Nucleophilicity of Acetylide ion

[hooli]

I'm trying to understand the nucleophilicity of alkynes, my book lists the pKa's of acetylene (HCCH), ethylene (CH2CH2), and methane (CH3CH3) as 25, 44, and 50 respectively. This leads to the conclusion that HCCH -acetylene- is the strongest acid and hence the weakest base. The book then goes on to say that acetylide anions are highly basic (compared to ethyl anion and methyl anion assumably) and is thus the strongest nucleophile.
This seems to me to be a contradiction, the book clearly states that methyl anion is more basic than vinyl anion which is in turn more basic than acetylide anion. Then proceeds to say that actylide ion is highly nucleophilic. How does this work?

 

[chem_tr]

Hello,

It is possibly because of the fact that pi system in vinyl and extended pi system in acetylide causes the basicity to be lower than that of methyl. Pi bonds tend to love electron pairs, so basicity is lowered.

However, please let me inform you that nucleophility is not the same thing as basicity. Nucleophility is rather the desire to pull electrons to produce a negatively (or more partially negatively) charged species. Basicity is, very briefly, the desire for protons.

I hope I could be of some help.

 

[movies]

Acetylene is more acidic because the lone pair that you are left with after deprotonation resides is in a low energy sp orbital, so the anion is more stabilized. This stabilization is less for ethylene (sp² orbital) and ethane (sp³ orbital) because these are higher energy orbitals (they contain more p-character).

I think you are misreading the statement about nucleophilicity, however. Certainly the ethylene anion and ethyl anion are also highly nucleophilic, but acetylene is also a good nucleophile relative to your run-of-the-mill nucleophiles (e.g. primary amines, hydroxide, enolates) because it has a full negative charge and is relatively unhindered by sterics since there aren't any groups around. It's kind of like a spear while other nucleophiles are like a club.

And remember what Chem_tr said, nucleophilicty is not the same as basicity!