Preparation of Alkyl Halides

Following are two methods commonly used to prepare alkyl halides.
Hydrogen halide addition to an alkene
Halogen halides add across carbon-carbon double bonds. These additions follow Markovnikov's rule, which states that the positive part of a reagent (a hydrogen atom, for example) adds to the carbon of the double bond that already has more hydrogen atoms attached to it. The negative part adds to the other carbon of the double bond. Such an arrangement leads to the formation of the more stable carbocation over other less-stable intermediates.

Reaction of alcohols with sulfur and phosphorous halides
Alcohols can be converted to alkyl halides by reaction with thionyl chloride, SOCl2•, phosphorous trichloride, PCl3•, phosphorous pentachloride, PCl5•, or phosphorous tribromide, PBr3. For example, ethyl chloride or ethyl bromide can be prepared from ethyl alcohol via reactions with sulfur and phosphorous halides. 

Grignard Reaction

In a Grignard reaction, an alkyl halide reacts with magnesium metal in an anhydrous ether solvent to create an organometallic reagent. 

The Grignard reagent is highly reactive and is used to prepare many functional groups. An example is the preparation of a carboxylic acid by reaction with carbon dioxide and mineral acid.

E2 mechanism

Elimination reactions can also occur when a carbon halogen bond does not completely ionize, but merely becomes polarized. As with the E1 reactions, E2 mechanisms occur when the attacking group displays its basic characteristics rather than its nucleophilic property. The activated complex for this mechanism contains both the alkyl halide and the alkoxide ion.
Following is the complete mechanism for the E2 elimination reaction: