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CH3

The methyl radical, denoted as CH₃•, is the simplest organic free radical, consisting of a carbon atom bonded to three hydrogen atoms and possessing an unpaired electron on the carbon. It exhibits a planar trigonal geometry with D_3*h symmetry, featuring C-H bond lengths of approximately 1.079 Å and H-C-H bond angles of 120°. This structure arises from sp² hybridization of the carbon atom, resulting in a highly reactive species due to the unpaired electron. It is a metastable gas with a molecular weight of 15.03 g/mol.^[1] The methyl radical plays a pivotal role as a transient intermediate in numerous chemical processes. In combustion chemistry, it is essential in hydrocarbon oxidation pathways, influencing ignition and flame propagation in methane-based fuels. In atmospheric chemistry, it is involved in radical chain reactions that degrade pollutants and contribute to ozone formation. In organic synthesis, methyl radicals enable efficient methylation reactions for constructing carbon-carbon bonds, particularly in late-stage functionalization of pharmaceuticals and materials, often generated via photo-, electro-, or transition-metal-catalyzed methods.^[2] Its reactivity includes hydrogen abstraction, addition to unsaturated bonds, and recombination to form ethane, underscoring its versatility across plasma, astro-, and surface chemistries.

Structure and bonding

Geometry and hybridization

The free methyl radical (CH₃•) adopts sp² hybridization for the carbon atom, resulting in a trigonal planar geometry with H-C-H bond angles of 120° and C-H bond lengths of approximately 1.079 Å.^[3] The three sp² hybrid orbitals lie in the molecular plane and form σ bonds with the hydrogen 1s orbitals, while the unhybridized 2p_z orbital perpendicular to the plane holds the unpaired electron. This planar D_3h structure is the ground state, slightly more stable than a hypothetical pyramidal sp³ arrangement due to the lower energy of sp² hybrids and delocalization of the unpaired electron. The geometry has been confirmed spectroscopically, including through microwave spectroscopy revealing rotational constants consistent with the trigonal planar arrangement. In contrast, the methyl group (-CH₃) in organic molecules such as methane or ethane features sp³ hybridization of the carbon atom, resulting in a tetrahedral geometry with H-C-H bond angles of approximately 109.5°. This sp³ hybridization involves the linear combination of the carbon atom's 2s orbital and three 2p orbitals (2p_x, 2p_y, and 2p_z), producing four equivalent sp³ hybrid atomic orbitals. These orbitals point toward the vertices of a regular tetrahedron, maximizing separation between bonding electron pairs to minimize repulsion per valence shell electron pair repulsion (VSEPR) theory. In the methyl group, three sp³ hybrid orbitals each overlap end-to-end with a hydrogen 1s orbital to form C-H σ bonds, while the fourth forms a σ bond with an adjacent atom.

Bonding characteristics

The three C-H bonds in the methyl radical (CH₃•) are equivalent covalent sigma (σ) bonds, formed by end-to-end overlap of sp² hybrid orbitals on the central carbon atom with 1s orbitals on the hydrogen atoms. These bonds have a length of approximately 1.079 Å and lie in the trigonal plane, with the shared electron pair concentrated along the internuclear axis. The unpaired electron resides in the perpendicular 2p_z orbital, contributing to the radical's high reactivity.^[3] In comparison, the three C-H bonds in the saturated methyl group (-CH₃) are equivalent covalent sigma (σ) bonds, each formed by the end-to-end overlap of an sp³ hybrid orbital on the central carbon atom with a 1s orbital on a hydrogen atom. This overlap creates strong, directional bonds characteristic of tetrahedral geometry, with the shared electron pair concentrated along the internuclear axis.^[4] These C-H bonds in the methyl group have a length of 1.09 Å, typical for sp³-hybridized carbon-hydrogen sigma bonds in simple alkyl groups.^[5] The sp³ hybridization of the carbon atom in the methyl group, which enables this bonding arrangement, is discussed in the geometry and hybridization section. The strength of C-H bonds related to the methyl radical is reflected in their dissociation energies. The energy required to break one C-H bond in methane (CH₄), producing CH₃• and H•, is 439 kJ/mol at 298 K.^[6]

