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Hydrogen-atom transfer (HAT) is a reaction in which a proton and an electron move together from one substrate to another in a single elementary step. In the common abstraction pattern A–H + B• → A• + B–H, a radical removes hydrogen from an A–H bond, leaving the substrate as a new radical that can form a bond, undergo oxidation, or enter another catalytic step.
In synthesis, HAT is a way to activate otherwise unreactive C–H bonds. Photocatalytic HAT methods have been used to convert ethers, alcohols, protected amines, alkanes, alkenes, alkylaromatics and aldehydes into products containing new C–C, C–Si, C–N, C–S or C–halogen bonds. The exact substrate scope, selectivity, catalyst loading and light source are protocol-specific, and a product that looks like an HAT product does not by itself prove that HAT was the mechanism.
What happens in a hydrogen-atom transfer reaction?
HAT moves the two constituents of a hydrogen atom—a proton and an electron—together. The defining feature is that the transfer occurs between two substrates in one kinetic step, rather than through separately observable electron-transfer and proton-transfer events. A useful general representation is:
A–H + B• → A• + B–H
Here, A–H is the bond being activated and B• is the hydrogen-atom abstractor. The abstractor gains hydrogen to become B–H, while A–H becomes the radical A•. If A–H is a C–H bond, the result is a carbon-centered radical that can be intercepted by a second reagent.
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“Hydrogen atom transfer (HAT) is a chemical transformation consisting of the concerted movement of two elementary particles – a proton and an electron – between two substrates in a single kinetic step.”
Marco A. Capaldo, Davide Ravelli and Maurizio Fagnoni, European Journal of Organic Chemistry review (2017).
HAT is commonly discussed as a subclass of proton-coupled electron transfer (PCET). The labels describe related but not identical mechanistic situations: PCET is the broader family in which proton and electron motion are coupled, while HAT specifically denotes transfer of a hydrogen atom between two reacting partners. A mechanistic study must establish whether the proton and electron move concertedly; observing a radical product is not sufficient evidence on its own.
Why creating a radical is useful
Most C–H bonds are not naturally good handles for cross-coupling or substitution. HAT turns one into a radical without first installing a halogen, boron group or other prefunctionalized handle. The radical can then add to an alkene, couple with an aryl or heteroaryl partner, react with a nitrogen, sulfur or silicon source, or be converted into an oxidized product. This is the basis of many direct and late-stage C–H functionalization strategies.
How a synthetic HAT cycle is organized
A productive reaction usually combines four events. The order and identity of the intermediates depend on the method, but the design logic is consistent.
- Generate an abstractor. Light excitation, an oxidant or reductant, and a catalyst create a species capable of removing hydrogen from the chosen substrate.
- Abstract hydrogen. The abstractor and the substrate undergo HAT, producing the substrate radical and the reduced or hydrogenated abstractor.
- Trap the radical. A reaction partner captures the radical to make a new bond or to set up an oxidation, elimination or rearrangement.
- Close the catalytic cycle. The catalyst is regenerated by electron transfer, energy transfer, reverse HAT, or—depending on the system—sequential proton and electron steps.
These steps compete with radical dimerization, hydrogen abstraction from the wrong C–H bond, reduction or oxidation of the product, and catalyst quenching. A protocol succeeds when the desired trapping event is faster or more selective than those alternatives.
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Direct and indirect photocatalytic HAT
Photocatalysis supplies a controlled way to make reactive excited states, but “photocatalytic HAT” covers two distinct arrangements.
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|---|---|---|---|
| Direct photocatalytic HAT | The excited photocatalyst itself abstracts hydrogen from the substrate. | The catalyst’s excited-state energy, polarity and geometry directly determine which C–H bonds it can reach. | Quenching experiments and kinetic evidence are needed to distinguish direct HAT from an alternative excited-state process. |
| Indirect photocatalytic HAT | Light and a catalyst first generate a separate thermal abstractor, often through electron transfer, energy transfer or PCET. | The abstractor can be tuned independently from the light-absorbing catalyst, broadening the available selectivity patterns. | The observed net reaction may also be compatible with single-electron transfer followed by proton transfer, so products alone do not establish HAT. |
Both arrangements can use batch vessels or flow photoreactors. Reviews describe ultraviolet, visible-light and solar-light examples, as well as medium-pressure lamps, but no single wavelength, lamp power or reactor geometry is universal. The correct light source is the one validated for the catalyst, concentration, vessel and scale in the specific procedure.
What products can HAT enable?
