Chapter 14, Problem 14.20P

Interpretation Introduction

(a)

Interpretation:

The given compound is to be identified as aromatic, antiaromatic, or nonaromatic.

Concept introduction:

Heterocyclic compounds are defined as cyclic compounds which consist of at least one heteroatom, mostly nitrogen, sulfur, or oxygen. Heterocyclic compounds can be aromatic, antiaromatic, or nonaromatic. In some heterocyclic compounds, a noncarbon atom contributes to a p atomic orbital to the aromatic $\text{π}$- system. Thus, a heteroatom can contribute a lone pair of electrons to the $\text{π}$- system to attain aromaticity.

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 $\text{n}\ge 1$.

Answer to Problem 14.20P

The given compound is antiaromatic.

Explanation of Solution

The given compound is:

It is a heterocyclic compound with an oxygen atom. The hybridization of the oxygen atom must be ${\text{sp}}^3$, and its geometry must be tetrahedral. But since it is directly attached to an ${\text{sp}}^2$ hybridized carbon atom, it should attain planar geometry close to the bond angle ${120}^{\circ }$. Thus, it is a highly strained ring. Due to the high ring strain, it is assumed to be planar with one of the lone pair of electrons on the oxygen atom in conjugation with the ring $\text{π}$- system.

The total number of electrons in this $\text{π}$- system would be $4$ which is an anti-Huckel number, and hence, the given compound must be antiaromatic.

Conclusion

The given heterocyclic compound is antiaromatic if the total number of electrons in that $\text{π}$- system is either $4,8,12,16,20,\text{and so on}\text{.}$

Interpretation Introduction

(b)

Interpretation:

The given compound is to be identified as aromatic, antiaromatic, or nonaromatic.

Concept introduction:

Heterocyclic compounds are defined as cyclic compounds which consists of at least one heteroatom mostly nitrogen, sulfur, or oxygen. Heterocyclic compounds can be aromatic, antiaromatic, or nonaromatic. In some heterocyclic compounds, a noncarbon atom contributes a p atomic orbital to the aromatic $\text{π}$- system. Thus, a heteroatom can contribute a lone pair of electrons to the $\text{π}$- system to attain aromaticity.

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 $\text{n}\ge 1$.

Answer to Problem 14.20P

The given compound is aromatic.

Explanation of Solution

The given compound is:

It is a heterocyclic compound with a nitrogen atom. The hybridization of the nitrogen atom must be ${\text{sp}}^2$, and its geometry must be planar as it is directly attached to an ${\text{sp}}^2$ hybridized carbon atom. There is one lone pair on the nitrogen atom. The lone pair on the nitrogen atom is not in conjugation with the ring $\text{π}$- system as it resides in an ${\text{sp}}^2$ hybridized orbital that is perpendicular to the $\text{π}$- system.

Thus, the total number of electrons in this $\text{π}$- system is $10$ which is a Huckel number, and hence, the given compound must be aromatic.

Conclusion

The given heterocyclic compound is aromatic if the number of electrons in that $\text{π}$- system is either $2,6,10,14,18,\text{and so on}\text{.}$

Interpretation Introduction

(c)

Interpretation:

The given compound is to be identified as aromatic, antiaromatic, or nonaromatic.

Concept introduction:

Heterocyclic compounds are defined as cyclic compounds which consist of at least one heteroatom mostly nitrogen, sulfur, or oxygen. Heterocyclic compounds can be aromatic, antiaromatic, or nonaromatic. In some heterocyclic compounds, a noncarbon atom contributes a p atomic orbital to the aromatic $\text{π}$- system. Thus, a heteroatom can contribute a lone pair of electrons to the $\text{π}$- system to attain aromaticity.

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 rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals.

Answer to Problem 14.20P

The given compound is nonaromatic.

