[carotis]

Hello there! 
I am new on board and I sincerely hope you can give me a hand because I am kindo frustrated. 

Here it goes: I have  4-Bromophenylacetylene. This one http://www.chemicalbook.com/ChemicalProductProperty_EN_CB7440678.htm

First I treat it with LDA. What should happen I think, is the deprotonation of the ethine, so the H+ goes off.

Well, I was wondering... if  I give H2O the next  step after then what I thin would occur is the reaction with H2O and Li to LiOH and the H+ attach to the ethine again.

But that would make no sense. So I think there must be a substitution with Br to (-)Ethine-Ph-OH and HBr but it would be double negative charge on both ends. So... somebody know the solution of my misere problem?

P.S: It's such a pain in the ass with the verification letters 

[Babcock_Hall]

Can a nucleophile approach 180 degrees from the bromine?

[hxj1985620]

LDA only act for the deprotonation of the ethine, which can't remove the bromine. Only 4-Bromophenylacetylene can be got if u quench the reaction with H2O

[clarkstill]

One could imagine lithium-halogen exchange if an excess of LDA were used:

https://en.wikipedia.org/wiki/Organolithium_reagent#Lithium-Halogen_exchange

These are more common when BuLi is used, rather than LDA, but it's definitely a possibility. Have you tried this experiment? What products do you get?

[orgopete]

Carotis, can you post a jpg of the actual problem? I agree that as stated, it doesn't make a lot of sense. It could be an error or it could be something missing? It can also be helpful to tell us the source of the discussion, organometallic chemistry, acetylene chemistry, etc. (assuming it is from a class).

[carotis]

THank you for all the answers people, you are awesome.

[carotis]

Here is the pic 

Note: This reactions goes this way under the assumption that Lithium is still in the solution. I think it also stood in the lecture book that if the reaction goes under protonic solvent like water then it would fail... I dunno but if it's the case right here. But lets just consider both possibilities

[orgopete]

I was hoping to see the stoichiometry of the reaction as to whether a benzene intermediate might have been indicated, a transition metal for an Ulman-like coupling, or whether another reagent was missing from the reaction, like formaldehyde to lengthen the chain. In the absence of any of these, I too would write a return of the starting acetylene.

[TheMantis]

Be careful.  the LDA could be nucleophilic enough to undergo a nucleophilic aromatic substitution to displace the bromide.  Anionic amines are generally nucleophilic enough to do this even if there are no deactivating groups onto the ring.  You could get a serious side product.

I would recommend a different reaction condition.

[orgopete]

Be careful.  the LDA could be nucleophilic enough to undergo a nucleophilic aromatic substitution to displace the bromide.  Anionic amines are generally nucleophilic enough to do this even if there are no deactivating groups onto the ring.  You could get a serious side product.

I would recommend a different reaction condition.

Re nucleophilic aromatic substitution
LDA won't directly substitute for the bromine. It could by first forming a benzyne. I would anticipate the acetylide anion to add to a benzyne before diisopropylamine, however without seeing how the problem was written in the book, I don't think that was the purpose of the LDA, especially as benzynes give mixtures.

This problem has the appearance of an addition to an aldehyde, ketone, or another electrophile that should be quenched with water, but such a reagent is missing.

[Babcock_Hall]

Perhaps the whole point of the problem was to show that water would be a problem.