Nitrile chemistry

This post is an add-on to the post on carbonyl chemistry.

A cyanide molecule looks like this, with a C≡N triple bond - which, by the way, is also one of the strongest molecular bonds in general. We can also have cyanide ions, which have the same structure but with a negative charge. That charge also makes cyanide ions very good nucleophiles. which means they can engage in all kinds of reactions, from substitutions or addition-elimination reactions, usually to form a nitrile bond.

A nitrile has this structure:

It's very straightforward - an alkyl group bonded to a nitrile group. I've included the lone pair on the nitrogen because ocxcasionally the nitrogen will act as a nucleophile, perhaps to accept a proton, like in this reaction:

Here, we're converting a nitrile into an amide. What I love about this reaction is that all we need is water and an acid/base catalyst, and the reaction can proceed. Everything else is just a series of proton transfers within the molecule, and the formation of certain intramolecular bonds. The example I've given is for an acid-catalysed reaction, by the way - you can tell by the hydronium ion - if it was base catalysed, I'm pretty sure we'd have a hydroxide ion attacking in the third step instead, and the proton transfer from the oxygen to the nitrogen would come later.

There are a few other notable reactions, such as this one to go from a nitrile to a ketone. To do this, we need a very strong base, like butyllithium (BuLi), but to keep things simple, I'll use a regular alkyl group R in place of a butyl group specifically:

I've broken up the mechanism into two to highlight that we form an imine intermediate, which is where we have a C-C=N-H functional group. Obviously to get that hydrogen, we'd need to source it from a hydronium ion, no surprises there. 

The rest of the mechanism is as dull as dishwater; all we're doing is successively protonating the nitrogen before expelling it to form ammonia...where it then steals another proton.

You might see some similarities between this reaction and the Grignard reagent reaction I mentioned last time, when we converted an ester into an alcohol via a ketone. And in theory, we can do this as well for this reaction to form an imine. However, I was taught to use an organolithium reagent, so I'll keep it simple for now.

We can do a very similar reaction to reduce nitriles into amines, this time using LiAlH₄ in place of the organolithium:

Now, I'm not sure if this mechanism is correct, but between my lecture slides and two other websites, none of them can agree entirely on what the mechanism is. I'm assuming my assumption on how the mechanism works is somewhat right, and I've only included it here as a reference point if I don't get the right mechanism before then. But the important parts - how we form the N-Al bonds, how we reduce the triple bond into a single-bonded alkene, and how we eventually protonate the nitrogen to form the amine - are conveyed here.

One final mechanism worth sharing is how we'd form an aldehyde. To do this, we use DIBAL, along with an aqueous workup, as shown below:

Now I'll be honest, I don't know if I need to know the whole mechanism for this, or whether saying "H₃O⁺ workup" is sufficient. Either way, you might have noticed we've formed an imine, so the rest of this reaction will be the exact same as for the ketone synthesis.

I think I can stop here for now.