CSEC Chemistry: Differentiating between Alkanes and Alkenes

As we discussed in a previous post, alkanes are saturated compounds because they have the maximum number of hydrogen compounds in their structures while alkenes are unsaturated because they contain double bonds between carbon atoms, and thus, do not contain the maximum number of hydrogen atoms in their structure.

The chemical significance of this is that because alkanes are saturated, they will undergo substitution reactions while alkenes, unsaturated, will undergo addition reactions. Thus, we can differentiate between alkanes and alkenes by testing for the presence of unsaturation.

Alkenes will rapidly decolourize an acidified solution of potassium manganate (VII). The potassium manganate (VII) oxidizes the alkene to a diol (a compound with two hydroxyl groups) and the KMnO4 is converted to brown MnO2. This reagent (also a common oxidizing agent) has no effect on alkanes.

The reaction above (shown two ways for better understanding) occurs rapidly, and there is a colour change from purple (potassium manganate [VII]) to pale pink.

Alkenes are also able to rapidly decolourize a solution of bromine in 1,1,1-trichloroethane from red-brown to colourless. This reaction occurs even in the absence of sunlight.

The reactions shown above are both addition reactions. Also note that alkanes will not react with halogens unless in the presence of bright light or sunlight. Even in the presence of light, alkanes do not react as rapidly as alkenes.