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DJF90
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Lead is very easy to get hold of. As you're a UK member as I seem to recall, theres a seller on ebay offering 1kg of lead milling media, for approx
£3, with approx £4 for postage.
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chemoleo
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Thread Topped
4-11-2009 at 15:19 |
chemoleo
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What are the best reaction conditions to achieve full condensation of a primary amine and benzaldehyde?
This of course forms a Schiff's base - but somehow the yields suck (20%, if that) if this done in aq medium, at pH 8 and RT.
[Edited on 4-11-2009 by chemoleo]
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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entropy51
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Are you trying to isolate the Schiff base or do you want to reduce to the secondary amine?
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chemoleo
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For now complete conversion to the imine is what is desired. Reduction afterwards maybe useful (Na cyanoborohydride) - but obviously is of little
point if the initial yield of imine formation is so bad! Does anyone have protocols for the quantitative conversion of aldehyde to imine?
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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entropy51
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If you want to reduce, there's no need to isolate the imine. Shaking the amine and benzaldehyde in ethanol with catalyst and hydrogen will give the
secondary amine.
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chemoleo
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| Quote: | | Shaking the amine and benzaldehyde in ethanol with catalyst and hydrogen will give the secondary amine |
Catalyst?
If it serves to push the reaction to the right (reducing the initial imine to amine) then it may be useful.
Otherwise - again - what are the optimal conditions to condense benzaldehyde with a primary imine?
Many thanks.
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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Sedit
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If your imine is of simple nature such as that formed from ammonia then hydrolysis back to the aldahyde in this case will be a problem. The reaction
itself forms H2O and the equlibrium lies far to the left leaving you with your starting aldahyde however more stable forms of imine can be done with
substituted amines such as methyl or ethylamine where they are more H2O tolerant and the shiffbase is stable.
My suggestion on increasing yeilds is to form the imine in dry alcohol in the presence of a mol sieve to absorb the generated H2O and push the
equalibrium to the right although as I understand it without H2O at all the reaction will not proceed. Look on Rhodiums archives for 10,000 files or
so on reductive amination reactions and the mechanics of such because if theres anything that them archives are good for this is it. The entire data
base pretty much centers around amination of ketones and what not to yeild the amines.
Im quite sure there are others here that may either prove me wrong or be able to explain the whole concept much more gracefully then myself but
personaly I say add an alcoholic NH3 or MeNH2 solution to your aldahyde to form the imine and if the results still appear to low start considering mol
sieves and drying agents to shift the equilibrium to the right more.
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Nicodem
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If you use sodium cyanoborohydride or sodium triacetoxyborohydride (STAB), then you do not need to isolate any imine. NaBH3CN does not reduce
benzaldehydes while STAB reduces them only very slowly compared to its speed at reducing iminium ions. NaBH3CN reductive aminations of aldehydes and
ketones are actually performed in acidic H2O/MeOH media. You can not use the usual hydrogenation with Pd-C catalysis because much of the so formed
benzylamine gets debenzylated, though it is actually possible to selectively stop the reduction at benzylamine stage if you slightly poison the
catalyst with traces of thiophene. But with primary amines the formation of N,N-dibenzyl products can also be a problem even with a poisoned catalyst
(it depends on the amine you will be using and its excess). With NaBH3CN or STAB this should not be a problem even with a stoichiometric ratio of
reactants.
Imines of benzaldehydes are otherwise easy to form. You can either let the mixture stir over the night in the presence of MgSO4 in nonpolar solvents
such as toluene, 1,2-dichloroethane, ethers, etc., then filter and rotavap. This generally gives a fairly pure imine. The other method is to use a
catalytic amount of tosylic acid (or an amine salt) and remove water from the mixture of amine and aldehyde with water immiscible solvents such as
hexane, benzene, toluene, etc by using a Dean-Stark trap (for benzaldehyde you don't even need any catalyst; see Klute's post on his preparation of
the Schiff base from PhCHO and PhCH2CH2NH2). For hard to make imines (from ketones) generally SnCl4 is used as acid catalyst and water scavenger (but
this is overkill for aldehydes which easily form imines and would probably just result in tar formation).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being unworthy of the scientist. Some of the
greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their scientific temper, not in the
scientific defense of the theory. - Weston La Barre
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chemoleo
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Thanks - so water is the problem - just what I feared  Will report back if it can
be improved.
