How many pi electrons does benzene have, and is it aromatic according to Huckel's rule?

• A. Six pi electrons; aromatic
• B. Four pi electrons; nonaromatic
• C. Six pi electrons; nonaromatic
• D. Eight pi electrons; aromatic

Answer: (A)

The key idea is Huckel’s rule for aromaticity: a planar cyclic molecule with a fully conjugated pi system is aromatic if it has 4n + 2 pi electrons.

Benzene has six pi electrons, coming from the three double bonds that create a continuous ring of p orbitals. Those six electrons fit 4n + 2 with n = 1, so the ring is aromatic. The other electron counts would either give 4n (which leads to antiaromatic behavior if the ring is planar and fully conjugated) or 8 (another non–4n + 2 count), neither of which describes benzene. Therefore, six pi electrons and aromatic best describe benzene.