Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Isocyanate synthesis via halide displacement
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 19-11-2020 at 07:37
Isocyanate synthesis via halide displacement


In the Kolbe nitrile synthesis, nitriles are formed from alkyl halides via reaction with an alkali cyanide. Could a similar reaction take place between alkyl halides and alkali isocyanates?

I.E. KNCO + BzBr --> BzNCO + KBr

The difference between cyanate and isocyanate salts are still somewhat confusing to me so this may not even make sense from a theoretical perspective.
View user's profile View All Posts By User
chemship1978
Harmless
*




Posts: 28
Registered: 8-6-2018
Member Is Offline


[*] posted on 19-11-2020 at 07:44


Take a look at this, it covers exactly what you were wondering about :)
https://scihub.wikicn.top/10.1021/cr60135a001
View user's profile View All Posts By User
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 19-11-2020 at 07:57


Thank you. For those wondering, the paper claims:

The reaction between organic halides or sulfates and salts of cyanic acid was
one of the first methods used for the preparation of isocyanates. Wurtz (108),
in 1849, prepared alkyl isocyanates by this method.
RzS04 + KCNO -+ RXCO
Slotta and Lorenz (93) expressed the opinion that the reaction between the
alkyl sulfate and potassium cyanate was the best method for the synthesis of
methyl or ethyl isocyanate.

edit: With this in mind, it seems like isocyanates might be an alternative to the gabriel synthesis of primary amines from alkyl halides. Thoughts?

[Edited on 11-19-2020 by njl]
View user's profile View All Posts By User
ArbuzToWoda
Hazard to Self
**




Posts: 98
Registered: 15-7-2020
Member Is Offline


[*] posted on 19-11-2020 at 09:15


Quote: Originally posted by njl  


edit: With this in mind, it seems like isocyanates might be an alternative to the gabriel synthesis of primary amines from alkyl halides. Thoughts?

[Edited on 11-19-2020 by njl]


Maybe, but why do we need to come up with alternatives? We've already got the praised sodium diformylamide that I know karlos is a huge fan of. That's pretty cheap, high yielding and reliable.
View user's profile View All Posts By User
DraconicAcid
International Hazard
*****




Posts: 3947
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline

Mood: Semi-victorious.

[*] posted on 19-11-2020 at 09:41


I'm pretty sure there's no such thing as an alkali isocyanate. A cyanate can only be "iso" by covalently bonding to something through the nitrogen instead of the oxygen, but the anion itself isn't bonded.



Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
View user's profile View All Posts By User
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 19-11-2020 at 09:43


Other than the fact that expanding the communities arsenal is a good thing by itself:

The NaDFA requires sodium methoxide
NaOMe can only feasibly be made with sodium metal (not OTC and not cheap)
NaDFA requires pure formamide (not OTC)
Homemade formamide could be used but needs pretty extensive purification (5 to 7 distillations according to karlos)
NaDFA is more water sensitive than (iso)cyanate salts

Sodium isocyanate can almost literally be had by burning piss with baking soda.
View user's profile View All Posts By User
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 19-11-2020 at 09:48


@draconicacid that's what I am confused about. I have read in some papers that the isocyanate ion with the charge on the nitrogen does not exist, that both exist in equilibrium, that the isocyanate is favored, etc. Both Wikipedia and Pubchem give somewhat conflicting answers. When I google potassium isocyanate there are links about both -NCO and NCO-.
View user's profile View All Posts By User
DraconicAcid
International Hazard
*****




Posts: 3947
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline

Mood: Semi-victorious.

[*] posted on 19-11-2020 at 09:53


If you put the charge on the N instead of the O, that's not equilibrium, that's resonance. -O-C(triple bond)N <-> O=C=N-

Two different drawings of the same ion.




Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
View user's profile View All Posts By User
Texium
Administrator
********




Posts: 3842
Registered: 11-1-2014
Location: Salt Lake City
Member Is Offline

Mood: Triturated

[*] posted on 19-11-2020 at 11:59


To elaborate on what DraconicAcid is saying, you can think of resonance structures as different aspects of a molecule's true structure. Originally it was termed resonance because it was believed that molecules that exhibit it shift rapidly or "resonate" between the different structures. They do not actually do this. The true structure is constant, but it is a gray area that is some blend of the different resonance structures.

