Chapter 14, Problem 14.16P

Interpretation Introduction

Interpretation:

The stable configuration of [16]annulene is to be drawn and whether it is aromatic, antiaromatic, or nonaromatic is to be suggested.

Concept introduction:

The monocyclic hydrocarbons with alternate single and double bonds are called annulenes. They are commonly named as [n] annulenes, where ‘n’ represents the number of carbon atoms in the ring. The stability of [n] annulenes depends on their angle strain between alternate single and double carbon-carbon bonds. To avoid this angle strain in [n] annulenes, a carbon-carbon double bond arranges in alternate cis and trans fashion.

The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has an overlapping p-orbital which could contribute to the conjugation. Huckel’s rule for aromaticity states that if a species possesses a pi system of molecular orbitals constructed from p-orbitals that are fully conjugated around a ring, then such a system is:

1) Aromatic if the number of electrons in that $\text{π}$ - system is $\text{2, 6, 10, 14, 18}$ and so on. (These numbers are called Huckel numbers).

2) Antiaromatic if the number of electrons in that $\text{π}$- system is $\text{4, 8, 12, 16, 20}$ and so on. (These numbers are called anti-Huckel numbers).

3) All other species are considered nonaromatic.

In other words, if a species contains an odd number of pi electrons, then it is said to be aromatic. If it contains an even number of pi electrons, then it is anti-aromatic.