We know that types that have actually a lone pair the electrons room Lewis bases (like ammonia).The structue of $\ceHCN$ is $\ceH-C#N:$; nitrogen has actually a lone pair that electrons and also it is tho acidic. Why?

We understand that species with weird electron counts such together nitrogen dioxide (having odd electrons) dimerise come acheive better stability. The structue that $\ceHNC$ is $\ceH-N#C$. Therefore why it does no dimerise?  The usual rule is one concern per short article unless strong related; while your questions are not really connected I’ll still answer both of them since it’s only two.

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Why is $\ceHCN$ no a base?

Well — the is a base. Girlfriend seem to be thinking that gift an acid and being a base is a dichotomy for part reason; rest assured that aside indigenous the very basic Arrhenius concept no acid-base concept predicts the bases can not be acids and also vice-versa.

You correctly allude out the there is a lone pair ~ above nitrogen, and also that lone pair can indeed act as a (Brønsted-Lowry) base. The just ‘difficulty’ is the the carbon side of the $\ce^-C#N$ reality is more basic than the nitrogen side; thus protonation will occur on carbon first.

$$\ceH-C#N^+-H H-C#N + H+ ^-C#N + 2 H+\tag1$$

Equation $(1)$ shows the complete acid-base equilibrium.

Why is $\ceNCH$ not an odd-electron compound?

I take it the liberty that rephrasing your 2nd question. If you count electrons as is done in equation $(2)$, you will certainly realise the $\ceCNH$ comes in at ten electrons — an also number — as with $\ceHCN$.

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$$\sum n_\cee- = n_\cee-(\ceC) + n_\cee-(\ceN) + n_\cee-(\ceH)= 4 + 5 + 1 = 10\tag2$$

When writing the Lewis structure, you have to remember official charges. Indeed, your description of $\ceCNH$ is bad; instead you should write the Lewis framework as $\ce^-C#N^+-H$. All valences space satisfied, over there is no reason for the link to dimerise. Assuming 5 electrons to it is in on nitrogen (and thus four to it is in on carbon) would certainly generate a an extremely unstable diradical which would violate the octet rule in a bad way (nine electron on nitrogen). Reduced electron counting violations that the octet preeminence are fine however going over the electron counting for a key group element does not happen.

However, over there is a great tendency for the compound to de- and reprotonate as composed in equation $(3)$: