Explain why the terminal hydrogen in acetylene is acidic?

Question

when in fact the electronegativity values of hydrogen and carbon are almost the same/

Answer 1

Ah isn't this just the question among questions. And I see nobody even attempted . . . even though the easy questions pounced on so quickly. Well I'll give you my best shot.

AKA Ethyne

So first of all just a general thought on organic compound acidity. They can be classified as acids with strenghts varying due to their ability to form a stable conjugate base. So this is why for example a carboxylic acid if you take a hydrogen off of the oxygen you get a negative charge. But that negative charge is able to "resonate" across the two oxygens and there fore they each effectively only have 1/2 a negative charge and are "happy". So the ability to form a stable product after H removal is the key. In general these organic compounds are very weak acids and it may be more useful to think of their acidity relative to eachother rather than something like HCl. But the bottom line is that their acidity is key in all sorts of organic and metabolic reactions.

So when you pull off or remove a hydrogen you are leaving behind a pair of electrons ( a negative charge). So it gets a bit complicated but the ethyne is SP hybrized since it is triple bonded. This means that it has a larger amount of S-character and less P character relative to an SP2-doublebond or and SP3-Singlebond.

So the reason it is acidic is due to the fact that the electrons in an S-orbital are much closer to the nucleus which contains the positively charged protons. This means that the molecule is overall lower energy (more stable) because of larger s-character. This means that thinking of it in terms of an equilibrium reaction:

general (use for all)
HA <--> A: - + H+

ethyne (acetylene)
H-C:::C-H <--> H-C:::C: - + H +

it will drive the equilibrium further to the right if you have a more stable A:- or in this case H-C:::C: - .

So if that didn't make sense it is because you have a more stable (lower energy) product when you remove the H.

Answer 2

The electronegativity values will tell you nothing about the bond strength. The bond is weak, and so protons can be lost.