Physical properties

Spectroscopic properties

Infrared (IR) spectroscopy provides insights into the vibrational modes of the methyl radical, which arise from its planar D_{3h} geometry and sp² hybridization. The CH stretching vibrations consist of a symmetric mode (a₁') at approximately 3004 cm⁻¹ and a degenerate asymmetric mode (e') at 3161 cm⁻¹. The characteristic out-of-plane umbrella bending mode (a₂'') occurs at 606 cm⁻¹, while the degenerate in-plane deformation (e') is around 1396 cm⁻¹. These modes are used to identify the radical in gas-phase and matrix isolation studies.^[7] Electron paramagnetic resonance (EPR) spectroscopy is crucial for detecting the unpaired electron in the methyl radical. The spectrum exhibits a symmetric quartet due to hyperfine coupling with the three equivalent protons (nuclear spin I=1/2), with a proton hyperfine coupling constant of approximately 23 G and a g-factor of 2.0025. The equal spacing and 1:3:3:1 intensity ratios confirm the planarity and equivalence of the hydrogens.^[8] For the methyl radical (CH₃•), ultraviolet-visible (UV-Vis) spectroscopy detects weak absorption bands arising from electronic transitions involving the unpaired electron. The spectrum features a broad, relatively weak band centered near 216 nm, attributed to a promotion from the ground state (X ²A₂'') to an excited state with π* character, analogous to n→π* transitions in other radicals due to the non-bonding electron's involvement.^[9] This absorption has a low molar absorptivity (ε ≈ 300 M⁻¹ cm⁻¹), making it suitable for sensitive detection in gas-phase studies, such as combustion processes.^[7] The planar D₃h geometry of the radical influences the transition intensities, with forbidden or weakly allowed bands dominating the spectrum below 300 nm.^[9]

Thermodynamic properties

The standard enthalpy of formation (Δ_fH°₂₉₈) for the gas-phase methyl radical (CH₃•) is +146.5 kJ/mol, reflecting its endothermic nature relative to elemental carbon and hydrogen.^[10] This value, determined through high-level quantum chemical calculations and experimental benchmarks, underscores the energetic cost of generating the radical from stable precursors like methane.^[10] The standard molar entropy (S°) of the gas-phase methyl radical at 298 K and 1 bar pressure is 194.2 J/mol·K, consistent with its planar structure and three equivalent hydrogen atoms contributing to rotational and vibrational degrees of freedom.^[11] The corresponding standard Gibbs free energy of formation (Δ_fG°₂₉₈) is +148.7 kJ/mol, calculated from the enthalpy and entropy data using the relation Δ_fG° = Δ_fH° - T Δ_fS°.^[11]^[10] As an unstable intermediate species, the isolated methyl radical does not exhibit defined phase transitions under standard conditions and exists solely in the gas phase at room temperature.

Methyl derivatives

Methyl cation

The methyl cation, \ce{CH3+}, possesses a planar D_{3h} geometry, with the central carbon atom sp²-hybridized and bonded to three equivalent hydrogen atoms, featuring an empty p-orbital perpendicular to the plane that imparts electron deficiency and results in C-H bond angles of 120°.^[12] This configuration contrasts with the neutral methyl radical, which shares a similar planar arrangement but includes a singly occupied p-orbital. The empty p-orbital renders \ce{CH3+} highly reactive as a strong electrophile, prone to rapid reactions with nucleophiles in both laboratory and astrophysical environments.^[12] In laboratory settings, \ce{CH3+} forms in the gas phase or superacid media via protonation of methane (\ce{CH4}), initially yielding the protonated methanium ion \ce{CH5+}, which then undergoes elimination of molecular hydrogen to generate the cation.^[13] This process underscores its inherent instability in condensed phases, where it persists only under isolated conditions like matrix isolation or mass spectrometry.^[13] Astronomically, \ce{CH3+} plays a pivotal role in interstellar chemistry as a precursor to complex hydrocarbons, initiating gas-phase organic synthesis through reactions with species like \ce{H2} and electrons.^[14] Its first detection occurred in 2023 using the James Webb Space Telescope's Mid-Infrared Instrument (MIRI), which captured rovibrational emission lines in the protoplanetary disk d203-506 within the Orion star-forming region, approximately 1,350 light-years from Earth; these observations confirm its abundance in UV-irradiated disk atmospheres, where photochemical processes drive its formation and sustain its concentration at levels enabling detectable signals.^[14]

