A Hydrogen Bonded to an sp Carbon Is Acidic

We have seen that an sp carbon is more electronegative than an sp carbon, which is more electronegative than an sp carbon (Section 2.6). 

Because the most acidic compound is the one with its hydrogen attached to the most electronegative atom (when the atoms are the same size), ethyne is a stronger acid than ethene, and ethene is a stronger acid than ethane. 

We have also seen that, if we want to remove a proton from an acid (in a reaction that strongly favors products), we must use a base that is stronger than the base that is formed when the proton is removed (Section 2.5). 

For example, NH2 is a stronger base than the acetylide ion that is formed, because NH3 (pA a = 36) is a weaker acid than a terminal alkyne (p Tg = 25). Recall that the weaker acid has the stronger conjugate base. Therefore, an amide ion ( NH2) can be used to remove a proton from a terminal alkyne to prepare an acetylide ion. 

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The amide ion caimot remove a hydrogen bonded to an sp or an sp carbon. Only a hydrogen bonded to an sp carbon is sufficiently acidic to be removed by the amide ion. Consequently, a hydrogen bonded to an sp carbon sometimes is referred to as an acidic hydrogen. The acidic property of terminal alkynes is one way their reactivity differs from that of alkenes. Be careful not to misinterpret what is meant when we say that a hydrogen bonded to an sp carbon is acidic. It is more acidic than most other carbon-bound hydrogens but it is much less acidic than a hydrogen of a water molecule, and water is only a very weakly acidic compound (pA a = 15.7). 

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