If you’re due to sit your A level organic chemistry exam, you’ll need to have a good grasp of aromatic chemistry. Continue reading for an overview of the key concepts so you can enter the exam better prepared.
In this post:
Aromatic compounds: what you need to know
While you may not be asked to name obscure aromatic compounds, you should at least be familiar with some of the most common ones. As benzene is the simplest aromatic compound, your revision should mainly focus on organic compounds that contain a benzene ring.
You’ll need to understand the structure of benzene and the various reactions it undergoes, including hydrogenation. Plus, you must be able to explain why the predicted enthalpy difference for benzene hydrogenation is higher than the measured value.
You’ll also need to know why the delocalised electrons in the benzene ring plane make the compound more stable. This is where the concept of pi bonds comes into play. These bonds are responsible for the chemical stability of benzene and other benzene-containing compounds.
Aromatic chemistry has a wide range of applications, from manufacturing dyes to synthesising medicines. The compounds formed can be complex, but they’re typically derived from a few fundamental reactions of aromatic compounds.
What is a pi electron cloud?
Pi bonds are covalent bonds that occur when two lobes of an orbital, typically p-orbitals, laterally overlap with the two lobes of an orbital of another atom. This then forms double bonds or triple bonds, such as in the case of aromatic compounds that have alternating double bonds in the ring. The illustration below shows the delocalised pi bonds in benzene.
In 𝛑 bonds, the two overlapping orbitals have the same nodal plane at which the density or probability of electrons is zero. This nodal plane passes through the nuclei of bonded atoms, creating a conjugated system that contributes to the stability of the molecule. It also lowers the energy of the molecule.
As you can see in the illustration above, the 𝛑 bonds form two doughnut-shaped electron clouds parallel to the benzene ring and each other. The electrons move around the ring as opposed to staying in just one atom. You can therefore represent the skeletal formula of a benzene ring with a circle inside instead of alternating double bonds, like this:
What are the criteria that determine aromaticity?
Many compounds derived from benzene, for example eugenol, have sweet or pleasant fragrances. They were therefore classified as aromatic compounds when they were first discovered in the 19th century.
However, not all aromatic compounds have benzene rings in them. Camphor, for instance, is a highly aromatic compound but it does not contain a benzene ring.
Aromatic compounds that contain benzene are called arenes. The functional group is known as an aryl group, which is a benzene ring connected to an R group. It may also contain other functional groups and aryl groups bonded together.
For example, naphthalene, which is the active ingredient of mothballs, has two aryl groups (benzene rings) bonded together, as shown in the diagram below.
To be classed as aromatic, an organic compound must meet four basic criteria:
- It must contain a cyclic or ring molecule, specifically benzene (aryl group) – compounds that contain a non-benzene ring or another type of cyclic group are classified as either antiaromatic or nonaromatic.
Some examples of antiaromatic compounds are shown below. In this graphic, A represents pentalene, B is biphenylene, and C is cyclopentadienyl cation. As you can see, a cyclic group can take on several polygonal shapes, which may either be planar or non-planar.
- The molecule must be planar – all the atoms in a molecule must lie on the same plane.
- Delocalisation of the pi electrons – this means the electrons are not tied to one particular atom but can move around within the pi bond cloud.
- It must follow Huckel’s rule – this is a set of algorithms that can help to establish whether a compound is aromatic, antiaromatic or nonaromatic. For aromatic compounds, you can determine the number of pi electrons by using the following algorithm: N= 4n+2, where n=0 or any positive integer.
How are aromatic compounds named?
Aromatic compounds names are based on the benzene ring. You can either use the common names of simpler substituted benzene derivatives, or their more systematic IUPAC names (International Union of Pure and Applied Chemistry). This naming convention takes into account the position of the substituent.
Some of the most common substituents are Br, Cl, and NO2, which are named bromobenzene, chlorobenzene, and nitrobenzene respectively. If any additional substituent becomes part of the ring, the following prefixes or their abbreviations can be used before the name of the additional substituent, depending on where it’s added.
For instance, if bromine is added to the ortho position of chlorobenzene, the new compound is named o-bromochlorobenzene. Some of the common names for benzene derivatives can also be used as the primary name or as the basis for names of other derivatives with additional substituents.
The IUPAC system accepts some common names of simpler derivatives like phenol, benzoic acid, and benzaldehyde. However, as the compound becomes more complex, the more systematic naming system is used. For example, the IUPAC name for the explosive trinitrotoluene (TNT) is 2-methyl-1,3,5-trinitrobenzene.
What are the chemical reactions of aromatic compounds?
Just like alkenes, aromatic compounds or arenes have double bonds. They undergo similar chemical reactions including substitution reactions, hydrogenation and coupling reactions. However, aromatic compounds don’t undergo additional reactions due to the delocalised electrons in the benzene ring.
Electrophilic substitution reaction – in this reaction, the aromatic group acts as a nucleophile. This means it attracts a positive ion (electrophile or lacking an electron) as a substituent, hence why it’s also known as electrophilic aromatic substitution. Here is an example of an electrophilic substitution reaction:
Nucleophilic substitution reaction – a nucleophilic substitution reaction is virtually the opposite of electrophilic substitution. In this reaction, a nucleophile displaces a good leaving group on the aromatic ring. Typically, the substituents located on the ortho and para positions are the ones that get replaced.
Hydrogenation – in this reaction, the benzene ring or aromatic group is converted into a saturated ring (without double bonds). The delocalised pi bonds, however, resist this reaction. Hydrogenation typically requires a catalyst and even then it’s still a very slow process.
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