Cyclopentadiene - An aromatic compound?

Question

Is cyclopentadiene what you would call an aromatic compound?

Answer 1

No. It does not obey Huckel's rule for aromaticity (i.e. 4n + 2, where n = any nonnegative integer). Solutions to Huckel's rule are 2, 6, 10, etc. (plugging in 0, 1, 2, etc. into the above equation). To be aromatic, a molecule must have 2, 6, 10, etc. electrons which are involved in the pi-bonds/pi-system of the ring. The cyclopentadiene has 4 electrons in the pi orbitals of the ring (two each from the two double bonds). The fifth carbon (the only one not double bonded to another carbon) HAS NO lone pair of electrons, and cannot contribute to the aromaticity.

Interestingly, the compound pyrrole, identical in structure to cyclopentadiene, but with a nitrogen atom replacing one of the carbon atoms (consider it as replacing the one carbon which IS NOT double bonded to any others) IS INDEED AROMATIC. The lone pair of electrons of the nitrogen atom are able to delocalize into the ring, yielding SIX TOTAL ELECTRONS delocalized into the pi-system and granting pyrrole the designation of AROMATIC. The oxygen equivalent of both of the above, furan, also has properties of aromaticity, as does thiophene (the sulfur analog).

Answer 2

For a substance to be aromatic, you must ensure the following conditions: a) All carbons participating must be of sp² hybrid. b) The number of pi electrons (n) must be a single number like 0, 2, 4 (4n+2 must be equal to the pi electron count). For benzene, which is aromatic, 4n+2=6 and n is 1. Cyclopentadiene is not aromatic, because it has a sp³ carbon with two hydrogens. However, if you remove a hydrogen, you will have a carbanion with a sp² hybrid, and your 4n+2 will be 6, which means n=1.

Answer 3

No. It is simply a diolefin with 4 pi-electrons. Treat it with a base and abstract a proton, and it becomes cyclopentadienyl anion with 6 pi-electrons (4n + 2), which is aromatic.

Answer 4

no, because it is cyclic coumpound