As noted in the history section, the tropylium cation (C7H7+) was one of the first carbocations to be discovered, due to its aromatic stability. Tropylium's carbocation is so stabilized that the molecule can be isolated and sold as a salt. On the other hand, the anti-aromatic cyclopentadienyl cation (C5H5+) is destabilized by some 40 kcal/mol. The cyclopropenium cation (C3H3+), 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. Given the role of carbocations in many reaction schemes, such as SN1 for example, choosing the conjugation of starting materials can be a powerful method for conferring kinetic favorability or disfavorability, as the rate constant for any given step is dependent on the step's activation energy according to the Arrhenius equation.