For nomenclature of alkenes in IUPAC system, the longest chain of carbon atoms containing the double bond is selected.
Numbering of the chain is done from the end which is nearer to the double bond. The suffix ‘ene’ replaces ‘ane’ of alkanes.
It may be remembered that first member of alkene series is: CH2 (replacing n by 1 in CnH2n) known as methene but has a very short life.
As already mentioned, first stable member of alkene series is C2H4 known as ethylene (common) or ethene (IUPAC). IUPAC names of a few members of alkenes are given below :