Aromatic Compounds

Sam covers key information about aromatic compounds for the B/B and C/P portions of the MCAT. He covers the properties of aromatic compounds, how to differentiate between aromatic and antiaromatic compounds, and the common aromatic molecules that you need to be familiar with.

• [02:12] Definition of an Aromatic Compound
• [04:09] Criteria for Aromaticity
• [06:06] How to Count Pi Electrons
• [07:48] Criteria for Antiaromaticity
• [09:58] Polycyclic and Heterocyclic Aromatic Compounds
• [12:10] Common Aromatic Compounds
• [15:02] Properties of Aromatic Compounds
• [20:23] Aromatic Rings in Biology

What is an Aromatic Compound?

An aromatic compound is a compound that contains one or more rings with pi electrons that are delocalized all the way around the ring or rings.

Definition of terms:

• Ring – A structure of elements that are bound together to form a circular ring.
• Pi electrons – Electrons in a pi bond. Typically this is electrons in either a double or triple bond.
• Delocalization – Electrons can move freely around the ring and are not found on any specific point. Delocalization of electrons makes a molecule more stable than normal.

Criteria for Aromaticity

A compound must meet three criteria to be considered aromatic:

1. The compound must be cyclic.
2. Every atom in the compound is conjugated. In other words, every atom must be connected to a double bond or to an atom that has a lone pair of electrons.
3. The number of pi electrons in the compounds must be equal to 4n+2, where n is any whole number. This is called Hückel’s rule.

Calculating the Number of Pi Electrons

To count pi electrons, you must consider the number of double bonds and the number of lone pairs in the molecule’s ring.

Each double bond has 2 pi electrons. Count how many double bonds there are and then multiply that number by 2. The product is the number of pi electrons in the molecule’s double bonds.  When counting lone pairs, only atoms within the ring structure should be considered. If one of the atoms in the ring has a lone pair or two lone pairs of electrons, that counts as 2 pi electrons. Tally the total number of pi electrons from the double bonds and lone pairs. If the resulting number is equal to 4n+2, then the compound is aromatic.

Criteria for Antiaromaticity

Antiaromatic compounds look like aromatic compounds, except that these compounds are highly unstable. An antiaromatic compound must meet 3 criteria:

1. Compound structure must be cyclic.
2. Every atom is conjugated.
3. The number of pi electrons is equal to 4n, where n is any whole number.

So, antiaromatic compounds share the first two criteria with aromatic compounds. However, the number of pi electrons in antiaromatic compounds is equal to 4n or multiples of 4.

Polycyclic vs Heterocyclic Aromatic Compounds

Polycyclic aromatic compounds have multiple aromatic rings in a single compound. These are big, bulky aromatic compounds.

Heterocyclic aromatic compounds have elements other than carbon in their ring or rings – typically this is nitrogen. Different elements can add to aromaticity to the ring by containing lone pairs of electrons.

Examples of Aromatic Compounds

For the MCAT, you must memorize the names and structures of common aromatic compounds.  It will be a test of recognition so it’s best to familiarize yourself with a few examples.

• Benzyne – It is a 6 membered ring that contains 6 carbons and 3 double bonds.
• Toluene – This compound looks like benzyne but it has a protruding stick which represents the additional methyl group it contains.
• Phenol – The structure looks like a benzyne ring with a hydroxy group attached.
• Aniline – It looks like a benzyne ring but with an amino group (NH₂) attached to it.

Properties of Aromatic Compounds

Aromatic compounds are nonpolar and insoluble in water. With regards to stability, aromatic compounds are quite stable and not very reactive.

Interestingly, aromatic compounds become fluorescent when exposed to UV light due to the presence of conjugated pi electrons. When excited by UV light, pi electrons can jump more easily to a high energy level. Once excited electrons fall down to a lower energy level, they emit photons. This phenomenon explains the aromatic compounds’ fluorescent property.

Lastly, it is worth noting that nitrogen containing heterocyclic compounds are basic. Aromatic rings with nitrogen inside can pick up and bond to hydrogen atoms or protons. The only circumstance where a nitrogenous heterocyclic compound is not basic is when its lone pair of nitrogen electrons participates in the conjugated pi system.

Aromatic Rings in Biology

Aromatic rings can be found in amino acids, DNA, and even the electron transport chain.

There are 3 amino acids that have aromatic rings: tryptophan (2 rings), tyrosine (1 ring), and phenylalanine (1 ring). They appear frequently in protein-protein binding sites on proteins because of cation-pi interactions. Aromatic rings in amino acids can interact with positively charged amino acids. A cation-pi interaction is formed, which keeps these two groups close together.

Purines and pyrimidines are bases that show up in DNA and RNA. Pyrimidines are heterocyclic aromatic compounds. Examples of these are cytosine, thymine, and uracil. As heterocyclic compounds, the other element present in their structure is nitrogen. Alternatively, these are known as nitrogenous bases. Purines such as adenine and guanine are polycyclic aromatic compounds. Like pyrimidines, purines have nitrogen within their structure. They are also considered nitrogenous bases.

Aromatic molecules present in the electron transport chain are ubiquinone, NAD+, and FAD. All three of them can easily lose or gain electrons without compromising stability thanks to their aromatic rings. They are considered to be excellent electron carriers.

Ubiquinone acts as a carrier within the electron transport chain. It can accept two electrons which turns itself into ubiquinol. As it travels through the electron transport chain, it is oxidized and becomes ubiquinone once again.

Both NAD+ and the FAD carry electrons to the electron transport chain. NAD+ is heterocyclic while FAD is polycyclic. When NAD+ gains 2 electrons it becomes NADH. When FAD is reduced, it turns into FADH2.