Relative acidities of organic molecules

[thestrokes14]

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Can someone explain why the acidity of alkynes (referring to the hydrogen neighboring the C-C triple bond) is greater than that of alkenes which is greater than that of alkanes?

Thanks!

 

[thestrokes14]

Actually, I think I may have found the answer. To those who are curious:

the alkynes have greater s orbital character (sp hybridized orbitals). S orbitals are located closer to the positive nucleus, and thus stabilize the corresponding negative charge upon deprotonation.

Do pi orbitals any role in this acidity business?

 

[soby10]

I always think of acidity as looking for the most stable conjugate base. So sp hybridized CH is more acidic than sp2 and sp2 is more acidic than sp3. Your right to say that the main reason is hybridization effects, the higher the s character the stronger the acid bc the closer the lone pair is held to the the nucleus and the more stable the conjugate base. So pi bonds play a role bc they contribute to the overall s and p hybridization. Pi bonds are the side to side overlap in 2p orbitals makes the bond shorter. Pi bonds have more s character a shorter bond length and thus the negative charge is closer to the nucleus in the conjugate base of a sp( where there are 2pi bonds and a sigma bond) hybridized bond.