In this lesson, we will learn:
- The definition of an aromatic compound.
- The properties of the important compound aromatic chemistry is based on.
- How to correctly draw and describe aromatic rings in organic chemistry.
- How to name aromatic compounds using IUPAC systematic nomenclature.
- Cyclic compounds are very common in organic chemistry, and one of the most important cyclic compounds is benzene, formula C6H6. Its structure is a six carbon cyclohexene ring with three double bonds, alternating with single bonds around the hexagonal ring structure. Each carbon atom in the ring is bonded to one hydrogen atom.
- This alternating single/double bonds layout is extremely stable because the single/double bonds can alternate positions between themselves – the double bonds are not truly 'fixed' in one place.
- It is more accurate to say the double bond electrons are delocalized throughout the whole ring, which makes the whole molecule resistant to chemical attack. The free interchanging between the two resonance forms mean benzene is normally written with the double bonds as a circle inside the hexagonal ring. This property is called aromaticity and compounds that behave like this are aromatic compounds.
- As already stated, aromatic rings are very stable – atoms bonded to the carbons in the ring (in benzene they are hydrogen atoms) can be replaced or reacted, but the carbon-carbon bonds that make the ring are extremely strong and hard to disrupt because the three electron double bonds behave as a single unit, not in one particular place.
- Aromatic rings are extremely important in chemical reactivity. A compound that contains a benzene ring is known as an aromatic compound. You can also refer to a benzene ring bonded to part of a larger molecule (so it has lost a hydrogen atom in bonding to this) as a phenyl substituent.
- Naming aromatic compounds is not clear-cut because many old names of compounds, from before IUPAC systematic naming was created, are still popular and accepted today – "preferred" and systematic names both exist and are considered correct. Also, an aromatic compound can be named referring to the benzene ring by the prefix "phenyl" or the suffix "-benzene". A general guide that provides correct systematic names can be used below:
- Treat benzene as a 6 member alkene carbon chain.
- Treat all groups below alkenes as lower priority, using the suffix –benzene if these are the only groups present in the molecule.
- Treat all carbon chains longer than 6 carbons or groups higher than alkenes as higher priority, using the prefix phenyl- for these compounds.