With this in mind, the true structure of a free cyanate anion floating around in solution would have a partial negative charge on the oxygen and a partial negative charge on the nitrogen, with a net charge of -1 distributed between those two atoms. The C-O bond order would be 1+x and the C-N bond order would be 2+y where x+y = 1. In other words, the C-O bond is somewhere between a full single bond and a full double bond, while the C-N bond is somewhere between a full double bond and a full triple bond. That is, until something comes along and reacts with either the N or the O, pulling electron density towards that end of the ion and changing the electronic structure.




Come check out the Official Sciencemadness Wiki
They're not really active right now, but here's my YouTube channel and my blog.
View user's profile Visit user's homepage View All Posts By User
clearly_not_atara
International Hazard
*****




Posts: 2332
Registered: 3-11-2013
Member Is Offline

Mood: Big

[*] posted on 19-11-2020 at 12:19


Quote: Originally posted by ArbuzToWoda  

Maybe, but why do we need to come up with alternatives? We've already got the praised sodium diformylamide

Overall for primary amines diformylimide is safer, but the isocyanate method allows for the direct three-component synthesis of the carbamate esters if it is performed in alcohol instead of water. Isocyanates undergo other interesting reactions as well, but due to their toxicity it is not a good target for amateurs.




[Edited on 04-20-1969 by clearly_not_atara]
View user's profile View All Posts By User
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 19-11-2020 at 16:44


Thanks atara for the insight. While isocyanates are dangerous, I would never isolate one. I think that despite the possible dangers associated with some of these compounds their diverse reactivity makes synthesis worthwhile. Carbamates, amines, ureas, amides, etc. can all in principle be made from isocyanates.
View user's profile View All Posts By User
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 20-11-2020 at 10:53


I found another paper that claims the reaction of alkyl chlorides and isocyanate does yield organic isocyanates, but they tend to trimerize quickly. I assume bulky alkyl groups are less likely to trimerize, and apparently acid can also catalyze the cyclization.

For those interested, I will link some more information that I found interesting.

https://sci-hub.scihubtw.tw/10.1021/cr60279a002

https://www.chemie.uni-konstanz.de/typo3temp/secure_download...

View user's profile View All Posts By User
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 8-2-2021 at 09:20


Silver cyanate seems like a logical starting point given the insolubility of silver halides.

https://en.wikipedia.org/wiki/Silver_cyanate

Can be prepared from silver nitrate and urea.
View user's profile View All Posts By User
clearly_not_atara
International Hazard
*****




Posts: 2332
Registered: 3-11-2013
Member Is Offline

Mood: Big

[*] posted on 8-2-2021 at 12:24


Quote: Originally posted by njl  
Thanks atara for the insight. While isocyanates are dangerous, I would never isolate one.

I mean, we all do Hofmann rearrangements, and the isocyanate decomposes in situ, no harm done. The problem is if you're producing an isocyanate from some kind of interesting (i.e. not ethyl) alkyl halide that won't dissolve in water, now there's a question: will the isocyanate manage to react with water before it volatilizes? It should, sure... it just requires a little forethought and don't just mix benzyl bromide and sodium cyanate in DMF or whatever.

In other words, it's possible to do it safely, but it's also possible to do it not-so-safely. I think the reason that people don't jump at the chance to use isocyanates is because they have this intimidating reputation, what with the reports of allergic sensitization & so forth.




[Edited on 04-20-1969 by clearly_not_atara]
View user's profile View All Posts By User
njl
International Hazard
*****




Posts: 601
Registered: 26-11-2019
Location: under the sycamore tree
Member Is Offline

Mood: ambivalent

[*] posted on 8-2-2021 at 13:38


True. I wasn't planning on trying this anytime soon. Perhaps alkyl halide in acetone, dripped into an appropriately stirred aqueous solution of NCO-, would be safer. Vapors can be drawn through a trap just like distillations of ammonia, HNO3, HCl, BzCl, or other irritants. Except that these are more than irritating. Anyway. Alkyl halide drips into NCO-, nucleophilic lovemaking ensues forming the isocyanate, isocyanate is hydrolyzed with evolution of CO2. Left in the reaction mixture is water, amine, halide, halide counter ion, and side products. From there remove acetone and steam distill out amine.
View user's profile View All Posts By User

  Go To Top