Methyl cation
CH3+Three bonds, flat, and an empty p orbital. Look edge-on.
- C–H 1.10 Å
- ∠HCH 120°
- planar
The methyl cation is the simplest carbocation: a carbon with only three bonds and a positive charge. Carbocations don’t sit on a shelf. They exist briefly in the middle of reactions, and understanding their shape explains a lot of later chemistry.
With only three electron groups, the carbon is sp² hybridized. Its three sp² orbitals hold the C–H bonds, flat in one plane, 120° apart. Turn on Angles & lengths and look edge-on to confirm the molecule is completely flat.
The leftover p orbital stands straight up and down, like the p orbitals in ethene. Turn on p orbitals to see it. Here, though, the orbital is empty: it holds no electrons at all. That empty orbital is exactly why carbocations are so reactive. It’s a vacancy that any electron-rich molecule can donate a pair of electrons into. (That’s also why there’s no π-electron layer for this page: there are no π electrons to show.)
This is the first of three species in a row: compare it with the methyl radical (one electron in that p orbital) and the methyl anion (two).
Sources
3D structure, measurements and electron clouds computed with PySCF (B3LYP/def2-SVP). Computed values can differ from measured ones by a degree or so, or a few hundredths of an ångström.