Aromaticity
Aromatic molecules (compounds) are special molecules that meet a particular set of criteria and are especially stable.
In order to be aromatic, a molecule must be:
- The compound must obey Hückel's Rule (# of π-electrons = 4n + 2)
- The compound must contain a 'concentric ring of aligned p orbitals.' Resonance!
- The compound must be planar (big rings are not planar)
- The molecule must be cyclic
Example: some carbon-carbon containing organic and not-aromatic molecules
Aromaticity and Lone Pairs
Sometimes, cyclic molecules with heteroatoms (non-carbon and hydrogen atoms) can also be aromatic. There are three common heteroatoms that you should be on the look-out for:
- nitrogen: when sp3 hybridized, donates 2 pi electrons; when sp2 hybridized, donates zero pi electrons
- oxygen: sp3 hybridized, donates 2 pi electrons
- sulphur: sp3 hybridized, donates 2 pi electrons
Naming Aromatic Rings
As a reminder, remember the General Naming Rules:
- Identify and name the longest carbon chain
- If present, assign stereochemistry (R/S or E/Z)
- Identify and name any substituents on the carbon chain
- Give these substituents the appropriate 'address'
- Add substituent names as a prefix to the carbon chain
- Change suffix of the carbon chain if necessary (for functional groups)
Most of the time you'll be naming benzene derivatives, using benzene as the centre and naming the substituents coming off. Benzene has 6 carbon atoms, and therefore has 6 positions. Take for example the following compounds:
But, we like to complicate things in chemistry by introducing different names. Enter the ortho (1,2-substitution), meta (1,3-substitution) and para (1,4-substitution) notation. We can use these terms in place of the numbering. Especially since benzene is planar so the numbering doesn't matter. For example, the middle compound above (1-chloro-2-methylbenzene) can be rewritten as ortho-chloromethylbenzene.
Aromatic rings can be substituents on a carbon chain, for example in the following compounds shown below. This is only the case when the aromatic group is a lower priority than the main carbon chain.
Example: Aromaticity
Which of the following are aromatic?
Example: Aromaticity
Which of the following are aromatic?
