Elimination reaction with bad leaving groups If a halogen is present, as in case of 2-fluorobutane, which is reacted upon by alc. KOH then, shouldn't the product be according to Zaitsev's rule. 

Then, why does the removal of hydrogen takes place, does this happens in case of halogens only or in case of all the bad leaving groups?
Alkyl halides with bad-leaving groups (like $\ce{F-}$) having EWG at $\ce{\beta}$-position undergoes these reactions, also known as $\ce{E_{1CB}}$, viz. elimination unimolecular conjugate base.

Example:


aldol condensation





photo-induced decarboxylation




Reagent: bulky or strong bases or both (like $\ce{K+(^tBu)-, LDA, NaOH, KNH2}$, etc) along with some heating.

Mechanism: