Alkenes undergo electrophilic addition reactions, but with certain alkenes the question of orientation of addition arises. With the reaction of propene with hydrogen bromide, what will be the major product since two possibilities exist: 1-bromopropane and 2-bromopropane.
Alkenes can be simply classified into symmetrical or non symmetrical. To differentiate these two you need to look at the "alkenic" carbon atoms..and check which atoms they are bonded to. If the are bonded to CC and HH; or CH and CC then the alkene is non symmetrical. If the alkenic atoms are bonded to CC and CC; or CH and CH; or HH and HH [ie ethene] then the alkene is symmetrical.
When then considering the orientation of addition of hydrgoen halides, remember the statement: " unto those that have shall more be given"..
in other words the hydrogen atom of the hydrogen halide will end up being bonded to the alkenic carbon atom that already has most hydrogen atoms.