Radical (chemistry)

A radical has one or more unpaired electrons. In a simple atomic orbital or molecular orbital occupancy picture, an unpaired electron occupies a spatial orbital without an opposite-spin partner in that orbital. A dot such as ${\mathrm{H}}^{\bullet }$ marks this electronic property; it does not mean a missing electron or positive electric charge. Free radicals can be neutral or charged. Terminology also has a boundary: metal ions exhibiting paramagnetism are not normally called free radicals just because they have unpaired electrons.^[ref]

Compare three species containing the same single proton. The hydrogen atom ${\mathrm{H}}^{\bullet }$ has one electron in its $1s$ atomic orbital, and is a neutral radical. The hydrogen cation ${\mathrm{H}}^{+}$ has no electrons, so no unpaired electron. The hydrogen anion ${\mathrm{H}}^{-}$ has two electrons paired in its $1s$ atomic orbital. It is charged but is not a radical in its ground state. With $N_p$ protons, $N_e$ electrons, and positive elementary charge $e$, the independent electric charge count isThe electron pair and electric charge questions require different information. An odd $N_e$ ensures that not all electrons can be paired; an even $N_e$ alone does not ensure pairing, as the molecular orbital description of molecular oxygen illustrates.

The distinction matters whenever a chemical reaction mechanism uses dots and charge signs together. Homolysis gives one bonding electron to each fragment, while heterolysis assigns both to one fragment. A radical chain reaction can pass an unpaired electron from one species to another without generating net electric charge. Detection then exploits the unpaired electron's spin and magnetic moment, rather than merely measuring whether the sample carries an electric charge.

An oxygen-centred radical of form $\mathrm{RO}{\mathrm{O}}^{\bullet }$, often formed by rapid addition of dioxygen to a carbon-centred radical.

The radical $\mathrm{ClO}$ formed when a chlorine atom reacts with ozone. Its reaction partners and reservoirs depend on altitude and temperature; the chlorine monoxide dimer cycle is important under cold, activated polar conditions.

The chlorine peroxide structure formed by association of two ClO radicals with collisional stabilization. Its low-temperature stability and photodissociation provide a route for catalytic ozone destruction. In a laboratory flow system, changing among three ClO precursors and examining infrared spectra helped distinguish the predominant ClOOCl product from an alternative ClOClO isomer. Product assignment and ultraviolet absorption support the proposed intermediate, but do not alone measure an entire atmospheric cycle's ozone-loss rate.^[ref]

A neutral nitrogen-oxide radical, distinct from the nitrate anion. It can add to unsaturated organic molecules and is important in some night-time oxidation pathways.

A radical with an oxygen-centred peroxyl group, commonly formed when a carbon-centred radical adds molecular oxygen. Its reactions can recycle radicals, form an alkoxy radical, or produce a terminating organic nitrate branch, depending on the reaction partner and structure.

The NH free radical, with triplet ground term $X{}^3{\Sigma }^{-}$. Two unpaired electrons occupy perpendicular approximately nonbonding orbitals in a simple valence model.

The CH free radical, whose ground electronic term is $X{}^2\Pi$.

The methyl radical is the neutral radical ${\mathrm{C}{\mathrm{H}}_3}^{\bullet }$, with an unpaired electron. It carries the methane chlorination chain between its two propagation steps.

A radical ion is both a radical and an ion. Removing one electron from neutral benzene can produce ${{\mathrm{C}}_6{\mathrm{H}}_6}^{\bullet +}$, a radical cation; adding one can produce ${{\mathrm{C}}_6{\mathrm{H}}_6}^{\bullet -}$, a radical anion. These formulae specify net electric charge and an unpaired electron, without claiming that both reside on one chemical atom.^[ref]

A radical cation has positive net electric charge and an unpaired electron.

A radical anion has negative net electric charge and an unpaired electron.

A radical chain reaction regenerates a radical during propagation. Initiation supplies free radicals; propagation consumes one and produces another; termination removes chain-carrying free radicals. This explains why one initiating event can lead to many product-forming chemical reactions. The chain-growth polymerization article develops the corresponding growth of polymers.

Removal of propagating free radicals, for example by combination or disproportionation.

Removal of chain carriers without creating a replacement carrier. Pair recombination removes two free radicals per elementary event.

A step consuming a chain carrier and producing another while converting stable reactants. Two different carriers can have different concentrations even when their propagation fluxes agree.