After abstraction, the carbon-centered radical is directed toward a chosen reaction partner. The following outcomes are represented in photocatalytic HAT literature, although no one method supports every substrate or product class.
| Transformation | Typical radical event | Substrate classes reported in reviews |
|---|---|---|
| C–C bond formation | Radical addition to an alkene or coupling with an aryl or heteroaryl partner. | Ethers, alcohols, alkanes, alkylaromatics, aldehydes and other C–H substrates. |
| C–Si bond formation | Capture of the carbon radical by a silicon reagent. | Selected aliphatic and benzylic C–H substrates, depending on the abstractor and trapping reagent. |
| C–N bond formation | Reaction with an azide, nitrogen-centered reagent or related nitrogen source. | Activated and unactivated C–H sites in method-dependent substrate sets. |
| C–S bond formation | Trapping by a sulfur reagent to install a thioether or related sulfur functionality. | Substrates whose radical polarity and steric environment match the sulfur partner. |
| C–halogen bond formation | Halogen-atom transfer or reaction with a halogenating reagent after radical generation. | Aliphatic, benzylic and other accessible C–H positions in suitable protocols. |
| Oxidation | The radical is converted into an oxygenated, carbonyl or otherwise oxidized product rather than being cross-coupled. | Alcohols, aldehydes, ethers and related oxygen-containing substrates. |
These reactions are valuable for late-stage modification because a complex molecule can sometimes be functionalized at a C–H position without rebuilding the entire structure. Selectivity remains the central constraint: a molecule may contain several bonds that are thermodynamically or geometrically accessible, and the method must favor the desired one.
How selectivity is controlled
The abstractor’s electronic character
Hydrogen abstraction is sensitive to bond strength, polarity and the electronic match between the abstractor and the C–H bond. An electrophilic abstractor can favor electron-rich sites, while a nucleophilic or more neutral abstractor can show a different preference. The molecular structure of the abstractor also controls steric access to crowded positions.
Bond environment and accessibility
Primary, secondary, tertiary, benzylic, allylic and heteroatom-adjacent C–H bonds can have different abstraction rates. A weak bond is not automatically selected if it is buried, poorly oriented or followed by a slow trapping step. Intramolecular distance and conformational accessibility can therefore outweigh a simple bond-dissociation-energy ranking.
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Additives and reaction conditions
Solvent, acid or base additives, oxygen exclusion, concentration and the identity of the radical trap can change both the rate and the product distribution. Additives may alter catalyst quenching, proton availability or the lifetime of the radical. Conditions should be copied from a validated procedure before attempting to generalize them to a new substrate.
Competition experiments and controls
To test a proposed site preference, chemists often compare substrates containing competing C–H bonds, use radical traps or isotopic labeling, and examine whether oxygen or a quencher suppresses the reaction. These experiments can support a HAT pathway, but no single control proves concerted HAT in every system.
HAT versus related pathways
Several mechanisms can produce similar products. Distinguishing them matters because a method that works through single-electron transfer may have different scope, selectivity and functional-group tolerance from a true HAT process.
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| Pathway | Elementary event | What it can look like in a synthesis | What must be established |
|---|---|---|---|
| HAT | Proton and electron move together between partners. | An R–H substrate becomes a radical in one hydrogen-abstraction step. | Evidence for concerted transfer, such as kinetics, isotope effects, thermochemical analysis or carefully designed controls. |
| Single-electron transfer (SET) | Only an electron moves initially, producing a radical ion. | Deprotonation or fragmentation of the radical ion can ultimately give the same carbon radical or product. | Redox potentials, quenching behavior and evidence for the radical-ion intermediate. |
| Proton transfer (PT) followed by electron transfer | A proton moves first and an electron moves later. | A deprotonated substrate is oxidized or reduced in a subsequent step. | Base dependence, proton inventory and stepwise kinetic behavior. |
| Energy transfer | Electronic excitation energy moves without initial charge transfer. | An excited substrate undergoes bond cleavage or another reaction that yields radicals. | Triplet or excited-state evidence and dependence on sensitizer–substrate energy matching. |
The Chemical Reviews discussion specifically warns against assigning HAT merely from the net transformation. Mechanistic assignment should be tied to the actual catalyst cycle and the controls reported for that protocol.
Catalyst and abstractor families
Different HAT systems use different combinations of light absorbers, redox partners and abstractors. Reviews discuss the following families:
- Aromatic ketones: excited ketones can participate directly in hydrogen abstraction from suitable substrates.
- Xanthene dyes: dyes such as Eosin Y can participate in photoredox cycles that generate an abstracting species.
- Polyoxometalates: decatungstate is a prominent hydrogen-abstracting platform for aliphatic C–H functionalization.
- Uranyl salts: photoactive uranium(VI) complexes have been used as HAT catalysts in selected transformations.
- Metal-oxo porphyrins: high-valent metal–oxo species can abstract hydrogen in biomimetic or oxidation-oriented chemistry.
- Tris(amino)cyclopropenium radical dications: these highly oxidizing organic species provide another route to radical generation.
- Transition-metal and organic photoredox catalysts: these can generate a separate abstractor through electron or energy transfer.
“Catalyst” does not mean the same thing in every entry: one protocol may use a catalyst that directly removes hydrogen, while another uses a photocatalyst only to create a short-lived abstractor. Loading, additives and sacrificial oxidants or reductants therefore cannot be transferred from one method to another without validation.
