been observed in solution. Primary carbocations in the solution phase, even as transient intermediates (the ethyl cation has been proposed for reactions A carbenium ion is a positive ion with the structure RR′R″C+, that is a chemical species with carbon atom having only three covalent bonds, thus bearing a +1 formal charge. Carbenium ions are a major subset of carbocations, which is a general term for diamagnetic carbon-based cations. In parallel with carbenium ions is another subset of carbocations, the carbonium ions with the formula R5+. In c tertiary cations are stable and many are directly observable in superacid media. The stabilization by alkyl groups is explained by hyperconjugation. The donation of electron density from a β C-H or C-C bond into the unoccupied p orbital of the carbocation (a σCH/CC → p interaction) allows the positive charge to be delocalized. Secondary cations are usually transient. Only the isopropyl, s-butyl, and cyclopentyl cations have been observed in solution. 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 carbenium ions 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. The simple ethyl cation, C2H+5 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 ca A carbenium ion is a positive ion with the structure RR′R″C+, that is a chemical species with carbon atom having only three covalent bonds, thus bearing a +1 formal charge. Carbenium ions are a major subset of carbocations, which is a general term for diamagnetic The stability of alkyl-substituted carbocations follows the order 3° > 2° > 1° > methyl. This trend can be inferred by the hydride ion affinity values (231, 246, 273, and 312 kcal/mol for (CH3)3C+, (CH3)2CH+, CH3CH+2, and CH+3). The effect of alkyl substitution is a strong one: tertiary cations are stable and many are directly observable in superacid media. The stabilization by alkyl groups is explained by hyperconjugation. The donation of electron density from a β C-H or C-C bond into the unoccupied p orbital of the carbocation (a σCH/CC → p interaction) allows the positive charge to be delocalized. Secondary cations are usually transient. Only the isopropyl, s-butyl, and cyclopentyl cations have been observed in solution. 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 carbenium ions 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. The simple ethyl cation, C2H+5 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. Carbenium ions can be prepared directly from alkanes by removing a hydride anion, H−, with a strong acid. (Equivalently, the corresponding carbonium ions tend to eliminate H2.) For example, magic acid, a mixture of antimony pentafluoride (SbF5) and fluorosulfuric acid (FSO3H), turns isobutane into the trimethylcarbenium cation, (CH3)3C+. The tropylium ion is an aromatic species with the formula C7H+7. Its name derives from the molecule tropine (itself named for the molecule atropine). Salts of the tropylium cation can be stable, e.g. tropylium tetrafluoroborate. It can be made from cycloheptatriene (tropylidene) and bromine or phosphorus pentachloride. It is a planar, cyclic, heptagonal ion; it also has 6 π-electrons (4n + 2, where n = 1), which fulfills Hückel's rule of aromaticity. It can coordinate as a ligand to metal atoms. The structure shown is a composite of seven resonance contributors in which each carbon carries part of the positive charge. In 1891 G. Merling obtained a water-soluble salt from a reaction of cycloheptatriene and bromine. The structure was elucidated by Eggers Doering and Knox in 1954. On the other hand, the antiaromatic cyclopentadienyl cation (C5H+5) is destabilized by some 40 kcal/mol. Another aromatic carbenium ion is the cyclopropenyl or cyclopropenium ion, C3H+3. Although less stable than the tropylium cation, this carbenium ion can also form salts at room temperature. Solutions of such salts were exhibit conventional spectroscopic and chemical properties. The cyclopropenium cation (C3H+3), although somewhat destabilized by angle strain, is still clearly stabilized by aromaticity when compared to its open-chain analog, allyl cation. These varying cation stabilities, depending on the number of π electrons in the ring system, can furthermore be crucial factors in reaction kinetics. The formation of an aromatic carbocation is much faster than the formation of an anti-aromatic or open-chain carbocation.