent reaction of that adduct needs a distinct claim. The IUPAC radical-chemistry schematic separates radical addition from a following event. Harris and Smith's isoundecenoic-acid comparison shows why the familiar HBr product orientation cannot be deduced from the word “radical” alone.[ref-c2b9dbcddeff][ref-2657f5ee0772]
Knowledge Transfer¶
For a new case, map the radical reactant, unsaturated site, new bond and radical-bearing adduct; then ask what evidence supports that sequence. Only afterward transfer questions about how radicals form, how products are completed, whether a chain propagates, and what selectivity is measured. The live Reaction Mechanism is a pathway representation, staged Hydrogen-Atom Abstraction is a distinct possible later step, and Chemical Process is a broader organized process; no nearest necessary typed genus has been established, so this workspace stages the node unparented.[ref-75561e500d2c][ref-c2b9dbcddeff]
The portable step-versus-pathway distinction may invite a future prime inquiry. This entry itself remains domain-specific because unpaired-electron chemistry, covalent bonds and radical-bearing intermediates are necessary to recognize it; two different reagent families do not establish literal transfer to unrelated domains.
[^ref-5b799f614c4f]: IUPAC, “addition reaction,” Compendium of Chemical Terminology (Gold Book), A00133; official indexed definition of an overall addition reaction, original Pure and Applied Chemistry 66 (1994), p. 1081. It is not an isolated radical-attack definition; direct page access denied during this audit. https://goldbook.iupac.org/terms/view/A00133 [^ref-c2b9dbcddeff]: IUPAC, “oxidative addition,” Compendium of Chemical Terminology (Gold Book), O04367, radical-chemistry usage/example and separate following step; official indexed text, direct page access denied. The scoped entry does not imply every free-radical addition is oxidative. https://goldbook.iupac.org/terms/view/O04367 [^ref-40c74901b5c5]: M. S. Kharasch and Frank R. Mayo, “The Peroxide Effect in the Addition of Reagents to Unsaturated Compounds. I. The Addition of Hydrogen Bromide to Allyl Bromide,” Journal of the American Chemical Society 55(6), 2468–2496 (1933), DOI 10.1021/ja01333a041; publisher title/metadata only, full text not inspected. https://pubs.acs.org/doi/abs/10.1021/ja01333a041 [^ref-2973581e2521]: J. C. Smith and P. L. Harris, “Addition of Hydrogen Bromide to Olefines,” Nature 135, 187 (1935), DOI 10.1038/135187b0, original letter abstract. https://www.nature.com/articles/135187b0 [^ref-2657f5ee0772]: P. L. Harris and J. C. Smith, “Addition of hydrogen bromide to non-terminal double bonds. Isoundecenoic acid,” Journal of the Chemical Society (Resumed) (1935), 1108–1110, DOI 10.1039/JR9350001108. The original paper's opening report describes the tested 9-/10-bromoundecoic-acid mixture under contrasting oxidizing and anti-oxidizing conditions; source text was checked through the indexed original scan. https://doi.org/10.1039/JR9350001108 [^ref-75561e500d2c]: Elizabeth L. Tyson, Michael S. Ament and Tehshik P. Yoon, “Transition Metal Photoredox Catalysis of Radical Thiol–Ene Reactions,” Journal of Organic Chemistry 78(5), 2046–2050 (2013), DOI 10.1021/jo3020825, original manuscript indexed Abstract/Introduction/Schemes 1 and 3; PMC direct open challenged, PubMed abstract and captions checked. https://pmc.ncbi.nlm.nih.gov/articles/PMC3573243/ [^ref-cb6ce0edf13e]: Margareta Horvat, Kata Mlinarić-Majerski, Axel G. Griesbeck and Nikola Basarić, “Photoinduced decarboxylation of 3-(N-phthalimido)adamantane-1-carboxylic acid and radical addition to electron deficient alkenes,” Photochemical & Photobiological Sciences 10(4), 610–617 (2011), DOI 10.1039/c0pp00357c, directly inspected original PubMed abstract. https://pubmed.ncbi.nlm.nih.gov/21267496/
Neighborhood in Abstraction Space¶
Free-Radical Addition sits in a moderately populated region (54th percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.
Family — Organic Reaction Mechanisms & Kinetics (11 abstractions)
Nearest neighbors
- Frustrated Lewis Pair — 0.88
- Hydrogen-Bond-Donor Catalysis — 0.86
- Conjugated System — 0.86
- Reaction Mechanism — 0.85
- Hydrogen-Atom Abstraction — 0.85
Computed from structural-signature embeddings · 2026-10-08