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- Organic chemicals that have rings of atoms in
- their molecules. They may be alicyclic
- (cyclopentane), aromatic (benzene), or
- heterocylic (pyridine). Alicyclic compounds
- have localized bonding: all the electrons are
- confined to their own particular bond, in
- contrast to aromatic compounds, where certain
- electrons have free movement between
- different bonds in the ring. Aromatic
- compounds, because of their special
- structure, undergo entirely different
- chemical reactions. Heterocyclic compounds
- have a ring of carbon atoms with one or more
- carbons replaced by another element, usually
- nitrogen, oxygen, or sulphur. Alicyclic
- compounds have chemical properties similar to
- their straight-chain (aliphatic)
- counterparts. Pyridine, a heterocyclic
- compound, is a six-membered ring with five
- carbons and one nitrogen (C5H5N). Furan is a
- five-membered ring containing one oxygen
- (C4H4O). Uracil contains two nitrogens in a
- six-membered ring (C4H4N2O2). These
- heterocyclic compounds may be aliphatic or
- aromatic in nature.
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