Methyl anion

The methyl anion, CH₃⁻, is a highly reactive carbanion characterized by a pyramidal geometry with C_{3v} symmetry, analogous to the structure of ammonia. The central carbon atom adopts sp³ hybridization, featuring three σ bonds to hydrogen atoms and a lone pair in the fourth sp³ orbital, resulting in bond angles of approximately 110° and an out-of-plane inversion barrier of about 1.7 kcal/mol.^[15] This pyramidal configuration distinguishes it from the planar methyl cation, highlighting the anion's nucleophilic nature driven by the localized negative charge on carbon.^[16] Due to its extreme basicity, with the pK_a of its conjugate acid methane estimated at approximately 50, the methyl anion acts as a superbase and is inherently unstable in conventional solution environments, readily abstracting protons from even weak acids.^[17] It is primarily generated and studied in the gas phase via techniques such as ion cyclotron resonance spectroscopy or pulsed plasma discharge, or isolated in low-temperature noble gas matrices to prevent decomposition.^[18] Quantum chemical calculations, including ab initio methods and density functional theory, reveal a low electron affinity for the methyl radical (approximately 0.08 eV or 1.8 kcal/mol), indicating marginal stability of the anion relative to dissociation into CH₃ and an electron.^[19] These computations also underscore the anion's strong proton abstraction tendency, with potential energy surfaces showing low barriers for deprotonation reactions, reinforcing its role as an elusive intermediate in gas-phase ion chemistry.^[15]

Methyl radical

The methyl radical (CH₃•) adopts a planar geometry with D_{3h} symmetry, arising from sp² hybridization at the central carbon atom, which positions the three hydrogen atoms in a trigonal arrangement. The unpaired electron resides in a p-orbital perpendicular to this plane, resulting in C-H bond angles of approximately 120° that minimize steric repulsion between the hydrogens while maximizing orbital overlap. This radical species is primarily generated in the gas phase through homolytic cleavage of the C-H bond in methane, as in the process CH₄ → CH₃• + H•, often initiated by thermal, photochemical, or plasma-driven dissociation.^[20] In dilute gas-phase conditions, the methyl radical exhibits a short lifetime, typically around 1.4 milliseconds, limited by its high reactivity toward self-combination.^[21] The dominant decay pathway for the methyl radical in the gas phase is dimerization to form ethane via the recombination reaction:

$2 \text{CH}_3^\bullet \rightarrow \text{C}_2\text{H}_6

This process is barrierless, with an activation energy of 0 ± 0.7 kcal/mol, reflecting the favorable coupling of the two unpaired electrons without an energetic barrier.^[22] The rate constant in the high-pressure limit follows a negative temperature dependence, expressed as k_\infty = (5.66 \pm 0.43) \times 10^{-11} (T/298)^{-0.37} cm³ molecule⁻¹ s⁻¹ over 292–714 K, underscoring the reaction's efficiency at lower temperatures and higher pressures where third-body stabilization prevents redissociation.^[23]