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A practical workflow for planning an HAT synthesis
- Define the target bond. Decide whether the desired outcome is C–C, C–N, C–S, C–Si, C–halogen formation or oxidation, and identify the exact C–H site that must be activated.
- Classify the substrate. Note whether the site is primary, secondary, tertiary, benzylic, allylic, adjacent to oxygen or nitrogen, or part of an alkane, alkene, ether, alcohol, aldehyde or protected amine.
- Choose a documented abstractor class. Match the substrate and desired selectivity to a direct photocatalyst or to an indirect system that generates a suitable thermal abstractor.
- Check the mechanistic evidence. Read the original procedure for isotope effects, radical-trapping tests, quenching data, electrochemical information and any discussion of SET, PT or energy transfer alternatives.
- Match the light and reactor. Use the wavelength range, lamp type, optical path, concentration and vessel specified by the validated method. Do not assume that a visible-light setup can replace a UV setup, or that a batch result will transfer directly to flow.
- Control oxygen and heat. Follow the method’s instructions for degassing, atmosphere, cooling and irradiation distance. Oxygen can intercept radicals and change yields or product identity; intense lamps can also heat the reaction.
- Screen the trapping partner. A radical that forms efficiently can still be lost if addition, coupling or oxidation is slower than side reactions. Keep concentration and stoichiometry within the demonstrated range while changing one variable at a time.
- Confirm the product and pathway. Use appropriate spectroscopic and chromatographic analysis, and check for overoxidation, reduction, dimerization or functionalization at an unintended C–H site.
Light sources, scale-up and laboratory format
Photocatalytic HAT has been demonstrated with ultraviolet, visible and solar illumination. The apparent simplicity of “shine light on the reaction” is misleading: absorption by the catalyst, optical penetration, vessel material, stirring and heat removal all affect the reaction. A lamp that emits the right nominal color may still be unsuitable if the catalyst absorbs weakly at that wavelength or if the geometry creates dark zones.
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Medium-pressure lamps and flow photoreactors appear in the reviewed literature, but the reviews do not establish one universal lamp specification. For scale-up, flow can improve light penetration and heat management, while batch operation may be easier for small exploratory experiments. Any change in path length, concentration or irradiated volume should be treated as a new condition rather than a guaranteed scale translation.
Photochemical work also requires ordinary laboratory controls: shielding from ultraviolet exposure, suitable eye and skin protection, thermal monitoring, pressure awareness for sealed vessels and compatibility checks for solvents and reactor materials. Radical reactions should be run under the atmosphere and safety procedures specified for the particular reagents.
What the published evidence does—and does not—show
Capaldo, Ravelli and Fagnoni reported that more than 50 papers on photocatalytic HAT had appeared during the short period covered by their 2017 review, mainly from 2014 onward. That is a dated, period-bounded count, not a current total for all HAT chemistry.
The same review includes individual transformations reported with yields above 80%. Those yields belong to particular substrates and protocols; they are not an expected performance range for HAT reactions in general. The authors also identify catalyst loading as a limitation in a direct ketone method, showing why a high isolated yield does not by itself establish that a procedure is broadly practical.
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No broad current market-size, universal success-rate or field-wide performance statistic has been established for HAT synthesis. The meaningful comparison is method by method: substrate scope, C–H selectivity, evidence for HAT, catalyst and additive requirements, light format, product class, yield and functional-group tolerance.
Common failure modes and how to diagnose them
No conversion
Check whether the catalyst actually absorbs the selected light, whether the vessel blocks that wavelength, and whether oxygen or a reagent is quenching the excited state. Confirm catalyst concentration, substrate solubility and irradiation distance before changing the entire reaction design.
Reaction occurs at the wrong C–H site
The abstractor may favor a different bond because of polarity, steric access or faster intramolecular geometry. Changing the abstractor or the radical-trapping partner is usually more rational than simply increasing light intensity.
Product decomposes or is overfunctionalized
Lowering irradiation time, reducing oxidant or trap excess, decreasing temperature and improving heat removal can limit secondary radical chemistry. Analyze time-course samples to distinguish slow initiation from product destruction.
Low isolated yield despite substrate consumption
Look for radical dimerization, hydrodehalogenation, oxidation, reduction and chromatography losses. A successful HAT step may still require a faster or more selective trapping reaction, a different solvent or a less reactive work-up.
Results fail on scale-up
Recheck photon delivery, optical path length, mixing and temperature. Larger volumes can create gradients even when the stoichiometry is unchanged; a flow configuration or shorter optical path may be needed.
Bottom line
HAT is a concerted proton-and-electron transfer that converts an R–H bond into a reactive radical. In synthesis, that radical provides a direct entry to C–C, C–Si, C–N, C–S and C–halogen bond formation or oxidation, including late-stage modification of complex molecules. The useful question is not simply whether a product can be drawn from an HAT scheme, but whether the chosen abstractor, catalyst cycle, light source and trapping partner give the required selectivity—and whether experiments support HAT rather than SET, stepwise proton/electron transfer or energy transfer.
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