Acetylenic carbanion hybridization

By similar arguments, we would predict that vinyl carbanions exhibit sp hybridization and acetylenic carbanions show sp hybridization. Based on the idea that the greater the s character in an orbital the more easily it can accommodate... 

By similar arguments, vinyl carbanions are expected to exhibit sp hybridization, and acetylenic carbanions should show sp hybridization. Based on the idea that the greater the s character in an orbital, the more easily it can accommodate a negative charge, we expect carbanions in which the negative charge is associated with a pair of nonbonded electrons in an sp hybrid orbital to be more easily formed than those in which the electrons are in sp hybrid or sp hybrid orbitals. Similarly, electron withdrawal by induction should stabilize a carbanion. 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

Acetylene, where the carbon is sp hybridized with 50% s character, is much more acidic than ethylene (sp, 33% s), which in turn is more acidic than ethane, with 25% 5 character. Increased s character means that the electrons are closer to the nucleus and hence of lower energy. As previously mentioned, cyclopropyl carbanions are more stable than methyl, owing to the larger amount of s character as a result of strain (see p. 181). 

Abstraction of an acetylenic proton gives a carbanion that has the lone pair of electrons in the sp hybrid orbital. Electrons in this orbital are close to the nucleus, and there is less charge separation than in carbanions with the lone pair in sp2 or sp3 hybrid orbitals. Ammonia and alcohols are included for comparison note that acetylene can be deprotonated by the amide ( NH2) ion, but not by an alkoxide ion ( OR). 

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