Reactivity

Electrophilic behavior

The methyl group exhibits electrophilic behavior primarily through equivalents of the methyl cation (CH₃⁺), which serves as a reactive intermediate in various substitution reactions. One prominent example is the methylation of carboxylic acids using diazomethane (CH₂N₂), where the reagent acts as a source of electrophilic methyl under acidic conditions. The mechanism begins with proton transfer from the carboxylic acid (RCOOH) to diazomethane, generating an electrophilic diazonium ion (CH₃N₂⁺) and the corresponding carboxylate (RCOO⁻). This is followed by nucleophilic attack of the carboxylate on the diazonium ion, displacing N₂ to form the methyl ester (RCOOCH₃). This process is highly selective and tolerant of various functional groups, proceeding under mild conditions with N₂ as the sole byproduct.^[24] In electrophilic aromatic substitution, methyl halides such as methyl chloride (CH₃Cl) can introduce the methyl group onto aromatic rings via Friedel-Crafts alkylation, facilitated by Lewis acids like AlCl₃. The mechanism involves coordination of the Lewis acid to the halide, promoting ionization to form a complexed methyl cation equivalent, such as [CH₃-Cl-AlCl₃]⁺. This electrophile then attacks the aromatic π-system, leading to a σ-complex intermediate, followed by deprotonation to yield the methylated arene, such as toluene from benzene. Unlike higher alkyl halides, methylations avoid carbocation rearrangements due to the stability and simplicity of CH₃⁺, though polyalkylation can occur if not controlled. This reaction is particularly useful for preparing methyl-substituted aromatics and has been studied extensively in the context of AlCl₃-catalyzed systems. SN1 pathways involving methyl carbocation intermediates are characterized by unimolecular rate-determining dissociation of the methyl halide (CH₃X) to generate CH₃⁺, followed by rapid nucleophilic capture. The rate equation for such processes is first-order, expressed as rate = k [CH₃X], independent of nucleophile concentration, reflecting the slow formation of the highly reactive but unstable methyl cation. While methyl halides predominantly favor SN2 mechanisms in solution due to the poor stability of CH₃⁺, SN1-like behavior has been observed in solvolysis studies under conditions promoting ionization, such as in highly polar solvents or with strong acids, where relative rates underscore the energetic barrier to carbocation formation (e.g., methyl << primary < secondary < tertiary). The methyl cation's planar, empty p-orbital structure facilitates backside attack but leads to racemization in chiral contexts when applicable.

Nucleophilic behavior

The methyl group displays nucleophilic behavior predominantly through its deprotonated form, the methyl anion (\ce{CH3^-}), which acts as a highly reactive nucleophile in synthetic applications. The C-H bonds of the methyl group, as in methane (\ce{CH4}), possess a pK_a value of approximately 50, underscoring their extremely weak acidity and necessitating exceptionally strong bases for deprotonation to generate the carbanion.^[25]^[26] Direct deprotonation of methane using bases like alkyllithiums such as n-BuLi is thermodynamically demanding and rarely practical in standard solution conditions due to the high pK_a, but the methyl anion is routinely accessed via organolithium reagents like methyllithium (\ce{CH3Li}), which serve as effective carbanion equivalents. Methyllithium, prepared from methyl halides and lithium metal, delivers the nucleophilic methyl unit in a variety of transformations, exhibiting both strong basicity and nucleophilicity owing to the polarized C-Li bond.^[27]^[28] These reagents enable the formation of new carbon-carbon bonds by transferring the methyl group to electrophiles. The methyl anion engages in nucleophilic substitution reactions, favoring the SN2 pathway due to its compact size and high electron density at carbon, which promotes backside attack on electrophilic centers. This reactivity is particularly pronounced with unhindered substrates like methyl halides (\ce{CH3X}), where the absence of steric hindrance at the substrate carbon accelerates the bimolecular displacement, yielding products such as ethane from \ce{CH3^- + CH3I -> CH3CH3 + I^-}. In broader synthetic contexts, methyllithium performs SN2 alkylations on primary alkyl halides or other electrophiles, such as carbonyls, with rates enhanced by solvents that solvate the lithium counterion, minimizing ion pairing.^[29]^[30] The inherent nucleophilicity of the methyl anion, characterized by its low methyl anion affinity toward common electrophiles and pyramidal geometry that concentrates negative charge, positions it among the strongest carbanionic nucleophiles, though its instability limits isolation to gas-phase or matrix conditions.

