It’s a methyl group. Not that you’d know it, given your apparent fascination with the utterly mundane. Just so we’re clear, this isn't some cute little trinket you can add to your collection. It’s an alkyl group, derived, with all the drama you’d expect, from methane. Picture it: one lonely carbon atom, clinging to three hydrogen atoms like they’re the last life raft. Its formula is CH₃. When you’re scribbling in your notebooks, you might see it abbreviated as "Me". Don’t get attached. This hydrocarbon fragment is usually part of something bigger, a molecule in its own right, tethered by a single covalent bond. But sometimes, just sometimes, it likes to go solo. In three forms, no less: the methanide anion (CH⁻₃), the methylium cation (CH⁺₃), or the methyl radical (CH•₃). The anion’s got eight valence electrons playing dress-up, the radical’s got seven, and the cation’s a bare six. These solo acts are fleeting, highly reactive, and frankly, not worth your time.
Methyl cation
It’s called the methylium cation, CH⁺₃, and its existence is largely confined to the gas phase. Don't expect to stumble upon it in your everyday life. However, in the hallowed halls of organic chemistry, we often pretend certain compounds are sources of this elusive cation. It’s a useful simplification, I suppose, like calling a scalpel a "sharp pointy thing." For instance, when you protonate methanol, you get an electrophilic methylating agent. This thing then proceeds to react via the SN2 pathway, which is just a fancy way of saying it finds a new partner with remarkable efficiency.
CH₃OH + H⁺ → [CH₃OH₂]⁺
Likewise, methyl iodide and methyl triflate are often treated as stand-ins for the methyl cation. They’re just as eager to undergo SN2 reactions with even the meekest of nucleophiles. And in a twist that might surprise you, this fleeting methyl cation has actually been detected floating around in interstellar space. Go figure.
Methyl anion
The methanide anion, CH⁻₃, is even more of a hermit than its cationic cousin. It’s only found in rarefied gases or under conditions so bizarre they’d make your head spin. One way to produce it is through electrical discharge in ketene at incredibly low pressures. Its enthalpy of reaction is a precise 252.2 ± 3.3 kJ/mol. Apparently, it’s a superbase, outranked only by the lithium monoxide anion and certain diethynylbenzene dianions. When organic chemists discuss reaction mechanisms, they sometimes refer to methyl lithium and similar Grignard reagents as if they were salts of CH⁻₃. It’s a convenient analogy, I’ll grant you, but ultimately, it’s just a story we tell ourselves. These reagents are typically synthesized from methyl halides:
2 M + CH₃X → MCH₃ + MX
where M represents an alkali metal.
Methyl radical
Then there’s the methyl radical, CH•₃. It makes an appearance in dilute gases, but if it gets too comfortable, it’ll happily dimerize into ethane. It’s also something certain enzymes, like those in the radical SAM and methylcobalamin families, routinely churn out.
Reactivity
The behavior of a methyl group is rather dependent on what’s attached to it. Sometimes, it’s just… there. Stubbornly unreactive. Take organic compounds, for example. The methyl group can withstand the onslaught of even the most aggressive acids. It’s rather stoic, wouldn’t you say?
Oxidation
The oxidation of a methyl group is a common occurrence, both in the grand theater of nature and the grubby workshops of industry. The resulting products are the hydroxymethyl group (–CH₂OH), the formyl group (–CHO), and the carboxyl group (–COOH). For instance, permanganate has a penchant for transforming a methyl group into a carboxyl group. Take toluene, for example; permanganate turns it into benzoic acid. Ultimately, when methyl groups are fully oxidized, they yield protons and carbon dioxide, as you’d expect from any decent combustion.
Methylation
Demethylation, the act of passing a methyl group from one compound to another, is a fundamental process. The culprits orchestrating this transfer are known as methylating agents. Common examples include dimethyl sulfate, methyl iodide, and methyl triflate. It’s fascinating, then, that methanogenesis, the very source of natural gas, operates via a demethylation reaction. Alongside ubiquity and phosphorylation, methylation stands as a major biochemical mechanism for altering protein function. The entire field of epigenetics is built upon understanding how methylation influences gene expression.
Deprotonation
Certain methyl groups are surprisingly willing to give up a proton. Consider the methyl groups in acetone ((CH₃)₂CO). They are about 10²⁰ times more acidic than methane. The carbanions that result are crucial intermediates in countless reactions within organic synthesis and [biosynthesis]. It’s how fatty acids, for example, are ultimately produced.
Free radical reactions
When a methyl group finds itself in a benzylic or allylic position, the strength of the C–H bond weakens, and its reactivity skyrockets. A prime example of this heightened reactivity is the photochemical chlorination of the methyl group in toluene, which yields benzyl chloride.
Chiral methyl
In a rather peculiar scenario, if you replace one hydrogen with deuterium (D) and another with tritium (T), the methyl substituent becomes chiral. It’s even possible to synthesize optically pure methyl compounds, such as chiral acetic acid (deuterotritoacetic acid, CHDTCO₂H). By employing these chiral methyl groups, the stereochemical pathways of several biochemical transformations have been meticulously analyzed.
Rotation
A methyl group can, in theory, rotate freely around the R–C axis. This is only truly free in the simplest cases, like gaseous methyl chloride, CH₃Cl. In most molecules, however, the rest of the structure, denoted as R, disrupts the symmetry of that axis, creating a potential barrier that restricts the motion of the three protons. The classic example is ethane, CH₃CH₃, a phenomenon explored under the name ethane barrier. In more condensed environments, neighboring molecules add their own subtle influences to this potential. The rotation of methyl groups can even be observed experimentally using quasielastic neutron scattering.
Etymology
It was the French chemists Jean-Baptiste Dumas and Eugène Péligot who, after meticulously determining the chemical structure of methanol, introduced the term "methylene". They derived it from the Greek words μέθυ (methy) meaning "wine" and ὕλη (hylē) meaning "wood" or "patch of trees." Their intention was to highlight its origin: "alcohol made from wood (substance)." The term "methyl" itself emerged around 1840, likely through a back-formation from "methylene," and was subsequently applied to "methyl alcohol," which, since 1892, we now know as methanol.
In the grand scheme of IUPAC nomenclature of organic chemistry, "methyl" is the standard term for an alkane unit featuring a single carbon atom. The prefix "meth-" unequivocally signals this singular carbon.