Hydrogen atoms are replaced by other atoms or groups of atoms, such as halogens or hydroxyls. Olefins, or alkenes, have at least one double bond between two carbon atoms. Ethylene is the first member. Olefins react primarily by addition of molecules such as water, oxygen, and halogens. Acetylenes, or alkynes, have at least one triple bond between two carbon atoms.
Acetylene is the first and most important member of the series.
Acetylenic hydrocarbons have reactions similar to the olefins. Alicyclics have their carbon atoms arranged in a ring. These atoms may have single or double bonds between them. Triple bonds in alicyclics are rare, but they can occur in sufficiently large rings. Many naturally occurring organic compounds fall into this class.