Preparation by addition reactions
When reactions are used preparatively, it is desirable that a high yield of desired product is obtained. A disadvantage of preparing haloalkanes is that some alkenes give a mixture of two haloalkanes.
One haloalkane is formed if either the halogen-containing reactant is symmetrical or the alkene is symmetrical. See examples in the reactions below.
Two haloalkanes are formed if both the alkene AND the reagent are asymmetric. This is because HX can add in two different ways as shown below.
If two products are formed, the major product has the more electropositive portion of the reagent (H in the case of HX) attached to the carbon bearing the larger number of hydrogens.
One haloalkane is formed if either the halogen-containing reactant is symmetrical or the alkene is symmetrical. See examples in the reactions below.
CH3CH2CH2CH=CH2
CH3CH=CHCH3 + HCl → 
Two haloalkanes are formed if both the alkene AND the reagent are asymmetric. This is because HX can add in two different ways as shown below.
CH3CH=CH2 + HCl → 
If two products are formed, the major product has the more electropositive portion of the reagent (H in the case of HX) attached to the carbon bearing the larger number of hydrogens.
A secondary halide is formed in preference to a primary one.
A tertiary halide is formed in preference to a secondary or primary one.
A tertiary halide is formed in preference to a secondary or primary one.
You can learn in more detail about why this happens and about addition reactions in general in the Organic - Addition reactions module.