joe_aldehyde
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carbene reaction conditions?
under which conditions do carbene reactions occur?
i did an a/b extraction of a certain compound and used a saturated NaOH solution for this, non-polar compound was chloroform. could it be that
chloroform was converted to dichlorocarbene, which reacted with my product in solution? all was done at RT, no heat applied anywhere, just the
standard a/b procedure.
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sparkgap
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Yes, reacting chloroform with a strong, concentrated base will produce dichlorocarbene.  However, the base should also be able to dissolve in chloroform. Methinks if you did form dichlorocarbene, not a lot of it was formed.
(edit: typos)
sparky (~_~)
[Edited on 22-5-2005 by sparkgap]
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joe_aldehyde
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so you think it might have reacted with my precious substrate? i can't really tell if that happened at all, but somehow it looked like the
precipitate (on shaking in the sep.funnel) changed its look.
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sparkgap
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Only a tiny bit may have reacted I suppose; chloroform isn't appreciably soluble in a concentrated NaOH solution. You shouldn't lose a lot
of your substrate to carbene addition.
What was your "substrate" anyway?
sparky (~_~)
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joe_aldehyde
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it was an extract from San Pedro "Trichocereus pachanoi", huge specimen (~10kg). at first, i was wondering about the huge amount of
alkaloids that separated out from my methanolic extract. then i thought it might have been phenolic compounds that i initially isolated, but on
redissolution in VERY concentrated NaOH, the precipitate didn't dissolve at pH 12 so i guess it can't quite be phenolic compounds forming
water-soluble phenolate salts. or do i need to go even further with the pH? i added solid NaOH until no more would dissolve.
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sparkgap
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Ah, I see, psychoactive cacti. 
Having a look at the structures of your substrates  , I can confidently say that
whatever dichlorocarbene may have formed, your substrates will be left untouched.
I don't know the answer to your other question, since I cannot recall their pKa's.
sparky (^_^)
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joe_aldehyde
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i thought that phenolates were in the general reagion of pKa=10. plus, carbenes are electronphiles that would form isocyanides with my amine
substrate! damnit!
[Edited on 22-5-2005 by joe_aldehyde]
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JohnWW
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In the absence of other reagents, what is the rate of dimerization of dichlorocarbene to tetrachloroethylene?
[Edited on 22-5-2005 by JohnWW]
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sparkgap
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Damn, I just plumb forgot about the isocyanide adducts, sorry.  But you should have
been greeted with an intolerable stench upon extraction since isocyanides as a family tend to reek.
Don't you have other solvents?
sparky (~_~)
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joe_aldehyde
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well after i realized that NaOH/CHCl3 might be incompatible i instantly switched to Cyclohexane Merck p.a. which was also handy
you're right about the smell - my fume hood doesn't pull the air too well so i oughta have noticed. so it seems i got away untouched, yee
hee (i actually felt a strange tingle in my bowels an hour later...slight nausea...i might have breathed some cyclohexane + mescalin base). some of
the precipitate left in the beaker crystallized in LONG BEAUTIFUL needles. i wonder what THAT is
and i tell you - i had to shake the sep funnel REAL hard to get all of the base compound into the cyclohex. phase!
[Edited on 23-5-2005 by joe_aldehyde]
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Sergei_Eisenstein
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NaOH, chloroform and carbenes:
http://clem.mscd.edu/~wiederm/oc2chp/oc2chpphenols/page4.htm
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joe_aldehyde
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thanks sergej so applying heat is a must. yeah!
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azaleaemerson
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What sort of carbene do you want? I know you can make carbenes by decomposition of sublimed molecules under red heat. But I remember hearing that
they mostly rearrange, rather than react with other molecules.
azalea
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joe_aldehyde
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i certainly don't want any carbenes. they ought to leave me alone during my a/b extractions!
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Magpie
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A test tube reaction that is supposed to be possible using aqueous methylamine is the formation of an isocyanide (R-NC). The procedure is to heat 1
mL methylamine soln with 3 mL 10% NaOH and 2 drops of chloroform. Presence of the isocyanide is to be detected by its smell.
Wikipedia says the reaction is:
R-NH2 + CCl2 + 2NaOH --> RNC + 2NaCl + 2H2O
CCl2 is a carbene formed from the chloroform and NaOH.
I tried this but it did not generate a foul stench as expected so presume no reaction. I was wondering if anyone else has tried this and had any
success (or not).
The single most important condition for a successful synthesis is good mixing - Nicodem
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stimo-roll
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addition PTC (TEBAC for example) may increase yeld.
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guy
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| Quote: | Originally posted by Magpie
A test tube reaction that is supposed to be possible using aqueous methylamine is the formation of an isocyanide (R-NC). The procedure is to heat 1
mL methylamine soln with 3 mL 10% NaOH and 2 drops of chloroform. Presence of the isocyanide is to be detected by its smell.
Wikipedia says the reaction is:
R-NH2 + CCl2 + 2NaOH --> RNC + 2NaCl + 2H2O
CCl2 is a carbene formed from the chloroform and NaOH.
I tried this but it did not generate a foul stench as expected so presume no reaction. I was wondering if anyone else has tried this and had any
success (or not). |
You need to do that in EtOH, or use a phase transfer catalyst.
Orgsyn procedure
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Magpie
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Yes, I have done a carbene experiment with PTC in organic class where CCl2 (from chloroform) is reacted with cyclohexene. The particular case I
pointed out in the above question is a qualitative test tube test from my old (1962) lab manual. It does not call for a PTC per se. I was wondering
if the methylamine itself was perhaps serving that function.
The single most important condition for a successful synthesis is good mixing - Nicodem
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chemrox
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alkaloids
Weren't you trying to isolate the alkaloids? I started writing a critique and realized I'd gotten totally lost. Would you mind quickly outlining the
process you used, including which non-polar solvent(s)? The carbene discussion sounds like you got sidetracked by the chemistry . ..which may be well
worthwhile ..
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chemrox
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BTW:
There's a danger of forming inert carbonates ..I think this is especially a risk using methanol for the initial workup ...
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