Alkyl-substituted carbocations follow the order 3° > 2° > 1° > methyl in stability, as can be inferred by the hydride ion affinity values (231, 246, 273, and 312 kcal/mol for (CH3)3C+, (CH3)2CH+, C2H5+, and CH3+). The effect of alkyl substitution is a strong one: tertiary cations are stable and many are directly observable in superacid media, but secondary cations are usually transient and only the isopropyl, "s"-butyl, and cyclopentyl cations have been observed in solution. There is seldom any experimental support for primary carbocations in the solution phase, even as transient intermediates (the ethyl cation has been proposed for reactions in 99.9% sulfuric acid and in FSO2OH-SbF5), and methyl cation has only been unambiguously identified in the gas phase. In most, if not all cases, the ground state of alleged primary carbocations consist of bridged structures in which positive charge is shared by two or more carbon atoms and are better described as side-protonated alkenes, edge-protonated cyclopropanes, or corner-protonated cyclopropanes rather than true primary cations. Even the simple ethyl cation, C2H5+, has been demonstrated experimentally and computationally to be bridged and can be thought of as a symmetrically protonated ethylene molecule. The same is true for higher homologues like 1-propyl and 1-butyl cations. Neopentyl derivatives are thought to ionize with concomitant migration of a methyl group (anchimeric assistance); thus, in most if not all cases, a discrete neopentyl cation is not believed to be involved.