Alkanes are the simplest structurally and least reactive hydrocarbon species containing only carbon and hydrogen. The distinguishing feature of alkanes is the lack of double or triple bonds - they are saturated hydrocarbons. The lack of reactivity of alkanes under laboratory conditions makes them quite uninteresting in synthetic organic chemistry.
Saturated hydrocarbon compounds only contain single C-C and C-H sigma bonds. A significant amount of energy is contained within these strong bonds, meaning the oxidation of alkanes can produce huge amounts of heat. This makes alkanes excellent fuels with their most significant reaction being combustion.
Alkenes and alkynes are families of hydrocarbon compounds that contain one or more C-C multiple bonds. In contrast to alkanes, the alkenes and alkynes are highly reactive and provide useful starting materials in synthetic organic chemistry.
Figure 1: Order of hydrocarbon reactivity
Alkenes are relatively stable compounds but are more reactive than alkanes due to the reactivity of the carbon-carbon pi bond. Most reactions of alkenes involve the reaction of an electrophile with the nucleophilic pi bond to form two new, stronger, sigma bonds.
Alkynes are usually more reactive than alkenes because their triple C-C bond contains two pi bonds. This means there are more loosely held pi electrons available for reaction with electrophiles.