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I am told that $\ce{HCN}$ is a Lewis acid.

From what I understand a Lewis acid possesses (an) empty orbital that is accessible to attacking nucleophiles.

Does $\ce{HCN}$ have any empty orbital(s)? So how can it behave as a Lewis acid? I know prototypical Lewis acids are for example boron trifluoride, and these have an empty p-orbital.

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  • $\begingroup$ You don't have to say that it is specifically a Lewis acid. $CN$ bond, as nitrogen is more electronegative, and due to the presence of triple bond, causes the carbon to become partially positively charged. It thus attracts the bonded pair of electrons with the hydrogen, and just like in hyperconjugation, an $H^+$ ion is released. In fact, this hydrogen is even more acidic than alpha hydrogens. $\endgroup$ – Gummy bears Jan 16 '15 at 16:36
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    $\begingroup$ Could you say in what reaction it would act as Lewis acid? It's quite relative. $\endgroup$ – Mithoron Jan 16 '15 at 19:57
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    $\begingroup$ HCN has a low empty orbital in the form of the antibonding sigma orbital H-CN. $\endgroup$ – Marlo Stanfield Jan 24 '15 at 19:47
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Low-lying, empty antibonding orbitals will suffice. The reaction of carbon dioxide and water to form carbonic acid serves as a parallel example. Water donates one of the lone pairs on oxygen towards bond formation, water is the Lewis base in this case. Carbon dioxide has empty, low-lying $\ce{\pi}$* orbitals that serve to accept these electrons; carbon dioxide is a Lewis acid in this example.

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