Aromatic and Antiaromatic Ions

Rings containing an odd number of carbon atoms can be aromatic or antiaromatic, if they are planar and have a conjugated p orbital on each ring atom. To have an even number of electrons in their odd number of p orbitals, these species must be ionic. They must be carbocations or carbanions. 

Because a carbocation is sp2 hybridized, with trigonal planar geometry and an empty p orbital, this ion has a cycle of three p orbitals. (Remember that it is not the number of orbitals that determines whether a compound is aromatic or not, but rather the number of electrons in the pi MOs.) The cyclopropenyl carbocation has two electrons in its three pi MOs, so it fits Hiickel s rule and should be aromatic. In fact, cyclopropenyl carbocations are significantly more stable than other carbocations, even though they have considerable angle strain. For example, most carbocations react rapidly with water, a weak nucleophile. In contrast, tri-ferf-butyl-cyclopropenyl perchlorate, a carbocation salt, is stable enough to be recrystallized from water. 

Another example is provided by the acidity of cyclopentadiene. This compound is approximately as strong an acid as water and is many orders of magnitude more acidic than other hydrocarbons because its conjugate base, with six pi electrons, is aromatic. 

The cycloheptatrienyl carbocation, also known as the tropylium cation, has six pi electrons. It is also aromatic and is quite stable. In fact, 7-bromo-l,3,5-cycloheptatriene actually exists as an ionic compound. 

In contrast, the cyclopentadienyl carbocation, which has four pi electrons and is antiaromatic, is quite unstable. Thus, 5-iodo-l,3-cyclopentadiene is unreactive under conditions in which iodocyclopentane reacts rapidly by an SN1 mechanism. 

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