A carbenium ion is a positive ion with the structure RR′R″C+, that is, a chemical species with a trivalent carbon that bears a +1 formal charge. In older literature the name carbonium ion was used for this class, but now it refers exclusively to another family of carbocations, the carbonium ions, where the charged carbon is pentavalent. The current definitions were proposed by the chemist George Andrew Olah in 1972, and are now widely accepted. Carbenium ions are generally highly reactive due to having an incomplete octet of electrons; however, certain carbenium ions, such as the tropylium ion, are relatively stable due to the positive charge being delocalised between the carbon atoms. Nomenclature Carbenium ions are classified as primary, secondary, or tertiary depending on whether the number of carbon atoms bonded to the ionized carbon is 1, 2, or 3. (Ions with zero carbons attached to the ionized carbon, such as methenium, , are usually included in the primary class). Reactivity Stability typically increases with the number of alkyl groups bonded to the charge-bearing carbon. Tertiary carbocations are more stable (and form more readily) than secondary carbocations, because they are stabilized by hyperconjugation. Primary carbocations are highly unstable. Therefore, reactions such as the SN1 reaction and the E1 elimination reaction normally do not occur if a primary carbenium would be formed. However, a carbon doubly bonded with the ionized carbon can stabilize the ion by resonance. Such cations as the allyl cation, , and the benzyl cation, , are more stable than most other carbocations. Molecules which can form allyl or benzyl carbeniums are especially reactive. Carbenium ions can also be stabilized by heteroatoms. Carbenium ions may undergo rearrangement reactions from less stable structures to equally stable or more stable ones with rate constants in excess of 109 s−1. This fact complicates synthetic pathways to many compounds. For example, when pentan-3-ol is heated with aqueous HCl, the initially formed 3-pentyl carbocation rearranges to a statistical mixture of the 3-pentyl and 2-pentyl. These cations react with chloride ion to produce 3-chloropentane and 2-chloropentane in a ratio of approximately 1:2. Types of carbenium ions Alkylium ions Carbenium ions can be prepared directly from alkanes by removing a hydride anion, , with a strong acid. For example, magic acid, a mixture of antimony pentafluoride () and fluorosulfuric acid (), turns isobutane into the trimethylcarbenium cation, . Aromatic carbenium ions The tropylium ion is