This is due to the following reason :-
This is in accordance with sytzeff rule that whenever two alkenes are theoretically possible during a dehydrohalogenation reaction , it is always the more highly substituted alkene which predominates .
It can also be best understood by the following illustrations showing the stabilizes of primary(1°) , secondary (2°) and tertiary (3°) carbocations.
The reaction involves the formation of carbocation as shown above. The stabilities of the relative carbocations are shown below.
It is clear from the above structures that the uppermost carbocation is most stable and it leads to the product formation.
Thus from complete discussion we come to the conclusion that it gives following reaction .
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