ChemTalk

Aromatic Compounds

aromatic compound examples

Core Concepts

In this tutorial, you will learn what makes a compound aromatic, the power of aromaticity, common examples of aromaticity, anti-aromatic compounds, and important aromatic ring reactions. 

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Vocabulary

  • Aromatic: Cyclic, planar structures that have a continuous conjugated system and satisfy Hückel’s law.
  • Anti-aromatic: Cyclic, planar structures that have a continuous, conjugated system and have 4n electrons.
  • Hückel’s Law: Describes a structure with 4n + 2 pi electrons

Aromaticity 

Aromatic compounds consist of compounds that are cyclic, possess a conjugated system, satisfy Huckel’s law, and are commonly planar. 

As the name suggests, cyclic structures form rings, as opposed to branching chain structures. Aside from a ring shape, aromaticity involves a continuous pi system. This means that each atom with the ring has an available p orbital, or is sp2 hybridized; thus is resonance stabilized. 

aromatic cyclical
aromatic continuous pi system

Hückel’s law states that aromatic compounds must have 4n + 2 pi electrons (2, 6, 10, 14, etc). Pi electrons are electrons that participate in pi bonds, seen in double and triple bonds, and are a part of a p orbital. The 4n +2 provides the number of pi electrons associated with aromaticity, not how to calculate the number of pi electrons within a ring. 

aromatic huckel's law

To determine the number of pi electrons in a ring, count the number of double bonds, negative charges, and lone pairs that contribute to the pi system. Then, multiply by 2.

Planar molecules are molecules that are flat as a result of each atom being located on the same plane. In the case of aromatic compounds, each atom is sp2 hybridized; therefore, they are planar. Per valence bond theory, the ring structure must be planar for the p orbitals to overlap and allow for the continuous movement of electrons.

aromatic planar

The Power of Aromatic Rings

When each of these 4 factors combines, the result is a structure far more stable than the sum of each factor. To demonstrate this, let’s observe the change in enthalpy associated with fully hydrogenating benzene. Specifically, this involves reacting hydrogen gas with benzene to yield cyclohexane with no double bonds. When measured empirically through calorimetry, the reaction is exothermic and releases 208 kJ/mol.

aromatic benzene hydrogenation

C6H6 + H2 → C6H12

∆Hrxn = -208 kJ/mol

Now let’s observe the enthalpy change in hydrogenating cyclohexene (one double bond) and cyclodiexene (two double bonds). For one double bond, 120 kJ/mol releases from the reaction, and for two double bonds, 234 kJ/mol releases. We can see a trend where about twice the energy releases from hydrogenating the cyclohexadiene than the cyclohexene.

cyclohexene hydrogenation

C6H10 + H2 → C6H12

∆Hrxn = -120 kJ/mol

cyclohexadiene hydrogenation

C6H8 + H2 → C6H12

∆Hrxn = -234 kJ/mol

If benzene were simply cyclohexane with three double bonds, we would expect the enthalpy of hydrogenation to equal about 360 kJ/mol for each of the three double bonds. Instead, as mentioned, only 208 kJ/mol of energy releases. Chemists call this 150 kJ/mol difference in benzene’s “resonance energy” which makes it substantially more stable than “cyclohexatriene”. As we can see, aromaticity provides added stability beyond that indicated by conventional insight related to molecular structure.

Aromatic Ring Examples

Benzene serves as the most recognizable aromatic molecule, but many more aromatic structures exist. Importantly, though the pi electrons must have the capability of moving cyclically, multiple cycles can exist within the same aromatic system. For instance, molecules such as naphthalene and anthracene count as aromatic structures.

multicyclic aromatic compounds

Further, many ionic structures have aromaticity as well, so long as the total number of electrons follows Hückel’s Law. Notable examples include the cyclopropenyl cation, cyclopentadiene anion, and cycloheptatrienyl cation.

aromatic ions

Interestingly, due to the thermodynamic favorability of aromaticity, molecules like cyclopentadiene have rather acidic hydrogens (pKa = 16) attached to their sp3 carbon.

cyclopentadiene deprotonation

Additionally, heterocyclic compounds may also have aromaticity, such as furan, oxazole, and purine.

furan oxazone purine

Also you may notice that many of the heterocyclic compounds, as well as the cyclopentadiene anion, have four electrons on one of their ring members. Though this usually indicates sp3 hybridization, in this case, these atoms still have sp2 hybridization, with a lone electron pair in a p orbital.

Anti-Aromatic Compounds

While aromaticity provides some added stability, there also exist similar compounds that have added instability. Chemists call these compounds “anti-aromatic,” and like their stable counterpart, they have a planar, cyclic structure with a continuous pi system. However, instead of following Hückel’s Law, these compounds have 4n pi electrons. The classic example of an anti-aromatic compound is cyclobutadiene, which is so unstable that it degrades near-instantaneously under non-extreme conditions.

cyclobutydiene

Indeed, larger would-be anti-aromatic compounds, such as cyclooctatetraene, instead develop a puckered structure. This increases steric hindrance between the member carbons, but such pucker has considerably more stability than a planar anti-aromatic structure.

cyclooctatetraene pucker

Aromatic Substitution Reactions

Due to the unique chemistry of aromaticity, chemists have engineered many uses for them. However, this generally involves some substituted aromatic compound. This primarily occurs through electrophilic aromatic substitution, allowing for the attachment of groups such as halides, nitros, acyls, and hydrocarbons.

electrophilic aromatic substitution

Substituents can also attach to the ring through nucleophilic aromatic substitution. This allows for the attachment of alcohols, aminos, and others. 

nucleophilic aromatic substitution

Conjugated carbonyls undergo very predictable reactions such as aldol reactions or Michael Additions