Hydrocarbons are not acidic. We have already established that methane has a pKa of about 48 (p. 170 above)—it’s essentially impossible to deprotonate. Alkyllithium are for this reason among the strongest bases available. But some hydrocarbons can be deprotonated, the most important example being alkynes—you saw on p. 171 that acetylene has a pKa of 25 and can be deprotonated by NH₂ − (as well as other strong bases such as BuLi). The difference is one of hybridization—an idea we introduced with the nitrogen bases above. Making the acetylide anion, whose negative charge resides in an sp orbital, is much easier than making a methyl anion, with a negative charge in an sp³ orbital, because electrons in sp orbitals spend a lot of their time closer to the nucleus than electrons in sp³ orbitals. C–H bonds can be even more acidic than those of acetylene if stabilization of the resulting anion is possible by conjugation. Conjugation with a carbonyl group has a striking effect. One carbonyl group brings the pKa down to 13.5 for acetaldehyde so that even hydroxide ion can produce the anion. You will discover in Chapter 20 that we call this the ‘enolate anion’ and that the charge is mostly on oxygen, although the anion can be drawn as a carbanion.

It is interesting to compare the strengths of the carbon, nitrogen, and oxygen acids of similar structure below. The ketone (acetone) is of course least acidic, the amide is more acidic, and the carboxylic acid most acidic. The oxyanion conjugate bases are all delocalized but delocalization onto a second very electronegative oxygen atom is much (~10 pH units) more effective than delocalization onto nitrogen, which is 4 pH units more effective than delocalization onto carbon.

Nevertheless, the effect of conjugation on the carbon acid compared with methane is enormous (~30 pH units) and brings proton removal from carbon within the range of accessible bases The nitro group is even more effective: nitromethane, with a pKa of 10, dissolves in aqueous NaOH. The proton is removed from carbon, but the negative charge in the conjugate base is on oxygen. The big difference is that the nitrogen atom has a positive charge through out. If the anion is protonated in water by some acid (HA) the ‘enol’ form of nitromethane is the initial product and this slowly turns into nitromethane itself. Whereas proton transfers between electronegative atoms (O, N, etc.) are fast, proton transfers to or from carbon can be slow.

Carbon acids are very important in organic chemistry as they allow us to make carbon bonds and you will meet many more of them in later chapters of this book.

مواضيع ذات صلة

Organometallics as bases

How to make organolithium reagents

Making organometallics, How to make Grignard reagents

Organometallic compounds contain a carbon–metal bond

Lewis acids and bases

pKa in action—the development of the drug cimetidine

Why do we need to compare acid strengths of O and N acids?

Substituents affect the pKa

The ‘pKas’ of bases

Nitrogen compounds as acids and bases

Delocalization of the negative charge stabilizes the conjugate base

Constructing a pKa scale