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Topic: diphenylacetylene synthesis  (Read 13760 times)

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polly

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diphenylacetylene synthesis
« on: April 16, 2004, 09:09:27 PM »
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)
« Last Edit: April 16, 2004, 10:11:46 PM by polly »

GCT

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Re:diphenylacetylene synthesis
« Reply #1 on: April 17, 2004, 02:51:50 PM »
Where's the attachment?  What exactly are the reactants

polly

  • Guest
Re:diphenylacetylene synthesis
« Reply #2 on: April 18, 2004, 07:35:42 PM »
second attempt...please let me know if not able to read - thanks!
« Last Edit: April 18, 2004, 09:23:07 PM by polly »

GCT

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Re:diphenylacetylene synthesis
« Reply #3 on: April 19, 2004, 10:49:27 AM »
I'll give you a hint.  Which hydrogen is more acidic, allylic or vinyl?

GCT

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Re:diphenylacetylene synthesis
« Reply #4 on: April 19, 2004, 11:23:39 AM »
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

  • Guest
Re:diphenylacetylene synthesis
« Reply #5 on: April 19, 2004, 06:17:49 PM »
Actually with the 2 butene with the reaction reagents will result in a conjugate alkene.


Offline Mitch

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Re:diphenylacetylene synthesis
« Reply #6 on: April 19, 2004, 06:47:03 PM »
you mean a pseudo-allene structure?
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polly

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Re:diphenylacetylene synthesis
« Reply #7 on: April 19, 2004, 07:44:16 PM »
Can you draw it?

GCT

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Re:diphenylacetylene synthesis
« Reply #8 on: April 20, 2004, 01:55:27 PM »
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.

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