Explanation of Solution

The given compound is:

It is an oxygen containing a heterocycle consisting of a seven-membered ring with three double bonds. Due to geometrical constrains, the lone pairs of electrons on the oxygen atom are not in conjugation with the $\text{π}$- system. Because of this, the conjugation breaks, and the system no longer exists as a planar system. The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals. Thus, this compound must be nonaromatic.

If the molecule is assumed to be planar (flat), then too it would contain $6$$\text{π}$ electrons, making it antiaromatic.

Conclusion

The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals.

Interpretation Introduction

(d)

Interpretation:

The given compound is to be identified as aromatic, antiaromatic, or nonaromatic.

Concept introduction:

Heterocyclic compounds are defined as cyclic compounds which consists of at least one heteroatom mostly nitrogen, sulfur, or oxygen. Heterocyclic compounds can be aromatic, antiaromatic, or nonaromatic. In some heterocyclic compounds, a noncarbon atom contributes a p atomic orbital to the aromatic $\text{π}$- system. Thus, a heteroatom can contribute a lone pair of electrons to the $\text{π}$- system to attain aromaticity.

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 rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals.

Answer to Problem 14.20P

The given compound is nonaromatic.

Explanation of Solution

The given compound is:

It is a sulfur containing a heterocycle consisting of a six-membered ring with two double bonds. There are two lone pair of electrons on the sulfur atom, but the sulfur atom is ${\text{sp}}^3$ hybridized. Also, the carbon atom in the ring which is between two double bonds is also ${\text{sp}}^3$ hybridized. Thus, the $\text{π}$- system does not form a complete ring. Hence, the given compound is nonaromatic.

As the $\text{π}$- system is not conjugated, the molecule is nonaromatic.

Conclusion

The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals.

Interpretation Introduction

(e)

Interpretation:

The given compound is to be identified as aromatic, antiaromatic, or nonaromatic.

Concept introduction:

Heterocyclic compounds are defined as cyclic compounds which consists of at least one heteroatom mostly nitrogen, sulfur, or oxygen. Heterocyclic compounds can be aromatic, antiaromatic, or nonaromatic. In some heterocyclic compounds, a noncarbon atom contributes a p atomic orbital to the aromatic $\text{π}$- system. Thus, a heteroatom can contribute a lone pair of electrons to the $\text{π}$- system to attain aromaticity.

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 rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals.

Answer to Problem 14.20P

The given compound is nonaromatic.

Explanation of Solution

The given compound is:

It is a straight chain compound with an oxygen atom and two double bonds. The above compound is not a cyclic compound. The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals. Thus, the $\text{π}$- system does not form a complete ring. Hence, the given compound is nonaromatic.

Conclusion

The rule for aromaticity or antiaromaticity applies only if the system is planar, cyclic and has overlap of p-orbitals.

Interpretation Introduction

(f)

Interpretation:

The given compound is to be identified as aromatic, antiaromatic, or nonaromatic.

Concept introduction:

Heterocyclic compounds are defined as cyclic compounds which consists of at least one heteroatom mostly nitrogen, sulfur, or oxygen. Heterocyclic compounds can be aromatic, antiaromatic, or nonaromatic. In some heterocyclic compounds, a noncarbon atom contributes a p atomic orbital to the aromatic $\text{π}$- system. Thus, a heteroatom can contribute a lone pair of electrons to the $\text{π}$- system to attain aromaticity.

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 $\text{n}\ge 1$.

Answer to Problem 14.20P

The given compound is aromatic.

Explanation of Solution

The given compound is:

It is a heterocyclic compound with a nitrogen atom in which one six-membered and one five-membered rings are fused. The hybridization of the nitrogen atom is ${\text{sp}}^2$. The nitrogen atom has one lone pair of electrons which resides in the p-orbital. Thus, it is in conjugation with the ring $\text{π}$- system.

Thus, the total number of electrons in this $\text{π}$- system is $10$ which is the Huckel number, and hence, the given compound must be aromatic.

Conclusion

The given heterocyclic compound is aromatic if the number of electrons in that $\text{π}$- system is either $2,6,10,14,18,\text{and so on}\text{.}$