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.chemship1978 - 19-11-2020 at 07:44
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]ArbuzToWoda - 19-11-2020 at 09:15
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.DraconicAcid - 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.njl - 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.njl - 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-.DraconicAcid - 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.Texium - 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.clearly_not_atara - 19-11-2020 at 12:19
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.njl - 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.njl - 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.
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.njl - 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.
