Let's say you had an enatiomerically pure (and relatively simple) compound like 2-amino pentane. Would it be possible to convert this to a racemate
via thermal action alone? Say, heating to ~200°C in an inert solvent for 24 hours? Or would the necessary heat be so much that substantial
decomposition would occur first?phlogiston - 27-5-2016 at 11:48
For that particular compound I have no idea, but amino acids definitely do racemize when heated. The rate at which it happens is different for every
amino acid though, and is affected by pH among other things, so that is very likely to be the case for the compounds you are interested in.
For amino acids, it would take a lot of heating to obtain a racemic mixture from an enantiomerically pure amino acid if it is even possible. In a
'reasonable' timeframe, you'll only get partial racemization by heating alone.Dr.Bob - 27-5-2016 at 15:20
2-amino pentane would not easily racemize via just heating. Amino acids are more easily racemized than simple amines since they have electron
withdrawing groups which make the alpha proton more acidic, but even then, they are not easily racemized by heat, and the main time that they racemize
is when they are reacted to form esters, which is why they can racemize during coupling steps. Polysialate - 27-5-2016 at 21:34
Would it be more feasible if the amine was protonated or oxidized to a chloramine (then reduced again post-racemazation)?AvBaeyer - 28-5-2016 at 04:19
You might consider a trace of an aldehyde such as benzaldehyde as a racemization catalyst. This would form an imine (reversible under the right
conditions) which would be susceptible to racemization in the presence of excess amine. This is a "trick" used for resolution of racemic compounds but
there is no reason it should not work in reverse.
Would it be more feasible if the amine was protonated or oxidized to a chloramine (then reduced again post-racemazation)?
No upon heating (beware haloamines are explosive(*)):
CH3-CH2-NCl2 --> CH3-C#N + 2 HCl
CH3-CH(-NCl2)-CH3 --> CH3-C(=NCl)-CH3 + HCl
Reaction is favourised if a base is present
(*)
NNN'N'-tetrachloro-ethylene diamine and methyldichloroamine are potent explosive...reputed to be as powerful as nitroglycerine zed - 4-6-2016 at 14:53
Refluxing with a small amount of ketone, should do the trick.
Perhaps even not-refuxing, in the presence of a small amount of ketone.
Provided that trans-amination doesn't occur during the hydrolysis process.
I haven't seen transamination used as a preparative lab method, for amines and amino-acids, but I'm going to go look, right now.
OK, I looked. Didn't see an example in regular chemistry. Gotta be possible.
[Edited on 4-6-2016 by zed]
[Edited on 4-6-2016 by zed]Polysialate - 5-6-2016 at 18:24
No upon heating (beware haloamines are explosive(*)):
CH3-CH2-NCl2 --> CH3-C#N + 2 HCl
CH3-CH(-NCl2)-CH3 --> CH3-C(=NCl)-CH3 + HCl
Reaction is favourised if a base is present
(*)
NNN'N'-tetrachloro-ethylene diamine and methyldichloroamine are potent explosive...reputed to be as powerful as nitroglycerine
Wow. I read about nitrogen trichloride some time ago (back when megalomania's archive was still up) but I didn't think that same effect would occur to
less chlorinated amines. Yikes.
Refluxing with a small amount of ketone, should do the trick.
Perhaps even not-refuxing, in the presence of a small amount of ketone.
Would this still work if the amine was a secondary amine? Say 2-methylamino pentane? Or would all the iminium/enamine business muck things up?
[Edited on 6-6-2016 by Polysialate]zed - 8-7-2016 at 14:40
Well, the premise is that once an imine/enamine is formed, the bond is capable of shifting.
If the bond CAN shift to the center of asymetry, for no matter how fleeting a moment, racimization may occur. Because, when the double bond again
migrates, the molecule can then "choose" between configurations.
