[polly]

When starting with 2-butene rather then stilbene, another product other then 2-butyne is the major product.  What is the product and why does it form preferentially?

(Please see attachment- hope you can read it- hard to draw on paint)

[GCT]

Where's the attachment?  What exactly are the reactants

[polly]

second attempt...please let me know if not able to read - thanks!

[GCT]

I'll give you a hint.  Which hydrogen is more acidic, allylic or vinyl?

[GCT]

Allylic is more acidic, the hydrogen at the end of the 2-butene due to resonance stabilization of the conjugate base.  Thus the triple bond will eventually form there; at the end of the molecule.

[GCT]

Actually with the 2 butene with the reaction reagents will result in a conjugate alkene.

[Mitch]

you mean a pseudo-allene structure?

[polly]

Can you draw it?

[GCT]

Not an allene, but a conjugated system.  Not absolutely sure about the mechanism, however, the instead of forming the unstable allene it will probably be rearranged to the conjugated alkene one way or another.

Polly, try working out the steps for yourself.  At first the bromine diatomic is added to the double bond.  With the first elimination is a return to the 2 alkene (3 bromo).  However, the terminal carbon has become more acidic and through an elimination reaction with a base the end hydrogen will be pulled off and the double bond will form there.  Depending on the reaction conditions E1 or E2 will take place.  However I am thinking it is more of an E2; a strong base will be needed for E1 since a carbocation is formed.  Nevertheless the allene is relatively unstable and I presume that it will rearrange through some means to the more stable conjugated system, 1,3 diene.