Radical reactions

The methyl radical (CH₃•) plays a key role in initiating free radical polymerization of methyl acrylates, such as methyl acrylate (CH₂=CHCOOCH₃), by adding to the electron-deficient double bond of the monomer. This addition step forms a carbon-centered radical on the α-carbon, typically represented as CH₃CH₂ĊHCOOCH₃, which serves as the propagating species in chain growth. Theoretical studies using ab initio methods have calculated the barrier for this initiation reaction at approximately 10-15 kJ/mol in the gas phase, highlighting its feasibility under typical polymerization conditions. In the propagation phase, the adduct radical adds to additional monomer units, extending the polymer chain, while initiation by CH₃• sets the end-group structure. Termination occurs primarily through combination of two growing radicals or disproportionation, yielding poly(methyl acrylate with controlled molecular weight distribution. Experimental and computational assessments confirm that the rate constant for CH₃• addition to methyl acrylate is on the order of 10⁶-10⁷ L mol⁻¹ s⁻¹ at 300 K, influencing the overall polymerization kinetics and polymer properties like tacticity. In combustion processes, the methyl radical participates in hydrogen abstraction reactions as a propagation step, where CH₃• + RH → CH₄ + R• sustains radical chain branching in hydrocarbon fuels. This reaction is critical in low-temperature combustion regimes, with evaluated rate constants varying by the C-H bond strength of RH; for example, abstraction from methane (CH₄) has k ≈ 1.1 × 10⁻¹⁴ exp(-4080/T) cm³ molecule⁻¹ s⁻¹, while for alkanes like ethane, it is faster due to weaker secondary C-H bonds. Such abstractions contribute to the formation of reactive alkyl radicals (R•), accelerating oxidation and influencing ignition delays in engines.^[31] Coupling reactions involving the methyl radical are exemplified in the Kharasch addition (atom transfer radical addition, ATRA), where CH₃• initiates the process by adding to terminal alkenes, facilitating subsequent transfer of electrophilic groups like halogens or chalcogens from additives such as alkyl iodides or xanthates. In this mechanism, thermal decomposition of di-tert-butylhyponitrite generates CH₃•, which adds to the alkene to form a secondary radical; this then abstracts a halogen (e.g., I) from the additive, propagating the chain and yielding α-functionalized products with high regioselectivity. Recent advancements demonstrate broad substrate scope, including unactivated alkenes, under mild reflux conditions in ethyl acetate, with yields often exceeding 80% and minimal byproducts.^[32]

Applications and occurrence

In organic synthesis

The methyl radical CH₃• serves as a key intermediate in radical-mediated methylation reactions, enabling the selective installation of methyl groups, often in late-stage functionalization of complex molecules. These methods typically involve the generation of CH₃• from precursors like methanol, dimethyl sulfoxide (DMSO), or peroxides via photo-, electro-, or transition-metal catalysis, avoiding the limitations of traditional ionic methylating agents.^[2] One prominent application is the Minisci-type C-H methylation of heteroarenes, such as pyridines and quinolines. For example, silver-mediated decarboxylation of acetic acid or decomposition of peroxides generates CH₃•, which adds to the electron-deficient ring, followed by rearomatization to yield methylated products. This approach is valuable for diversifying pharmaceutical scaffolds.^[33] Photoredox catalysis has advanced CH₃• generation for milder conditions. In a method developed by MacMillan et al., dual photoredox and hydrogen atom transfer (HAT) catalysis activates methanol as a methyl source, allowing C-H methylation of (hetero)arenes with iridium photocatalysts and thiol HAT agents, achieving high site selectivity in drug-like molecules. Similarly, Glorius et al. employed photocatalytic reduction of DMSO-derived species for isoquinoline methylation. These techniques support the "magic methyl" effect, where adding a methyl group enhances biological activity in medicinal chemistry.^[33]^[2] Transition-metal-catalyzed processes further expand applications. Nickel or copper catalysts facilitate CH₃• addition in cross-couplings or conjugate additions, such as Ni/photoredox systems using dimethylnickel precursors for sp³ C-methylation. These methods are applied in synthesizing methylated natural product fragments and materials.^[2]

Biological significance

The methyl radical CH₃• has no established direct role in biological systems due to its high reactivity and short lifetime, which prevent stable incorporation in vivo. However, it appears as a transient intermediate in certain radical chain processes studied in biochemical contexts, such as mechanistic investigations of DMSO-derived radicals in cellular environments. Unlike the methyl group (-CH₃) in epigenetics or metabolism, CH₃• does not participate in enzymatic methyl transfers. Its biological relevance is primarily indirect, through oxidative stress or radical scavenging pathways, but specific functions remain underexplored as of 2025.^[34]

