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Why does a primary and a secondary nitro compound behave as an acid in the presence of strong alkali whereas tertiary nitro compounds do not? Can anyone explain it with reactions?

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  • $\begingroup$ How many $\alpha$ hydrogens are there on a tertiary nitro compound? $\endgroup$
    – bon
    Jan 29, 2016 at 13:04
  • $\begingroup$ there is no alpha hydrogen in tertiary nitro compound $\endgroup$ Jan 29, 2016 at 13:10
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    $\begingroup$ There is your answer then. $\endgroup$
    – bon
    Jan 29, 2016 at 13:10

2 Answers 2

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In order for a compound to act as an acid, it must have a proton which can be abstracted. As you correctly pointed out, tertiary nitro compounds have no $\alpha$ hydrogens and therefore they cannot be deprotonated at the $\alpha$ position.

Of course, they could be deprotonated elsewhere, but if the other groups are just alkyl groups then this isn't really feasible as they have such a high $\mathrm{p}K_\mathrm{a}$.

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Organic nitro compounds can tautomerise to give its aci-nitro form. As we know tautomers exist in a dynamic equilibrium, the enol form of nitro compound is called its aci-nitro form, which is present in a small amount in the equilibrium mixture. Aci-nitro form is the acidic form of a respective nitro compound, which is able to liberate an acidic hydrogen when needed.

Thus, when an alkali metal is added to a nitro compound, they react with the remaining aci-nitro form (which gets used up) and thus the equilibrium shifts right, producing more aci-nitro compounds. This results in complete dissolution of the alkali in the nitro compound, provided that the given amount of nitro compounds is able to synthesise required amount of aci-nitro form to completely dissolve the alkali.

The role of alpha hydrogen in the nitro compound is to take part in tautomerisation:

Keto-enol tautomerism of nitro compound

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    $\begingroup$ Please edit your post for grammar, punctuation, and style; use MathJax for typesetting; and avoid ethereal links. $\endgroup$ Jul 2, 2021 at 18:31

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