[Ak]

Hey all,

Im a little puzzled with this synthesis.... I know that NaNH2 normally pulls of an atom (like H), so does the HCCH add on where the Hydrogen comes off?...thts the best guess i have...Any help would be appreciated

[omegasynthesis999]

Hmm...NH2- is a pretty strong base. I don't know the pKa of the benzylic proton is higher or the pKa of the alkyne proton is higher. I've typically seen the NH2- deprotonate an alkyne proton in other reactions but you would have to check pKas to be sure. However if the alkyne proton is more acidic, then you will get the carbanion. This of course is a pretty strong nucleophile which would then react with your starting reactant.

[macman104]

The alkyne proton is much more acidic (25 for the alkyne vs. ~41 for the benzylic proton).  You pull of the proton of the alkyne and do an SN2 on the starting material, kicking out the bromide.

[spirochete]

In practice I wonder if you would get a mix of products if you threw those three molecules together all at once.  The benzyl bromide is activated toward nucleophilic attack and the SN2 with NaNH2 would be irreversible.  On the other hand acid base reactions are much faster than SN2 reactions.  I checked my book and they list this type of synthesis with the deprotonation as a seperate step.

[Ak]

Thanks for the help...im gonna have to go with macman's suggestion...it seems to make sense

[macman104]

On the other hand acid base reactions are much faster than SN2 reactions.  I checked my book and they list this type of synthesis with the deprotonation as a seperate step.

Indeed, I believe you would first deprotonate, and generate a "pot" of anion.  Then, you would drip the benzyl bromide into the solution to control possible side reactions.