Other reagents that react with alkenes by addition
Alkenes react with H2 by addition in the presence of a Pt (platinum) or Pd (palladium) catalyst. This reaction is referred to as hydrogenation.
All reactions that have elements as reactants are redox (oxidation-reduction) reactions. H2 is a reductant, and the alkene is said to have been reduced because both carbons of the double bond bear more H in the product.
Alkenes are converted into alcohols on reaction with H2SO4 and then with H2O. The reaction is initiated by reaction of the alkene with H+ to give a carbocation.
This can react with either –OSO2OH to give an alkyl hydrogensulfate ester ROSO2OH which then reacts with water (hydrolyses), or with H-OH. In either case, the overall outcome is addition of water to the alkene.
All reactions that have elements as reactants are redox (oxidation-reduction) reactions. H2 is a reductant, and the alkene is said to have been reduced because both carbons of the double bond bear more H in the product.
An alkane can be prepared from an alkene using hydrogenation as shown in Reaction 1,
Reaction 1: 
Alkenes are converted into alcohols on reaction with H2SO4 and then with H2O. The reaction is initiated by reaction of the alkene with H+ to give a carbocation.
This can react with either –OSO2OH to give an alkyl hydrogensulfate ester ROSO2OH which then reacts with water (hydrolyses), or with H-OH. In either case, the overall outcome is addition of water to the alkene.
Both alkenes below are symmetrical and give rise to only one product as shown below.
Reaction 3:
CH3CH2CH=CHCH2CH3 Reaction 4:
CH3CH=CHCH3