General Formula

Alkenes are a homologous series of hydrocarbons that contain at least one carbon to carbon double bond, C=C. The C=C bond is the functional group. Because they contain a double bond, alkenes are unsaturated: they can gain more atoms by addition across the C=C.Alkenes with one C=C and no ring have the general formula:C_nH_2nEach member of the series differs from the next by CH₂. The smallest alkene is ethene, C₂H₄, because a double bond needs at least two carbon atoms. Next come propene, C₃H₆, and butene, C₄H₈. Compared with the alkane with the same number of carbons (C_nH_2n+2), an alkene has two fewer hydrogen atoms.Worked example: an alkene has M_r = 70. Each CH₂ unit has M_r = 12 + 2 = 14, so 14n = 70 and n = 5. The formula is C₅H₁₀.Take care: cycloalkanes such as cyclohexane, C₆H₁₂, also fit C_nH_2n, because the ring uses up the same two hydrogen atoms that a double bond would. So a molecular formula that fits C_nH_2n does not on its own prove that a compound is an alkene; you need its structure or a chemical test.