Reagents that react with alkenes by addition
Alkenes react by addition with H2 to give an alkane. For this reaction to proceed at a reasonable rate, a metal catalyst must be present to activate the H2 toward reaction.
Alkenes react by addition with halogens (Br2, Cl2) to give dihaloalkanes with the halogen atoms on adjacent carbon atoms. A single product is formed because this reagent is symmetrical (both groups at BOTH doubly bonded carbons the same).
Alkenes react by addition with HX (HCl or HBr) to give monohaloalkanes. As this reagent is asymmetric, two different haloalkanes form if the alkene reactant is asymmetric (one or more groups at doubly bonded carbons are different).
Alkenes react by addition with concentrated sulfuric acid (H2SO4). Adding water to the sulfate product formed gives the alcohol. The H+ activates the double bond toward reaction. This reagent is asymmetric; therefore two alcohols form if the alkene is asymmetric.