Another question 
Can someone suggest simple ways for getting from R-Phe-Br (para) to R-Phe-CH2-Br? I could think of doing a Grignard with CH2O and subsequent
bromination, but surely there are better ways? R could be anything from Phe, NO2, CH3 etc - realising though that the nature of the para substituent R
may have effects on the reaction that someone may hopefully propose
Never Stop to Begin, and Never Begin to Stop...
Tolerance is good. But not with the intolerant! (Wilhelm Busch)
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S.C. Wack
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N-Methylbenzimine. Under a well-ventilated hood, a 250-mL Erlenmeyer flask is equipped with a magnetic stirrer and charged with an aqueous
solution (40% w/w, 100 mL) of methylamine. Freshly distilled benzaldehyde (26.5 g, 0.25 mol) is added to the stirred solution of methylamine at room
temperature. A mildly exothermic reaction occurs resulting in a milky white emulsion. The Erlenmeyer flask is stoppered and the mixture is stirred
overnight (15 hr). The milky emulsion is transferred to a separatory funnel, diethyl ether (200 mL) is added, and the organic phase is separated and
dried over potassium carbonate. The solids are removed by filtration, washed with ether (50 mL), and the combined filtrate and washings are
concentrated on a rotary evaporator. The residue is distilled through a 10-cm Vigreux column using a water aspirator to give 28.3 g (95%) of imine as
a water white liquid (bp 99-100°C/25 mm).
http://www.orgsyn.org/orgsyn/RxnTypes/section.asp?section=108
[Edited on 6-11-2009 by S.C. Wack]
"You're going to be all right, kid...Everything's under control." - Yossarian, to Snowden
"So what's the problem?" - http://visit-x.net/rammstein/ |
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Bronstein
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Molecular sieves
I read in one place that you must stir a liquid with molecular sieves in it very gently so that the sieves will not get destroyed. Is there any truth
in this? Have seen another experimental procedure where they state to stir such a liquid vigorously.
(The drying should be more effective if you can stir it fast, so they sieves are suspended in the liquid instead of just lying on the bottom?)
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marksev1
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I have a very stupid question, i used the search but didn't find any useful info so i'm asking it here....What is the advantage of having 29/32 joints
compared to say a size smaller...i don't mean in terms that the 29/32 is the most used...but are there any advantages in speed or effectivity of
distillation, i would think that only maybe very slighty faster if i'd use bigger joints...
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User
Hazard to Others
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If iam right..
Distilling for example one litre, it would take longer for the smaller setup to spit it out at the same purity (considering this equal to speed).
What a fine day for chemistry this is.
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gsd
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Molecular sieves
@ Bronstein
IMHO the Molecular sieves should be gently handled as they are brittle in nature. To ensure intimate solid liquid contact with such solids, packed
bed, trickle bed, fluid bed or spouted bed contacting techniques are used instead of vigorous stirring.
gsd
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bbartlog
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| Quote: | | are there any advantages in speed or effectivity of distillation, i would think that only maybe very slighty faster if i'd use bigger joints...
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Considering only the joint size in isolation, it would be unlikely to have much effect; the gas velocity through the joint would have to be pretty
high before backpressure and/or gas compression and expansion effects were measurable.
Of course, the size of the joint may affect the size of the column that you can put downstream, and that will definitely matter. Look at the size of
the various glassware components available in different joint sizes and you'll see what I mean. I think in choosing joint size you should look first
at the overall size of the glassware you want (considering price and all) and then settle on one common joint size that lets you buy things in that
general range.
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Sedit
International Hazard
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I have heard of Sulphur being used in place of lithium in birch reductions and it got me woundering about the structure of ammonia/sulphur complex
that forms. I know Lithium forms Li(NH3)x structures but how exactly would sulphur bond and do you think it would be possible to form this complex
simular to how Lithium bronze complexes are formed in a non polar solvent with anhydrous Ammonia feed into it.
It seems interesting because I can not find any information on this in the slightest only a few veuge references to it being used in S/NH3 systems.
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