Alkenes contain the unsaturated C=C functional group, which characteristically undergoes electrophilic addition reactions (as the pi electrons act as the nucleophile). This is driven by the conversion of the weaker π bond into 2 new, stronger σ bonds. The reactions of alkenes can seem a little bewildering as a wide variety of electrophilic reagents undergo this type of reaction providing access to products with various regioselectivities and stereoselectivities depending on the reagent and / or reaction conditions, but ultimately on the mechanism by which the reaction occurs.
There are potentially 3 factors associated with each of the addition reactions of alkenes:
Overall transformation (i.e. C=C is converted to what functional group)
Regioselectivity - see Markovnikov's rule (if the two atoms that add across the alkene are different, which side of the double bond will they add)
Stereoselectivity - syn- or anti-addition (preference to react in a certain 3D orientation, most easily revealed with cyclic alkenes)
Addition reactions follow the general reaction scheme:
For each reagent, identify the electrophilic atom as it will often be involved in the first step of adding to the π bond and it will add to the less highly substituted position so controlling the regioselectivity of the reaction.
Conceptually, addition is the reverse of elimination, which can most clearly be explained by the equilibrium between the dehydration of an alcohol and the hydration of an alkene.