Aliphatics & aromatics
There are three types of aliphatic hydrocarbons:
- alkane (CnH2n+2)
- only has single bonds (e.g., methane, CH4)
- alkene (CnH2n)
- has one or more carbon–carbon double bonds (e.g., ethene, C2H4)
- alkyne (CnH2n−2)
- has one or more triple bonds (e.g., ethyne, C2H2)
Alkanes are saturated because their carbon atoms have the maximum number of bonds (four each). Alkenes and alkynes are unsaturated, and so they are more reactive. All aliphatics are generally nonpolar molecules, and the larger ones (longer chains) have higher boiling points than the smaller ones.
Alicyclics, or cyclic aliphatic hydrocarbons, occur when the two ends of an aliphatic’s chain are joined together. There are cyclic alkanes (CnH2n), cyclic alkenes (CnH2n−2), and cyclic alkynes (these are rare).
Aromatic hydrocarbons are based on a ring of alternating single bonds and double bonds. This ring by itself is called benzene, and other hydrocarbons based on it (such as toluene) are collectively called aromatics.