Why do alkyl iodides on standing become violet or brown?
Answer : The strength of the C−X bond decreases in the order: C−F > C−Cl > C−Br > C−I; hence, the stability of haloalkanes containing the same alkyl group follows the order: alkyl fluorides > alkyl chlorides > alkyl bromides > alkyl iodides. Because of the least stability of alkyl iodides among alkyl halides, they easily decompose to iodine in the presence of light; therefore, develop violet or brown colour.
