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M.Zack
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[*] posted on 24-1-2014 at 09:51
Oximes


Hi everyone! i've been doing some research about the formation of oximes from aldehydes and was wondering what is the easiest way that also gives the best yields.

I am using hydroxylamine and sodium carbonate. ( would NaOH be better ) also is stirring necessary?

For the moment I am mixing my aldehyde in Etoh, adding hydroxylamine in H2O, adding sodium carbonate, stir about 4h then let everything cool over night in a ice bath.

What would be your suggestions?
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DJF90
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[*] posted on 24-1-2014 at 13:23


Standard procedure for oximes that I have seen is hydroxylamine hydrochloride and sodium acetate in an appropriate solvent (aqueous ethanol?). If you have read about oxime formation you'll know its fastest at pH 4-6, which is why the acetate buffer is used. From Inorganic Syntheses vol 3:

Quote:

Hydroxylammonium phosphate serves as a convenient reagent for the preparation of oximes; the saturated aqueous soIution has a pH of about 5, the pH of maximum rate of oxime formation.


It also gives the preparation of this salt, though I'm not sure how it qualifies as "convenient" given its aqueous solubility at 20 *C is 1.9 g/100mL, and 4.2 g/100mL at 40 *C.

You might want to re-name this thread to something a little less vague than "oximes".
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Rich_Insane
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[*] posted on 24-1-2014 at 15:36


Here is a procedure from OrgSyn: Veratronitrile

They use NaOH as a base. I have used sodium carbonate and sodium hydrogen carbonate in the past. Sodium carbonate seemed to work the best for me. I use a slight excess of it, and I got the best results when the reaction solvent was ethanol or methanol (iPrOH dissolved my product too much and I ended up with what looked like egg yolk).

If you use ethanol, I'd suggest adding water to it. Pure methanol worked well for me, but my aldehyde was not completely soluble in it.


Quote:

If you have read about oxime formation you'll know its fastest at pH 4-6, which is why the acetate buffer is used.


I was not aware of this. Does the text provide a reference? I've always assumed that the base selection is arbitrary so long as it deprotonates the NH2OH*HCl. I have used sodium acetate before with poor results -- but I was attempting a one-pot reaction to the aldehyde, so high temperatures were used alongside a copper sulfate catalyst (to this date, I have no idea how that article got published in Tetrahedron Letters. It seems to be complete bullshit).

My procedure went like this, but in retrospect I would've added the hydroxylamine dissolved in water:

Dissolve the aldehyde in methanol (I needed to use a lot of methanol). Add sodium carbonate and stir, add water if needed to dissolve the sodium carbonate. Then add the crystalline hydroxylamine slowly over a period of 10 minutes with vigorous stirring. Allow the reaction to run for 2-3 hours (I did not have TLC at the time, so this was just a guess) and then dilute with a volume of cold water equal to that of the methanol initially used. Let it sit overnight. Typically, a hard, waxy substance formed at the bottom of the vessel. I recrystallized from ethanol, but this might not be necessary.
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UnintentionalChaos
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[*] posted on 24-1-2014 at 16:05


Quote: Originally posted by Rich_Insane  
I have no idea how that article got published in Tetrahedron Letters. It seems to be complete bullshit.


You answered your own question. There are plenty of journals out there that need to be given as much scrutiny as patents (and this forum tends to give patents far more credit than they deserve, most likely).




