Out of a given set of compounds, I was asked to identify which would produce hydrogen gas on reaction with Na/K. One of the given compounds was:


I could not detect any acidic hydrogen in it, so concluded that it won't produce hydrogen. But the answer says the opposite.

So my questions are:

  1. Why does the above compound produce hydrogen gas?

  2. What can be the general set of rules to decide which hydrogen shall be acidic enough so that hydrogen gas is produced on reaction with an alkali metal?

  • 3
    $\begingroup$ This thing will produce a dianion, which according to Huckel feels quite well. As for the general set of rules, it constitutes much of organic chemistry. $\endgroup$ Jan 28 '19 at 13:32
  • $\begingroup$ What should be the ideal pKa value below which Na can produce hydrogen? $\endgroup$ Jan 28 '19 at 13:37
  • $\begingroup$ Actually in NaK it's rather potassium that is stronger reducing agent. Also it would be easier to say what doesn't react with such stuff, because most of organic compounds does. $\endgroup$
    – Mithoron
    Jan 29 '19 at 0:09
  • 1
    $\begingroup$ This is not an acid-base reaction. $\endgroup$
    – Zhe
    Jan 31 '19 at 15:29

First of all, this is not an acid-base reaction. Alkali metals are not bases, rather strong reducing agents. For example, in this site, you may find a excellent explanation. Also remember, metallic $\ce{Li}$, $\ce{Na}$, or $\ce{K}$ does not abstract acidic hydrogen from organic molecules, not necessarily. For example, metallic $\ce{Na}$ reacts with acetylene ($\mathrm {p}K_a$ of terminal alkyne-$\ce{H}$ is $25$) to give sodium acetylide, but it does not undergo aldol condensation with acetone with more acidic $\ce{H}$, $\mathrm {p}K_a$ of which is $19.3$ (https://owl.oit.umass.edu/departments/OrganicChemistry/appendix/pKaTable.html). The reaction of acetone and metallic $\ce{Na}$ rather undergoes dimerization (e.g., see here). Another example is the reaction of triphenylmethane (the $\mathrm {p}K_a$ of $\ce{H}$ on the central carbon is $33$) and metallic $\ce{K}$ in 1,2-dimethoxyethane, which has given solutions of triphenylmethyl potassium containing various cleavage and reduction products derived from triphenylmethane (Ref.1), but authors stated that no hydrogen was evolved in the course of the preparation.

However, the given compound has potential to undergo redox reaction with alkaline metal and release $\ce{H2}$ gas as a byproduct because the resultant dianion stabilized by resonance and show hint of aromaticity (Ref.2). Tentative mechanism is illustrated in following scheme:

Dianion Formation


  1. R. O. House, V. Kramar, “The Chemistry of Carbanions. I. The Reaction of Triphenylmethane with Potassium$^1$,” J. Org. Chem. 1962, 27(12), 4146-4149 (DOI: 10.1021/jo01059a007).
  2. G. Boche, H. Etzrodt, M. Marsch, W. Thiel, “The Dianion 1,2,3,4‐Tetraphenylcyclobutadienediide,” Angew. Chem. Int. Ed. Engl. 1982, 21(2), 133-133 (https://doi.org/10.1002/anie.198201331).

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