Question

What is the action of hydrogen gas on cyanogen?

Complete the following reactions:
`(CN)_2 + H_2 -> ?`
`(CN)_2 + 4H_2 -> ?`

Answer 1

Given that a nitrile (`"C"-="N"`) like cyanogen is similar to a comparable imine (which has `"C"="N"` bonds instead), `"H"_2` should behave similarly to how it behaves with imines.

It reduces imines into amines, so it should reduce nitriles (`"C"-="N"`) into imines (`"C"="NH"`), and then amines (`"C"-"NH"_2`), as it indeed shows here.

This parallels the reduction of alkynes (`"C"-="C"`) into alkenes (`"C"="C"`), and then alkanes (`"C"-"C"`).

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These types of hydrogenation reactions require a catalyst to cleave the `"H"_2` bond such that it can add cis to the substrate compound (in this case, the nitrile). Here is an example:

It likely depends on the catalyst whether or not a single `pi` bond will be removed, or all of them.

I will assume a palladium catalyst over carbon (as you would traditionally use on alkenes and alkynes for full reduction), and that the catalyst promotes reduction of one `pi` bond at a time without reducing all the way in one go.

`"N"-="C"-"C"-="N" stackrel("H"_2" "" ")(->) "N"-="C"-"CH"="NH"`
`color(white)(ssaaaaaaaaaaaa)""^("Pd/C")`

`"N"-="C"-"C"-="N" stackrel(4"H"_2" "" ")(->) "H"_2"N"-("CH"_2)_2-"NH"_2`
`color(white)(ssaaaaaaaaaaaa)""^("Pd/C")`

Either way, it accomplishes the same result at each step, just multiple times where applicable.

For the reduction of nitriles, elevated temperatures and pressures promote the reduction.