Chapter 14, Problem 14.21P

Interpretation Introduction

(a)

Interpretation:

The given species is to be identified as aromatic, antiaromatic, or nonaromatic based on Huckel’s rules.

Concept introduction:

Aromaticity is not limited to uncharged molecules because several molecular ions that obey Huckel’s rules are also aromatic.

The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals. 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 (${\text{sp}}^2$ hybridized), then the species is:

1) Aromatic if the number of electrons in that $\text{π}$- system is either $2,6,10,14,18,\text{and so on}\text{.}$ (these numbers are called Huckel numbers). 2) Antiaromatic if the number of electrons in that $\text{π}$- system is either $4,8,12,16,20,\text{and so on}\text{.}$ (these numbers are called anti-Huckel numbers). 3) All other species are considered nonaromatic. In other words, Huckel number is an odd number of pairs, whereas anti-Huckel number is an even number of pairs. The Huckel number corresponds to $4\text{n}+2\text{π}$, where $\text{n}\ge 0$. Anti-Huckel number corresponds to $4\text{n}$ where $\text{n}\ge 1$.

The hybridization of carbon atom bearing the positive charge is ${\text{sp}}^2$. The hybridization of carbon atom bearing the negative charge (carbanion) is ${\text{sp}}^3$.

Interpretation Introduction

(b)

Interpretation:

The given species is to be identified as aromatic, antiaromatic, or nonaromatic based on Huckel’s rules.

Concept introduction:

Aromaticity is not limited to uncharged molecules because several molecular ions that obey Huckel’s rules are also aromatic.

The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals. 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 (${\text{sp}}^2$ hybridized), then the species is:

1) Aromatic if the number of electrons in that $\text{π}$- system is either $2,6,10,14,18,\text{and so on}\text{.}$ (these numbers are called Huckel numbers). 2) Antiaromatic if the number of electrons in that $\text{π}$- system is either $4,8,12,16,20,\text{and so on}\text{.}$ (these numbers are called anti-Huckel numbers). 3) All other species are considered nonaromatic. In other words, Huckel number is an odd number of pairs, whereas anti-Huckel number is an even number of pairs. The Huckel number corresponds to $4\text{n}+2\text{π}$, where $\text{n}\ge 0$. Anti-Huckel number corresponds to $4\text{n}$ where $\text{n}\ge 1$.

The hybridization of the carbon atom bearing the positive charge is ${\text{sp}}^2$. The hybridization of the carbon tom bearing the negative charge (carbanion) is ${\text{sp}}^3$.