Reaction with asymmetric alkenes

As shown by the reaction of bromine with propene below, addition reaction products have

• all atoms of both reactants
• the same arrangement of carbons as the reactant
• one fewer site of unsaturation than the reactant
• single bonds to the parts of the reagent from carbon atoms that were doubly bonded in the reactant

As shown by the reactions of propene and butene with HCl, addition reactions can give rise to
 

• a single product if either the alkene or the reagent is symmetrical
  
  CH₃CH=CHCH₃ + HCl
   
• two products if both the alkene and the reagent are asymmetric.
  
  CH₃CH=CH₂ + HCl