2010 Destroyer ochem #110 Alkyne formation question

[fuqele]

Hey guys,

In this problem, there is a step in which 1,2-dibromopropane turns into an alkyne by 1)3NaNH3, NH3, 2) H2O. I can't find this reaction anywhere. Does anyone know what this reaction is called and its mechanism? Thanks in advance.

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[Neil45]

I think "double elimination from a dihaloalkane"? Here's a website: http://chemwiki.ucdavis.edu/Organic.../Preparation_of_Alkynes_by_Double_Elimination

Here's a mechanism from the same website:

 

[ttpharm]

Think about it. NH3 is the very happy. It wants 5 electrons/bonds. It has 3 bonds and a lone pair. Losing another H is very unlikely making it a very WEAK acid and therefore a VERY strong conjugate base. It would pull a H off like nothing to be happy again.

 

[fuqele]

Ahh, now it makes a lot of sense. Thanks!

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[NewBee12]

This a simple E2 step. It's called double Dehydrohalogenation