1. Allylic/Benzylic Halogens: The bromine atoms in stilbene dibromide are attached to carbons adjacent to the benzene rings (benzylic positions). This makes these carbons more acidic compared to typical alkane carbons due to the electron-withdrawing effect of the aromatic rings. This increased acidity facilitates the removal of the protons by the relatively strong base KOH.
2. Resonance Stabilization: The formation of the double bond after the first dehydrohalogenation creates a conjugated system with the benzene rings. This resonance stabilization makes the resulting alkene more stable, further promoting the second dehydrohalogenation step.
3. Leaving Group Ability: Bromine is a good leaving group, which makes the elimination reactions relatively facile.
4. Steric Considerations: Stilbene dibromide, with its bulky phenyl groups, might be less accessible to the bulky NaNH₂ base. KOH, being smaller, may be able to access the bromines more readily.
In summary: The combination of benzylic positions, resonance stabilization, good leaving group, and steric considerations make the dehydrohalogenation of stilbene dibromide relatively easier, allowing the use of the less strong base KOH to achieve the double elimination.
