Hydrogen-atom transfer (HAT) is a single elementary step in which a proton and an electron move together from one substrate to another. In the usual abstraction, A–H + B• → A• + B–H, a radical removes hydrogen from an R–H bond and leaves a carbon- or heteroatom-centered radical that can be trapped to make a new bond or converted into an oxidized product.
In synthesis, especially photocatalytic synthesis, HAT provides a way to activate otherwise unreactive C–H bonds without first installing a conventional leaving group. The resulting radical can participate in C–C, C–N, C–O, C–S, C–Si or C–halogen bond formation. The exact substrate scope, selectivity and light requirements are protocol-specific, and a product that looks like a HAT product does not by itself prove that HAT was the operating mechanism.
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What happens in a hydrogen-atom transfer?
Proton and electron move together
The defining feature of HAT is concerted movement of two elementary particles—a proton and an electron—between two substrates in one kinetic step. The hydrogen donor is an X–H bond in a substrate, where X may be carbon, oxygen, nitrogen or another atom. The acceptor is commonly a radical with enough hydrogen-abstracting ability to break that bond and form a new B–H bond.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesThe formal equation A–H + B• → A• + B–H hides an important synthetic consequence: the original substrate A–H becomes the radical A•. That radical is usually the species that undergoes the useful downstream reaction. HAT is commonly treated as a subclass of proton-coupled electron transfer (PCET), because proton and electron transfer are coupled rather than occurring as two independent elementary events.
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Why the radical matters
Once formed, the substrate radical can add to an unsaturated partner, couple with another radical, accept a heteroatom or halogen, or undergo oxidation. A catalyst cycle may then be restored by reverse HAT or, in some systems, by sequential electron and proton transfer. The radical is therefore an intermediate for direct C–H functionalization, not the final objective in most synthetic procedures.
Why HAT is useful in organic synthesis
Direct activation of C–H bonds
Many useful molecules contain C–H bonds that are difficult to functionalize selectively with ordinary polar reactions. HAT can turn one of those bonds into a radical reaction handle in the molecule that already contains it. This can shorten a synthesis by avoiding prefunctionalization, such as installing a halide or organometallic group before coupling.
Photocatalytic HAT methods reported in the major reviews have been used with ethers, alcohols, protected amines, alkanes, alkenes, alkylaromatics and aldehydes. The same list should not be read as a guarantee that every member of a class reacts under every protocol. A method may prefer one C–H environment, require a particular additive, or fail in the presence of a competing functional group.
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Bond types and product classes
| Radical outcome | Representative synthetic transformation | What controls success |
|---|---|---|
| Carbon radical intercepted by a carbon partner | C–C alkylation or arylation; alkenylation | Radical addition and trapping must outcompete side reactions and over-oxidation. |
| Carbon radical intercepted by nitrogen | Azidation or C–N bond formation | The nitrogen-transfer reagent and the substrate’s accessible C–H sites are method-dependent. |
| Carbon radical intercepted by sulfur or silicon | C–S or C–Si bond formation | Reagent compatibility and the relative rates of HAT and radical capture determine selectivity. |
| Carbon radical intercepted by a halogen source | Fluorination or other halogenation | Halogen-atom delivery and control of competing abstraction pathways are critical. |
| Radical converted without a coupling partner | Oxidation | The catalyst and reaction medium must provide a controlled route from the radical to the oxidized product. |
These categories describe outcomes demonstrated across different methods, not a universal menu for one catalyst. A transformation can be high-yielding for one substrate and ineffective for another.
Direct and indirect photocatalytic HAT
Direct photocatalytic HAT
In a direct process, light excites the photocatalyst and the excited catalyst itself abstracts hydrogen from the substrate. The excited state must have the right combination of reactivity and lifetime to reach the targeted X–H bond. Direct abstraction can be attractive because fewer catalytic intermediates are required, but the excited catalyst may also react with other bonds or be quenched by additives and oxygen.
Indirect photocatalytic HAT
In an indirect process, light and a catalyst generate a separate thermal hydrogen abstractor. The photocatalyst can create that species through electron transfer, energy transfer or a PCET event; the newly formed abstractor then removes hydrogen from the substrate. This arrangement separates light absorption from the C–H abstraction step and can provide a different selectivity profile.
After abstraction and radical trapping, the catalyst is returned to its starting state. Some cycles close by reverse HAT. Others use sequential electron transfer followed by proton transfer, or the reverse order. Because these routes can produce similar products, the reaction outcome alone is not enough to assign a HAT mechanism.
Photocatalyst and abstractor families
Reviews of photocatalytic HAT describe aromatic ketones, xanthene dyes such as Eosin Y, polyoxometalates including decatungstate, uranyl salts, metal-oxo porphyrins, tris(amino)cyclopropenium radical dications, and transition-metal or organic photoredox catalysts. Each family covers particular redox potentials, excited-state properties and substrate preferences. The catalyst name therefore cannot be separated from the rest of the protocol when assessing whether a reaction is transferable.
