Difference Between Aromatic Antiaromatic and Nonaromatic

Main Difference – Aromatic vs Antiaromatic vs Nonaromatic

Aromaticity is the property of conjugated cycloalkenes in which the stabilization of the molecule is enhanced due to the ability of the electrons in the pi orbitals to delocalize. Aromatic compounds are organic compounds composed of carbon and hydrogen atoms arranged in ring structures with delocalized pi electrons. Antiaromaticity is the presence of a cyclic molecule with a pi electron system having 4n electrons in it (where n=0, 1, 2, etc.). Antiaromatic compounds are highly unstable, thus reactive. Nonaromatic compounds are molecules that are no aromatic. The main difference between aromatic antiaromatic and nonaromatic is that aromatic means having a delocalized pi electron system with (4n +2) electrons and antiaromatic means having a delocalized pi electron system with 4 electrons whereas nonaromatic means there is no delocalized electron system in that molecule.

Key Areas Covered

1. What is Aromatic
     – Definition, Requirements to be Aromatic, Huckel’s Rule
2. What is Antiaromatic
     – Definition, Requirements to be Antiaromatic
3. What is Nonaromatic
     – Definition, Requirements to be Nonaromatic
4. What is the Difference Between Aromatic Antiaromatic and Nonaromatic
     – Comparison of Key Differences

Key Terms: Antiaromatic, Aromatic, Cyclic, Delocalization, Huckel’s Rule, Nonaromatic, Pi Electron System, Resonance Effect

What is Aromatic

Aromatic compounds are organic compounds composed of carbon and hydrogen atoms arranged in ring structures with delocalized pi electrons. Aromatic hydrocarbons are named as such due to their pleasant aroma. Aromatic hydrocarbons are essentially cyclic structures. These are also planar structures.

Aromatic compounds are highly stable due to the resonance effect. This means aromatic compounds are often represented as resonance structures containing single and double bonds, but the actual structure has delocalized electrons shared between all the atoms of the ring. Delocalization refers to the overlapping of p orbitals of adjacent atoms. This overlapping occurs only if the double bonds are conjugated. (When conjugation is present, every carbon atom of the ring structure have a p orbital.)

Figure 1: Resonance Structures of Benzene

For a molecule to be named as an aromatic compound, it should obey the Huckel’s rule. This rule can be given as follows.

• An aromatic compound must have 4n + 2 pi electrons (where n is a whole number = 0, 1, 2, etc.).

Generally, aromatic compounds are nonpolar. Therefore, they are immiscible with water. The carbon-to-hydrogen ratio is less in aromatic compounds. Most aromatic compounds undergo electrophilic substitution reactions. Due to the presence of delocalized pi electrons, the aromatic ring is rich with electrons. Therefore, electrophiles can attack this ring to share electrons.

Aromatic compounds are often obtained from petroleum oil. Polyaromatic hydrocarbons (PAH) are considered as environmental pollutants and carcinogens.

What is Antiaromatic

Antiaromatic compounds are molecules that are cyclic, planar and completely conjugated but consist of 4n pi electrons. These antiaromatic compounds are highly unstable, thus reactive. For example, cyclobutadiene is antiaromatic.

Figure 2: Cyclobutadiene is an Antiaromatic Compound

Antiaromatic compounds do not obey the Huckel’s rule. They are always less stable than the acyclic compounds having the same number of pi electrons. However, antiaromatic compounds have delocalized pi electron systems due to the presence of conjugated double bonds.

Antiaromatic compounds can be thermodynamically recognized by measuring the energy of the cyclic conjugated pi electron system. The energy will be always higher than the reference compound used for the comparison.

What is Nonaromatic

Nonaromatic compounds are molecules that lack one or more of the requirements to be aromatic: being planar and cyclic structure, completely conjugated system. Therefore, all aliphatic compounds are nonaromatic. Even some cyclic compound which are planar may be nonaromatic due to the lack of conjugated double bonds. For example, 1,3-cyclohexadiene is a nonaromatic compound because it lacks conjugation of double bonds though it is planar and cyclic.

Figure 3: 1,3-cyclohexadiene is a Nonaromatic Compound

Difference Between Aromatic Antiaromatic and Nonaromatic

Definition

Aromatic: Aromatic compounds are organic compounds composed of carbon and hydrogen atoms arranged in ring structures with delocalized pi electrons.

Antiaromatic: Antiaromatic compounds are molecules that are cyclic, planar and completely conjugated but consist of 4n pi electrons.

Nonaromatic: Nonaromatic compounds are molecules that lack one or more of the requirements to be aromatic: being planar and cyclic structure, completely conjugated system.

Stability

Aromatic: Aromatic compounds are stable.

Antiaromatic: Antiaromatic compounds are highly unstable.

Nonaromatic: Nonaromatic compounds are stable.

Delocalization

Aromatic: Aromatic compounds have delocalized pi electron system and 4n + 2 pi electrons.

Antiaromatic: Antiaromatic compounds have delocalized pi electron system and 4n pi electrons.

Nonaromatic: Nonaromatic compounds may or may not have delocalized pi electron system.

Pi Electrons

Aromatic: Aromatic compounds have 4n + 2 pi electrons.

Antiaromatic: Antiaromatic compounds have 4n pi electrons.

Nonaromatic: The number of pi electrons is not applicable for nonaromatic compounds.

Reactivity

Aromatic: Aromatic compounds are less reactive.

Antiaromatic: Antiaromatic compounds are highly reactive.

Nonaromatic: Nonaromatic compounds are less reactive.

Conclusion

The main difference between aromatic antiaromatic and nonaromatic is that aromatic means having a delocalized pi electron system with (4n +2) electrons and antiaromatic means having a delocalized pi electron system with 4 electrons whereas nonaromatic means there is no delocalized electron system in that molecule.

Reference:

1. “Aromaticity.” Chemistry LibreTexts, Libretexts, 18 Sept. 2016, Available here.
2. Pooja Thakral. “Aromaticity Antiaromaticity Non aromaticity.” LinkedIn SlideShare, 4 Dec. 2016, Available here.
3. “Antiaromaticity.” Wikipedia, Wikimedia Foundation, 23 Nov. 2017, Available here.

Image Courtesy:

1. “Benzene resonance structures” By Edgar181 – Own work (Public Domain) via Commons Wikimedia
2. “Cyclobutadiene structure2” By Jake V – Own work (Public Domain) via Commons Wikimedia
3. “1,3-cyclohexadiene” By Wickey-nl – Own work (Public Domain) via Commons Wikimedia