# The most acidic hydrogen among ethane, ethene, ethyne and allene

Among

$$\ce{CH3-CH3}\qquad\ce{CH2=CH2}\qquad\ce{HC≡CH}\qquad\ce{CH2=C=CH2}$$

I need to find out the one having highest acidic hydrogen.

As per my observations, there are two equivalent resonating structures possible in case of conjugate base of $$\ce{CH2=C=CH2}.$$ However, the correct answer is given as $$\ce{HC≡CH}$$ due to inductive effect of sp hybridization of carbon.

But the priority order in determining stability of a conjugate base is:

1. Aromaticity
2. Number of equivalent resonating structures
3. Resonance
4. Hyperconjugation
5. Inductive effect

As such, the correct answer should have been the allene. Where did I go wrong?

• Jun 21, 2020 at 13:44
• I did not downvote, but likely some people will if they seem to see a homework problem with no/too little work input from the asker. You might want to look at the link given be andselisk for some ideas, as well. Jun 21, 2020 at 13:58
• This "priority order" is hardly a law of nature. In my opinion, instead of trying to force everything into a fixed order, it's a better idea to get more experience with various functional group pKa's so that you have a better idea of how much each factor affects the pKa. That takes time. You might think that memorising some order is a quick shortcut to the top, and maybe it does work for quite a few cases, but I fear that this sort of approach comes with the risk of having to unlearn wrong things later on. Jun 21, 2020 at 14:04
• But that's just me. If you have to learn this sort of thing, it's ok, just make sure to take it with a pinch of salt. Especially because these sort of orders were worked out precisely by observing the pKa's of various compounds, so using them to predict pKa's is a little bit like circular logic. TLDR: Models only work in the contexts in which they were developed. Jun 21, 2020 at 14:14
• I agree with @orthocresol. The stability order here would be $\mathrm{sp \gt sp^2 \gt sp^3}$. In allene, $\ce{H}$ is on $\mathrm{sp^2}$ carbon, so most stable carbanion would be that from acetylene. Jun 21, 2020 at 14:26