Formation of chain carriers from stable reagents or an external source. Initiation events that create two free radicals contribute twice their event rate to the radical-number balance.

A radical chain in which hydrogen and chlorine radical carriers alternate between propagation steps. With M-assisted chlorine initiation and recombination termination, radical steady-state balances can give first order in H2 and half order in Cl2 without requiring equal propagation rate constants.

Light initiates methane chlorination by photodissociation of $\mathrm{C}{\mathrm{l}}_2$ through homolysis. The elementary bookkeeping isHere $h\nu$ denotes the photon energy. The first propagation step replaces a chlorine radical with a methyl radical; the second regenerates a chlorine radical. Adding the two propagation equations cancels both free radicals and gives $\mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{l}}_2⟶\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}+\mathrm{HCl}$: chloromethane and hydrogen chloride are made while the chain carrier is regenerated.^[ref]

For example, recombination $2{\mathrm{C}{\mathrm{H}}_3}^{\bullet }⟶{\mathrm{C}}_2{\mathrm{H}}_6$ of methyl radicals makes ethane and a covalent bond from two unpaired electrons, removing two carriers. Recombination of two ${\mathrm{Cl}}^{\bullet }$ also terminates carriers; in the gas phase, stabilization of the product requires disposal of released energy, for example to a third collision partner. The figure omits that partner. Termination competes with propagation, and further chlorination can consume chloromethane, so this cycle alone does not predict a pure product or a measured reaction rate.^[ref]

An unpaired electron does not set a universal reaction rate. A stable radical can retain radical character during isolation and storage under specified conditions; a persistent radical survives long enough under specified conditions to be observed. These descriptions concern lifetime and handling, rather than immunity to every possible chemical reaction. DPPH is a practical counterexample to the claim that all free radicals react immediately.

Two mechanisms help explain persistence. Spreading electronic character over several chemical atoms, represented by chemical resonance, can lower the energy relative to a localized description. Bulky surrounding groups can impede approach to a reactive site and slow recombination. Lower energy and slower chemical kinetics are different advantages: neither alone guarantees a long lifetime. A primary computational study explicitly separates these effects, using electronic and geometric descriptors; it does not directly measure every predicted lifetime.^[ref]

DPPH is a neutral organic radical used as a spin reference in electron paramagnetic resonance (EPR), also called electron spin resonance (ESR). Its unpaired electron remains bound within the chemical species: “free radical” does not mean a freely moving electron. The DPPH electron spin resonance experiment makes its magnetic moment observable.

The University of Iowa's Electron Spin Resonance, University of Iowa laboratory protocol (revised July 2007, Experiment and Resonance Data Analysis, printed pp. 2–6) specifies DPPH on cotton in a tube inside an RF coil, between Helmholtz coils. A bridge monitors RF absorption; an oscilloscope records this output and the voltage across a $1\Omega$ current-monitor resistor. Sweeping the coil current passes through the resonant magnetic field. The protocol describes two dips when the sweep revisits resonance; these are repeated crossings, not two distinct spectral lines.

Its calibration replaces the sample with a Hall effect probe, maps the magnetic field in $2\mathrm{mm}$ steps, and accounts for sample dimensions. At least four RF frequencies test the resonance relation. Sweep speed is adjusted to obtain comparable passages. Only a local peak-plus-background fit is prescribed, because the full shape is not Gaussian. The protocol also flags unexplained trailing spikes and possible electronics contributions. These controls constrain field and sweep errors; an empty-tube/cotton blank would be an additional background control, not a reported measurement in this protocol.^[ref]

For an isolated effective spin one-half with dimensionless positive factor $g$, the basic resonance model isHere ${\mu }_{\mathrm{B}}$ is the Bohr magneton, $h$ is the Planck constant, $\nu$ is the applied frequency, and $B_{\mathrm{res}}$ is the calibrated resonant magnetic field. A change in frequency should shift the resonance field proportionally in this model. The factor $g$ describes the electronic magnetic moment coupling, not the species' net electric charge.

A resonance supports the presence of detectable paramagnetism. It does not, by itself, identify an unknown radical: other species exhibiting paramagnetism, impurities, overlapping responses, and interactions with atomic nuclei can complicate interpretation. Conversely, absence of a detectable resonance need not mean absence of free radicals; a low concentration, a short lifetime, or an unsuitable measurement range can conceal them. The occupancy and chain diagrams are original bookkeeping illustrations; neither is a measured or simulated DPPH spectrum.