An organic synthesis problem

loom91

Using NBS to create a low concentration of bromine is an accepted method of performing allylic substitution. But what is that first reaction? Grignard reagents reacting with acetylene? I've never seen it before. What is it called?

Molu

chaoseverlasting

I dont know what the reaction is called, but grignards can react with a hydrogen on a sp hybridised carbon. It acts as a base for the acidic hydrogen.

chaoseverlasting

Ouch. I made a mistake. When you would do that reaction, you would get methane, as the CH3 group would take away the hydrogen on the sp hyb carbon.

chaoseverlasting

You could still over come that though. Instead of the gridnards thing, you could react it with CH3Cl in presence of NaOH, and proceed in the same way. That would give you your maelic acid in the end.

chemisttree

This would be an expensive way to make methanol...

chaoseverlasting

How would you get methanol? The base would react with the acidic hydrogen and then CH3 would attack at that position giving you 2-butyne. That you would reduce using hydrogen gas in poisoned by lindlars catalyst. NBS to get Br at the allylic position and hydrolysis to get you the compound you require.

loom91

I don't think hydroxide is a sufficiently strong base to be able to extract the very weakly acidic acetylenic hydrogen. You need a much stronger base, like hydride or amide. On the other hand, it is a sufficiently strong nucleophile to displace chlorine from chloromethane in a SN2 attack, forming methanol.

chaoseverlasting

No. Im sure of this reaction.

loom91

I strongly doubt it would work. Water is a significantly stronger acid than ethyne, therefore OH- (the conjugate base of water) will not be able to deprotonate ethyne to any significant extent. Consider this well known reaction: Adding Calcium Dicarbide (CaC2) to water results in protonation of dicarbide dianion to form ethyne. This indicates that the reaction

acetylene + OH- <-> acetylide + water

has a negligibly small equilibrium constant. Where did you find your reaction?

Molu

chemisttree

Review some basic organic chemistry...
Organic halides in the presence of hydroxide ion. SN2 displacement leads to the replacement of halogen with hydroxyl. The product is an alcohol.

Do you think you can generate a carbanion of acetylene (pKa=25) with hydroxide? pKa of water is 14.