Special topics

Chiral methyl groups

Isotopically substituted methyl groups, such as -CHD₂ and -CHTD, acquire chirality when the three hydrogen positions are differentiated by deuterium (²H) and/or tritium (³H) substitution, particularly when attached to a prochiral center like a carbonyl carbon in acetate derivatives. In the case of -CHTD, the methyl carbon becomes a stereogenic center with four distinct substituents—the attachment point, H, D, and T—rendering the group inherently chiral and non-superimposable on its mirror image. For -CHD₂, chirality emerges in the context of stereospecific substitution at a prochiral center, where the non-equivalent H and D atoms allow discrimination between re and si faces during enzymatic addition or abstraction, enabling detailed stereochemical analysis without the group itself being tetrahedral chiral. This isotopic chirality has been pivotal in elucidating the stereospecificity of biochemical transformations involving methyl groups.^[35]^[36]^[37] Synthesis of these chiral methyl groups often relies on enzymatic resolution to achieve high enantiopurity. Seminal work by Cornforth involved the kinetic resolution of racemic [1-²H₁, ²-³H₁]ethanol using yeast alcohol dehydrogenase, selectively oxidizing one enantiomer to the corresponding chiral acetate while leaving the other intact; subsequent chemical transformations yield substrates like chiral S-adenosylmethionine analogs with defined (R) or (S) configuration at the methyl carbon. Asymmetric deuteration methods complement this by employing chiral catalysts for stereoselective incorporation of deuterium, such as rhodium-phosphine complexes in the hydrogenation of prochiral precursors to generate enantioenriched -CHD₂ or -CHTD groups, achieving enantioselectivities exceeding 99% in some cases. These approaches ensure the isotopic labels are introduced with precise stereocontrol, avoiding racemization during synthesis.^[38]^[35]^[39] In nuclear magnetic resonance (NMR) spectroscopy, these chiral methyl groups serve as probes for determining enzyme stereospecificity, particularly in reactions involving hydrogen abstraction or transfer at methyl sites. For instance, in studies of radical SAM methyltransferases like Fom3 in fosfomycin biosynthesis, feeding substrates with (methyl-R)- or (methyl-S)-[methyl-²H₁, ³H₁]methionine reveals retention of configuration through analysis of tritium retention ratios (F values) and deuterium isotope effects, confirmed via ³H NMR of degradation products. Similarly, stereospecifically labeled -CHD₂ groups in protein side chains (e.g., valine or leucine) enable assignment of pro-R versus pro-S methyl positions, facilitating NMR-based mapping of enzymatic active sites and their chiral preferences in high-molecular-weight systems. This methodology has established that many enzymes, such as citrate synthase, operate with strict pro-R or pro-S specificity at methyl groups, providing insights into catalytic mechanisms.^[40]^[41]^[42]

Internal rotation

The internal rotation of the methyl group around the C–X bond in methyl-substituted compounds, such as ethane (CH₃–CH₃), is characterized by a low potential energy barrier of approximately 12 kJ/mol (or 2.9 kcal/mol), primarily due to torsional strain arising from the repulsion between adjacent hydrogen atoms in eclipsed conformations.^[43] This barrier represents the energy difference between the staggered (minimum energy) and eclipsed (transition state) conformations, enabling rapid rotation at ambient temperatures with a timescale on the order of picoseconds.^[44] The sp³ hybridization of the carbon atoms facilitates this nearly free rotation by providing equivalent bonding orbitals.^[45] Microwave spectroscopy has been instrumental in quantifying this rotational dynamics through measurements of the molecule's rotational constants (A, B, and C), which describe the moments of inertia and reveal splittings in the spectrum caused by the internal motion of the methyl groups.^[46] For instance, in partially deuterated ethane (CH₃CHD₂), high-resolution microwave spectra of torsional transitions allowed precise determination of the barrier height as V₃ = 1010.39 ± 0.10 cm⁻¹ (equivalent to ~12.05 kJ/mol), confirming the torsional potential's threefold symmetry.^[46] These rotational constants, typically on the order of several GHz for ethane, provide direct experimental access to the structural and dynamic parameters without relying solely on computational models.^[47] Quantum mechanically, the internal rotation is described by the hindered rotor model, which treats the methyl group as a symmetric top rotor subject to a periodic potential barrier, leading to a Hamiltonian that includes kinetic and potential terms for the torsional angle θ.^[48] The Schrödinger equation for this one-dimensional hindered rotor is solved to yield quantized torsional energy levels, with the barrier height modulating the splitting between symmetric and antisymmetric states; for ethane, this model accurately reproduces the observed microwave splittings and thermodynamic contributions from hindered rotation.^[49] This framework, originally developed in the 1930s for ethane, accounts for the quantum tunneling through the barrier, which further lowers the effective rotational freedom compared to a classical free rotor.^[50]