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[*] posted on 25-1-2014 at 07:35


The kinetics and mechanism of oxime formation has been studied in academia for over a century ;) An article considered to be a good summary of the subject published by Jencks in 1958 is in English and is attached :) The end results from the effort applied :cool:

Attachment: ja01511a053.pdf (245kB)
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Chemistry is our Covalent Bond
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Nicodem
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25-1-2014 at 09:33
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[*] posted on 24-7-2017 at 11:02


I agree with Rich_Insane , sometimes Sodium carbonate is a better choice.

http://www.orgsyn.org/demo.aspx?prep=CV2P0313

Org. Synth. 1931, 11, 54
DOI: 10.15227/orgsyn.011.0054
HEPTALDOXIME
[Enanthaldehyde oxime]

1. Procedure
In a 5-l. two-necked flask, fitted with a mechanical stirrer, a reflux condenser, a thermometer, and a separatory funnel, are placed an aqueous solution of 348 g. (5 moles) of hydroxylamine hydrochloride (Org. Syn. Coll. Vol. I, 1941, 318) in 600 cc. of cold water and 460 g. (4 moles) of heptaldehyde (Note 1). Stirring (Note 2) is started, and a solution of 265 g. (2.5 moles) of anhydrous c.p. sodium carbonate in 500 cc. of water is added at such a rate that the temperature of the reaction mixture does not rise above 45°. Stirring is continued at room temperature for an hour after the addition of the sodium carbonate solution is complete.
The oily layer on top of the reaction mixture is separated and washed with two 100-cc. portions of water (Note 3). The washed product is transferred to a 1.5-l. modified Claisen flask and distilled under reduced pressure from an oil bath. The first fraction contains a very small amount of water along with a mixture of heptanonitrile and heptaldoxime. The product is collected at 103–107°/6 mm. (temperature of the oil bath, 140–147°) (Note 4). The yield is 420–480 g. (81–93 per cent of the theoretical amount). The product solidifies slowly on cooling and melts at 44–46°. It can be used for reduction to n-heptylamine (p. 318) without further purification.
The product can be purified by recrystallization from 60 per cent ethyl alcohol, using approximately 70 cc. of the solvent to 25 g. of the distilled product. One such recrystallization (Note 5) gives white leaflets melting at 53–55° (Note 6) and (Note 7). The yield of recrystallized material from a single run is 315–320 g.

2. Notes
1. The heptaldehyde used boiled at 54–59°/16 mm.
2. Since the heptaldehyde and the aqueous solution of hydroxylamine hydrochloride form a heterogeneous mixture, it is necessary to provide rapid, efficient stirring in order to obtain good results.
Ethyl alcohol can be used to provide a homogeneous solution, but the yield seems to be diminished slightly owing to the presence of more high-boiling material.
3. The product is so insoluble in water that an ether extraction is hardly necessary to obtain all the product from the water solution if sufficient time is allowed for the separation of the two layers.
4. The temperature of the oil bath during distillation is important. The first fraction is cut as soon as a constant boiling point is reached, and this constancy of boiling point is obtained sooner if the temperature of the oil bath is regulated to a constant temperature before distillation is started. No more than 50 cc. (of which approximately 10 cc. is water) should come over in the first fraction. If the temperature of the bath is regulated carefully, practically all the product will distil at a constant temperature.
5. The product is dissolved by gentle heating and the solution is then cooled to 0° or below for several hours. The material remaining in the mother liquor (about 30 per cent of the total) may be recovered as impure, oily oxime by evaporation of the alcohol.
6. The melting point given was determined by the capillary-tube method and depended on the rate of heating. The melting point as given in the literature varies from 50° to 58°.
7. Cyclohexanone oxime can be prepared in the same percentage yields by a procedure which differs from the above only in that the reaction mixture becomes solid before the addition of the sodium carbonate is complete. After all the sodium carbonate has been added, steam is passed in until the oxime is melted, and the mixture is shaken vigorously for fifteen minutes at five-minutes intervals. Cyclohexanone oxime boils at 100–105°/10–12 mm. and melts at 87–88°.
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[*] posted on 28-7-2017 at 00:48


An interesting oxime synthesis involves the reaction of a ketone (possibly aldehydes as well) and virtually any alkyl nitrite in an alcoholic HCl solution. The oxime group actually forms alpha to the ketone carbonyl. The old name for these compounds was "isonitroso", because the group had the same atoms as a nitroso group, but different properties.



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