From Alkyl Halides by Dehydrohalogenation

Alkenes can be synthesized by reacting the corresponding alkyl halides with a suitable strong base that can abstract a proton from one of the carbon atoms, while the leaving group attached to the adjacent carbon leaves. The general mechanism is shown below. 

According to Zaitsev s rule, the major alkene product is the one that is the most highly substituted. Consider the following example involving 2-bromobutane with potassium hydroxide. 

CH3CH =CHCH3 disubstituted alkene major product(80%) 

If bulky bases are used, the least substituted alkene may predominate as the product. In the next example, the base used is /er/-butoxide ion. Since it is a bulky base it will preferentially abstract the less hindered hydrogen, leading to the formation of the least substituted alkene-the Hofmann product. 

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We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

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