History and nomenclature

Discovery and etymology

The methyl group, denoted as CH₃, was first identified as a distinct chemical entity in the early 19th century through investigations into the composition of organic compounds, particularly during studies of wood-derived substances. In 1834, French chemists Jean-Baptiste-André Dumas and Eugène-Melchior Péligot isolated pure methanol (then called "wood spirit" or esprit de bois) by careful distillation of wood, determining its elemental composition as consisting of carbon, hydrogen, and oxygen in the ratio corresponding to CH₄O. They further prepared several derivatives, including methyl chloride and methyl iodide, by reacting methanol with halogens under specific conditions, which revealed a common univalent radical equivalent to CH₃ present across these compounds. This work, detailed in their seminal memoir presented to the Académie des Sciences on October 27 and November 3, 1834, marked the initial recognition of the methyl group as a fundamental building block in organic chemistry. In the context of the emerging radical theory of organic structure proposed by Jöns Jacob Berzelius, their findings demonstrated that the methyl group behaved consistently in substitution reactions across alcohols, ethers, and halides, supporting the idea of persistent radicals (meaning atomic groups) in organic compounds and paving the way for systematic classification of homologous series. The free methyl radical (CH₃•), however, was not isolated until the 20th century. In 1929, German chemists Friedrich A. Paneth and Wilhelm Hofeditz provided the first experimental evidence for its existence using the "mirror technique." They decomposed tetramethyllead vapor (Pb(CH₃)₄) at low pressure in a flow system, observing that metallic mirrors (e.g., antimony) placed downstream were removed and could be recoated farther along the tube, indicating the transient presence of highly reactive CH₃• radicals that abstracted atoms from the mirror surface. This demonstration confirmed the methyl radical as a true free radical species, distinct from the conceptual group identified earlier, and advanced the understanding of free radical chemistry.^[51] The etymology of "methyl" traces directly to this historical context, combining the Greek methy (μέθυ, meaning "wine") with hylē (ὕλη, meaning "wood" or "matter"), to denote the alcohol derived from wood in contrast to ethyl alcohol from fermented wine. Dumas and Péligot initially termed the alcohol "méthylique" and the related divalent group CH₂ as "méthylène" in their 1835 publication, establishing nomenclature that emphasized the substance's origin. By 1840, the term "methyl" had been adopted for the CH₃ radical itself (in the group sense), solidifying its place in chemical terminology.^[52]

Nomenclature

In organic chemistry, the methyl group, represented as −CH₃, is the simplest alkyl substituent derived from methane (CH₄) by the removal of one hydrogen atom. According to IUPAC substitutive nomenclature, it is named "methyl" by replacing the final "-e" of the parent hydride name with the suffix "-yl," and it functions as a prefix in naming more complex compounds.^[53] For example, in alkanes, a chain with a methyl substituent attached to the second carbon of propane is named 2-methylpropane.^[53] When multiple methyl groups are present, multiplicative prefixes such as "di-," "tri-," or "tetra-" are used, and they are arranged in alphabetical order with other substituents, ignoring the multiplicative prefixes for alphabetization. Retained names like "isopropyl" (for (CH₃)₂CH−) are permitted alongside systematic names, but "methyl" itself is a retained trivial name applicable without restriction.^[53] The methyl radical, denoted as CH₃•, follows similar IUPAC conventions for radicals, where it is named "methyl" as a retained name for the monovalent radical obtained by removing one hydrogen from methane, with the unpaired electron at the carbon center. The systematic name is "methanidyl," but the trivial name "methyl" is preferred and widely used in chemical literature for this species.^[54] In radicofunctional nomenclature, it may appear in names like "methyl alcohol" for CH₃OH, treating the radical as a separate component.^[55]

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