Final answer:
In the reaction between chlorine and an alkene, chlorine adds across the double bond, resulting in an addition reaction that forms a dichloro compound, such as ethylene dichloride from ethylene. This is different from the reactivity observed with methane, where chlorine substitution leads to various products depending on the amount of chlorine.
Step-by-step explanation:
When chlorine, a halogen, reacts directly with the double bond of an alkene, the type of reaction that occurs is known as an addition reaction. During this reaction, the chlorine molecule (Cl₂) will break, and one chlorine atom will add to each of the carbons that were previously double-bonded, resulting in a dichloro compound. The structural formula of this reaction might depict the transformation of ethylene into ethylene dichloride, where each carbon from the double bond ends up bonded to a chlorine atom. An example of this process is shown in the reaction of ethylene with chlorine gas under mild conditions:
CH₂=CH₂ + Cl₂ → CH₂Cl-CH₂Cl
This reaction contrasts sharply with the reactivity seen with methane (CH₄), where the addition of Cl₂ leads to the substitution of hydrogen by chlorine, producing chloromethane (CH₃Cl) to potentially form a mixture of products, including CH₃Cl, CH₂Cl₂, CHCl₃, and CCl₄ depending on the amount of chlorine present. Halogens display varying reactivities, with fluorine being very reactive and iodine being less so. The halogenation of alkenes and alkynes is fairly common and differs substantively from halogenation reactions with alkanes.