[gralla55]

One of these starting materials reacts with lithium acetylide and forms the product. I'm thinking it has to be #3, because it seems like the only one with enough carbon atoms to form the product. Does that look correct? Still, if that's the case I'm not sure what kind of reaction it is... Any help would be appreciated!

[MissPhosgene]

What type of properties does lithium acetylide have? What type of properties do epoxides and carbonyls have? Think complementary...



You do have the number of carbons in the reactant correct.

[gralla55]

Thanks, so #3 was correct then? #2 would also have enough carbon atoms, but the double bond in #3 would take care of the -2 charge of acetylide.

I know epoxide rings are opened with nucleophiles and acids, and that acetylide could work as a weak acid, so now I'm suddently unsure again. Carbonyls will react with electrophiles? Thanks again.

[Doc Oc]

You're thinking purely of the acetylide, but you're forgetting why the lithium is there.

[gralla55]

I thought the lithium was there only to balance out the 2- charge?

[Doc Oc]

Acetylide is not -2 in charge, and even if it was a single Li wouldn't be able to stabilize that since Li is +1.

But you have the right idea, the Li is there to stabilize a negative charge.  Would something negatively charged be an acid or a base?  Nucleophile or electrophile?

[gralla55]

Well, something negatively charged would be a base and a nucleophile. I think the nucleophile has to have a spare hydrogen atom to oped the epoxie ring though, which would make #3 the right choice?

[Doc Oc]

Well, 3 is the correct answer, but you don't need an H to open an epoxide.  Since you have Li to stabilize the - charge on the acetylide once the epoxide opens the Li will stabilize the O- charge instead.  Then when you work the reaction up (usually an aqueous condition) the hydrogen will get slapped onto the alcohol.

But if you draw the products of the acetylide opening the epoxide vs attacking the aldehyde you'll see that the products are actually different.