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DJF90
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[*] posted on 3-11-2009 at 15:02


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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4-11-2009 at 15:19
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[*] posted on 4-11-2009 at 15:21


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]




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[*] posted on 4-11-2009 at 16:00


Are you trying to isolate the Schiff base or do you want to reduce to the secondary amine?
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[*] posted on 4-11-2009 at 18:14


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?



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[*] posted on 4-11-2009 at 18:41


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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[*] posted on 4-11-2009 at 18:48


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.




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[*] posted on 4-11-2009 at 21:53


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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[*] posted on 5-11-2009 at 01:24


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).




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[*] posted on 5-11-2009 at 16:46


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 :)




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[*] posted on 5-11-2009 at 16:56


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=10...

[Edited on 6-11-2009 by S.C. Wack]




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[*] posted on 11-11-2009 at 13:47
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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[*] posted on 16-11-2009 at 03:19


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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[*] posted on 16-11-2009 at 04:11


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).




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[*] posted on 16-11-2009 at 06:03


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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[*] posted on 17-11-2009 at 13:46


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...


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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[*] posted on 20-11-2009 at 08:21


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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[*] posted on 21-11-2009 at 05:52


Hi guys!

i recently got a load of old chemicals and some of them are perculiarly packaged. My 'aniline' is a liquid, so i assume it is in solution of something, any ideas what? I think it has the words 'tonc vapour' written beneath the word aniline. It is in a brown bottle and the liquid looks opaque/ does not let light through.

what im really wondering about is my supposed thionyl chloride. It is packaged in a little glass bottle inside one of those large plastic jar things, specifically for the purpose (both recepticles are BDH manufactured)... BUT!! inside the plastic container, surrounding the glass bottle is a small amount of liquid. I was wondering if there was some common way of packaging like this, like how thiosulphate is used to package bromine, or if you guys had any idea what might be going on.

The bottles are really old, with the outer label barely legible (there is a newer one stuck on top) and the inner one missing completely, i dont think its a spillage as there is no noticable odour, although it presumably could have degraded with general humidity. I havnt opened the bottle of SOCL2 yet, and i would rather not at the moment.

Any ideas would be much appreciated.
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[*] posted on 21-11-2009 at 06:02


Aniline is supposed to be liquid. It is likely dark in colour because thats something aniline does, but if you distill it it will come over colourless once more. It will however darken again upon standing for long periods I beleive.
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[*] posted on 21-11-2009 at 06:12


hah, well that was a stupid mistake! im not sure why but i had just go it into my head that aniline was solid, i think i must have been confusing it with something else. Thanks for that.. Fancy hazarding a guess as to what is going on with my thionyl chloride?
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[*] posted on 22-11-2009 at 08:45


I have a 90% DMSO 10% H2O mixture I wish to dry but as I understand partial pressure is needed in order to prevent decomposition of the DMSO and not having a vacuum means short path distillation is out of the question. However I have considered a couple of ideas that I wanted to get your take on which would be the better method.

Given the large difference in BP from the H2O and DMSO I thought about just distilling the water from the mixture and allowing it to cool in a dessicator. This sounds like it would be very easy but I am unsure how easy the DMSO decomposes to and what it decomposes to.

The other way I was thinking was to azeotropically dry it using EtOH or IpOH. Adding an effective amount of the given alcohol to the solution to aid in the removal of the water. Given the low BP of the H2O/-OH azeotrope this will keep the temperature pretty low as I dry it and should yeild pretty dry DMSO.

Anyone with first hand experiance here I would sure like to hear some suggestions.





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[*] posted on 22-11-2009 at 09:31


From Armarego - 'Purification of laboratory chemicals'

Dimethyl sulfoxide (DMSO) 167-68-51 M 78.1, m 18.0-18S0, b 75.6-75.8O/12mm,
190°/760mm, d 1.100, n 1.479. Colourless, odourless, very hygroscopic liquid, synthesised from
dimethyl sulfide. The main impurity is water, with a trace of dimethyl sulfone. The Karl-Fischer test is
applicable. It is dried with Linde types 4A or 13X molecular sieves, by prolonged contact and passage through a
column of the material, then distd under reduced pressure. Other drying agents include CaH2, CaO, BaO and
CaS04. It can also be fractionally crystd by partial freezing. More extensive purification is achieved by
standing overnight with freshly heated and cooled chromatographic grade alumina. It is then refluxed for 4h over
CaO, dried over CaH2, and then fractionally distd at low pressure. For efficiency of desiccants in drying
dimethyl sulfoxide see Burfield and Smithers [J Org Chem 43 3966 1978; Sat0 et al. J Chem SOC, Dalton
Trans 1949 1986].
Rapid purification: Stand over freshly activated alumina, BaO or CaS04 overnight. Filter and distil over
CaH2 under reduced pressure (- 12 mm Hg). Store over 4A molecular sieves.
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[*] posted on 22-11-2009 at 14:06


The issues with all of those is that they require a final distillation using a vacuum and where as I admit that would be the best way to go I just do not have the means to perform that.

However I mainly needed to know in order to create NaDMSO and Im starting to think that I may possibly be able to mix DMSO and NaOH and slowly distill the H2O off as it slowly forms and push the equilibrium to the right more simular to the alkoxide formation in I think it was Klutes thread.

This could possibly work better for me then trying to dry it first as I would accomplish two steps in one process.





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[*] posted on 22-11-2009 at 14:29


It happens I was reading of these anions recently and dimethyl sulfone anion appears to be preferred if one is to oxidize/reduce after the alkylation reaction... but I don't know if that is of interest?
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[*] posted on 22-11-2009 at 15:00


It was going to undergo pyrolysis to yeild the terminal alkene. Its basicly to extent the carbon chain.

Point being the Dimsyl ions are useful for a few different reactions and are a simple Super base but the synthesis of it is not as trivial as it first appears and one normaly needs hydrides or alkali amides to effect the deprotanation. I have reference stating low concentrations of NaDMSO being make from NaOH and DMSO but I feel this is due to it forming equilibrium with the formed H2O. Remove the water and the reaction should be pushed to the right yeilding NaDMSO.





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[*] posted on 22-11-2009 at 15:09


With a sump pump, 1/2hp-1hp, will I achieve any useful vacuum with an aspirator?
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