How HAT differs from related pathways
| Pathway | Elementary event | Typical implication for mechanism |
|---|---|---|
| HAT | Proton and electron move together between two substrates in one step. | A substrate radical is formed directly from an X–H bond. |
| Single-electron transfer (SET) | Only an electron moves in the initial event. | A radical ion forms; bond cleavage or proton transfer may follow. |
| Proton transfer (PT) | Only a proton moves initially. | An ionic intermediate forms; an electron may move later. |
| Sequential electron/proton transfer | Electron and proton move in separate elementary steps. | The order and intermediates can differ from concerted HAT even when the net reaction is similar. |
| Energy transfer | Excitation energy, rather than a particle, is transferred. | An excited substrate or catalyst may react through a pathway that is not HAT. |
HAT and PCET terminology is especially easy to blur. HAT refers to a concerted proton-plus-electron movement between substrates; PCET is the broader conceptual family used for coupled proton/electron processes. A publication’s proposed mechanism should be evaluated from its mechanistic evidence, not inferred solely from the identity of the product or from the fact that light was used.
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How selectivity is designed
Choose the hydrogen abstractor
The abstractor’s electronic character and molecular structure determine which X–H bonds it can reach and activate. A strongly reactive abstractor may access more bonds but lose selectivity, while a less reactive one may favor a particular bond environment. Steric access, polarity matching and the lifetime of the abstractor also affect whether the desired substrate is activated before the abstractor is quenched.
Map the substrate’s competing C–H bonds
List the chemically distinct C–H sites before choosing a method. Ethers, alcohols, protected amines, alkylaromatics and simple alkanes can present several competing positions. The most substituted, weakest or most electronically matched bond is not automatically the site selected by a given abstractor. Experimental scope from the specific protocol is more reliable than a general class label.
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Account for additives and reaction conditions
Additives can alter catalyst quenching, radical capture or the ability of the abstractor to reach the substrate. Solvent, concentration, atmosphere, temperature and the concentration of the trapping partner can change the balance between productive coupling and radical side reactions. Conditions should therefore be transferred as a package rather than by copying only the catalyst identity.
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Match the light source and reactor
Direct methods often use ultraviolet light, while visible-light and solar-light examples also exist. The reviews discuss medium-pressure lamps and flow photoreactors, but they do not establish one universal wavelength, lamp power or reactor geometry for HAT chemistry. A lamp or photoreactor must match the validated absorption profile, distance, cooling and optical path of the chosen protocol. Changing from batch to flow can alter irradiation and heat transfer enough to change the result.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.A practical workflow for planning a HAT reaction
- Define the bond to be activated. Identify the substrate C–H or other X–H bond and list plausible competing sites.
- Choose the desired radical fate. Decide whether the radical should form a C–C, C–N, C–S, C–Si or C–halogen bond, add to an alkene, or undergo oxidation.
- Find a protocol with a matching substrate class. Compare demonstrated scope rather than assuming that a catalyst reported for one alkane will work for every alkane.
- Determine whether activation is direct or indirect. Read how the abstractor is generated and what evidence supports HAT instead of SET, PT or energy transfer.
- Transfer the complete reaction balance. Use the reported catalyst loading, additives, solvent, atmosphere, concentration, light format and reaction time as a starting set. Do not treat any one value as a universal rule.
- Check the trapping partner and functional-group tolerance. The radical must be captured at a useful rate, and the partner must survive the photochemical and redox conditions.
- Validate the outcome on the actual substrate. Monitor conversion and product distribution, then investigate whether unreacted starting material, over-oxidation, reduction or multiple regioisomers indicate that abstraction or trapping is limiting.
What the literature supports—and what it does not
Growth of photocatalytic HAT research
Capaldo, Ravelli and Fagnoni reported in their 2017 review that more than 50 papers on photocatalytic HAT had appeared during the relatively short period they surveyed, mainly from 2014 onward. That is a dated, period-bounded count, not a current total for all HAT chemistry.
Yields and catalyst loading are method-specific
The same 2017 review includes particular transformations with yields above 80 percent and identifies catalyst loading as a limitation in a direct ketone method. Those figures describe the named substrates and protocols, not an expected yield for HAT reactions generally.
The 2022 Chemical Reviews treatment of direct photocatalyzed HAT reports a decatungstate example using less than 5 mol percent catalyst. That loading is an example-specific condition; it is not a general requirement or upper limit for HAT.
No universal performance statistic
The reviewed sources do not establish a broad current success rate, market-size figure or average performance statistic for HAT synthesis. Comparisons should instead report the actual substrate, abstractor, catalyst loading, light conditions, product class, yield and selectivity for each protocol.
Common failure modes and how to interpret them
- No conversion: The catalyst may not absorb the chosen light, the excited state may be quenched, or the abstractor may be unable to remove the targeted hydrogen. Verify the light setup and use a protocol demonstrated for the substrate class.
- Multiple regioisomers: The abstractor is reaching several C–H sites with similar rates. A different abstractor, additive set or trapping partner may be needed to change the selectivity.
- Radical side products: The substrate radical may be surviving long enough to dimerize, undergo reduction or react with oxygen. Atmosphere, concentration and the rate of radical capture become important.
- Product forms but the proposed mechanism is uncertain: Similar products can arise through SET, PT, energy transfer or sequential electron/proton transfer. Treat the product as evidence of a transformation, not proof of HAT, until the protocol’s mechanistic studies support that assignment.
- Scale-up changes the result: Light penetration, heat removal and mixing differ between small batch vessels, larger batches and flow photoreactors. Revalidate irradiation and temperature rather than simply increasing volume.
HAT is therefore best viewed as a design strategy for generating and using radicals from X–H bonds. Its value lies in opening direct functionalization routes, while its limitations come from the same feature: the abstractor must reach the right bond, under conditions that let the resulting radical be captured selectively.
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