Cyclic Compounds

In many organic compounds the carbon atoms form open chains, which may be straight or branched. These are aliphatic compounds. In cyclic compounds, some of the carbon atoms are joined in a ring.There are two kinds of cyclic hydrocarbon.Alicyclic compounds have a ring that is not aromatic. The cycloalkanes are the simplest examples: cyclopropane, C₃H₆, cyclobutane, C₄H₈, cyclopentane, C₅H₁₀, and cyclohexane, C₆H₁₂. They are named by adding the prefix cyclo to the name of the alkane with the same number of carbon atoms.Aromatic compounds (arenes) contain a benzene ring, C₆H₆, in which six electrons are delocalised around the ring.Closing a chain into a ring removes two hydrogen atoms, one from each end, to make the extra C-C bond. So cycloalkanes with one ring have the general formula C_nH_2n, the same as the alkenes. Cyclohexane and hex-1-ene are both C₆H₁₂, but cyclohexane is saturated: it has only single bonds and does not decolourise bromine water.Cyclic compounds are usually drawn as skeletal formulae. Cyclohexane is a hexagon, where each corner is a carbon atom carrying two hydrogen atoms. Benzene is a hexagon with a circle inside, showing the delocalised electrons.Worked example: give the molecular formula of methylcyclohexane. The ring has six carbons, C₆H₁₂. Replacing one H with a CH₃ group adds one C and two H, giving C₇H₁₄.