7.2.1 Structure and Formulae of Alkenes

Alkenes are a class of hydrocarbons that contain a carbon-carbon double bond. This double bond gives alkenes their unique chemical and physical properties. The general formula for alkenes is:

C_nH_2n

where "n" represents the number of carbon atoms in the molecule.

Due to the presence of the double bond, alkenes are considered unsaturated hydrocarbons. Unsaturated means that the molecule has the potential to undergo additional reactions to add more atoms. In the case of alkenes, they have two fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms. This unsaturation gives alkenes greater reactivity compared to alkanes.

The first four members of the alkene homologous series are ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈), and pentene (C₅H₁₀). These molecules differ in the number of carbon atoms but share the same general structure of a carbon-carbon double bond.

To represent alkene molecules, a condensed structural formula can be used. For example, C₃H₆ represents a three-carbon alkene with six hydrogen atoms. This condensed formula provides a simplified way to represent the arrangement of carbon and hydrogen atoms in the molecule. It can also be represented in the following form: