The reaction between 1-butene and chlorine gas is called halogenation, specifically chlorination.

Here's why:

* Halogenation refers to the addition of a halogen (like chlorine, bromine, or iodine) to an organic molecule.

* Chlorination is the specific case of halogenation using chlorine.

This reaction results in the formation of 1,2-dichlorobutane, where the chlorine atoms add across the double bond of 1-butene.