What distinguishes alkenes from alkanes during a chemical reaction with bromine?

Alkenes are distinguished from alkanes during a chemical reaction with bromine primarily by their ability to undergo an addition reaction due to the presence of a carbon-carbon double bond. In the case of bromine, alkenes will react readily to form dibromoalkanes. This reaction characteristic is due to the reactivity of the double bond, which can break and allow bromine atoms to add across it.

Alkanes, on the other hand, are saturated hydrocarbons with only single bonds between carbon atoms. They do not react readily with bromine under standard conditions as they lack the double bond necessary for the addition reaction. Therefore, only alkenes will show a positive reaction with bromine, which is noticeable through the decolourisation of bromine water, turning it from brown to colourless. This key distinction highlights that alkenes are more reactive than alkanes when exposed to bromine.