Sciencemadness Discussion Board

Synthesis of benzonitrile by P2O5 dehydration of benzamide - another perplexing failure

SplendidAcylation - 1-6-2020 at 17:47

Another addition to the fail collection!

A few months ago, I got quite excited when I found out that benzonitrile could be made by dehydrating benzamide, which can be made via thermal dehydration of ammonium benzoate, using P2O5.

The dehydration of the amide should be just the same as dehydrating formamide or acetamide, and the preparation of the amide itself, by heating the ammonium carboxylate, should also be as easy as the other ones, however the preparation of the ammonium benzoate is a lot more straight forward than either the formate or acetate, because the benzoate is much more stable, and less soluble in water.

So, with these things in mind, and a reckless desire to waste phosphoric anhydride, I went about it.

I made the ammonium benzoate simply by dissolving benzoic acid in an excess 25% ammonia solution.

The dissolution was extremely slow, so I ended up recrystallizing the benzoic acid first to increase its surface area; then it dissolved fairly quickly.

Once dissolved, I heated the solution in a dish on a water bath, once the water was all gone I had an almost quantitative yield of the salt.

The dry salt was then sealed in a glass tube and heated, immersed in sand, to around 200c for an hour.

Upon cooling, I found that the dry white salt had turned into a light brown, wet, waxy substance, presumably benzamide.
The substance melted readily, and was thus poured out of the tube.

I dried it on a water bath, and did a preliminary test; I placed some, along with phosphorus pentoxide, in a test tube, and heated it.

Immediately, the pleasant smell of benzonitrile was evident, alongside a few droplets on the glass, which burned with a smoky flame.

"Great!" I thought.
"This should scale up nicely"

So I aimed for around 25mL of benzonitrile, I don't remember the exact amounts used, but it was in that area.
I carefully calculated the required amount of P2O5, and set it up in an ordinary distillation apparatus on an air bath.

After hours of heating, with the smell of benzonitrile in the air, I got a yield of... 0g.
Not a single drop made it's way into the condenser, and I was left with a blackened charred mass in my boiling flask, which I eventually managed to remove.

It still haunts me to this day...
What could possibly have gone wrong?

I came up with a few ideas, none of which are really satisfactory;
Maybe I overheated it, and my benzonitrile turned into carbon, hence the charred mass in the flask

I may be wrong, but this just doesn't seem like a feasible possibility, the yield was just too low even considering overheating, I think...

Another idea was that maybe I still had ammonium benzoate, that my dehydration hadn't been successful...
This doesn't really seem likely, as even ammonium benzoate would be dehydrated by heating in the presence of P2O5...
It would require twice as much P2O5, but it would work.

I really want benzonitrile and this is the only way that seems doable, so I am tempted to try it again, but there's no point in trying another small scale test, as they have all been promising, and another large scale test seems like a waste of P2O5.

Has anyone had any success with this reaction?
Would really appreciate any suggestions as to why I got a less than 0.1% yield from a seemingly idiot proof reaction.

Thanks!

clearly_not_atara - 1-6-2020 at 20:57

As P2O5 is hydrated, it converts to metaphosphoric acid, which forms hydrogen bonds and possibly even salts with benzamide. These complexes decompose at a higher temperature in a less predictable way. Metaphosphoric acid is not so great for organic dehydrations despite lots of dehydrating power. P2O5 works well for sulfonic acids in particular, and also sulfuric acid.

Instead you could try heating benzamide over alumina at around 300 C. See halogen's excellent sleuthing here:

http://www.sciencemadness.org/talk/viewthread.php?tid=11137&...

[Edited on 2-6-2020 by clearly_not_atara]

mackolol - 2-6-2020 at 11:21

You can make benzonitriles from corresponding benzamides by heating with sulfaminic acid, which is way cheaper than P2O5 and is said to work. Here is an example:

50g o-Bromobenzamide and 35g (25g=theory) sulfaminic (sulfamic) acid is thoroughly mixed and heated in a Wurtz flask. At 250-255°C distillation begins, which is over at 285-295°C (takes approx. 1.5-2 hrs). The collected product is redistilled, yield 36g (80% of theory).

Pumukli - 4-6-2020 at 02:00

Yeah, I tried mackolol's suggested method.
I used nicotinamide as starting amide and used ammonium-sulfamate (in accordance with literature). The method worked but was not without failures. Simple on paper but need some tweaking of the heating regime/temperature ramp-up timing.

But I'd go this route if I wanted benzonitriles on the cheap that's sure.

SplendidAcylation - 4-6-2020 at 17:57

Quote: Originally posted by clearly_not_atara  
As P2O5 is hydrated, it converts to metaphosphoric acid, which forms hydrogen bonds and possibly even salts with benzamide. These complexes decompose at a higher temperature in a less predictable way. Metaphosphoric acid is not so great for organic dehydrations despite lots of dehydrating power. P2O5 works well for sulfonic acids in particular, and also sulfuric acid.

Instead you could try heating benzamide over alumina at around 300 C. See halogen's excellent sleuthing here:

http://www.sciencemadness.org/talk/viewthread.php?tid=11137&...

[Edited on 2-6-2020 by clearly_not_atara]


Interesting!
And there I was naively thinking it was about the simplest reaction conceivable :D

I'm currently attempting to make HCN (immediately dissolved in NaOH solution), by dehydration of formamide, essentially the same sort of reaction, but HCN doesn't contain a carbon-carbon bond as the other nitriles do, do I guess it may be quite different.

Chemi Pharma - 5-6-2020 at 10:39

Here's a study with all the various recent know methods to dehydrate amides into nitriles.

Attachment: amides to nitriles - various methods.pdf (106kB)
This file has been downloaded 393 times

mackolol - 5-6-2020 at 11:21

Quote: Originally posted by Pumukli  
Yeah, I tried mackolol's suggested method.
I used nicotinamide as starting amide and used ammonium-sulfamate (in accordance with literature). The method worked but was not without failures. Simple on paper but need some tweaking of the heating regime/temperature ramp-up timing.

But I'd go this route if I wanted benzonitriles on the cheap that's sure.


Could you possibly share some lab notes on this reaction? As I have looked on this, I had doubts if it will be straightforward too, but I'm glad to hear that it works.