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Author: Subject: Bisulfite adducts, iodide adducts, solubility (ketones, aldehydes)
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[*] posted on 24-7-2017 at 13:23
Bisulfite adducts, iodide adducts, solubility (ketones, aldehydes)

Whilst I have heard much about the use of bisulfite adducts and to a far lesser degree the use of NaI to form adducts with ketones and aldehydes and indeed have used a bisulfite adduct before to rid something of benzaldehyde,

What I'd like to know, is what sorts of solvents are these adducts generally soluble in, or insoluble in? and as for regenerating the carbonyl compound from the adduct, with bisulfite adducts base is used, is it better to use strong or weak bases, and strong or dilute solutions of the base used (E.g stronger bases like NaOH, KOH, alkoxides vs carbonate, bicarbonate, CaO etc.) ? and how does speed of addition affect yield/loss of the parent carbonyl compound. I've got a problem, in that my vacuum pump has, it seems permanently kicked the bucket and I need to get a new one asap before this carbonyl compound in question oxidizes and I'm left with nothing for my time, effort and finances allocated to the project. I have a water aspirator but whether or not it'll pull a sufficient vacuum to distill before the breakdown temperature (can't go above 140-150 'C tops afaik) I don't know.

So, is it possible to purify ketones or aldehydes generally, sans vac distillation, lest it prove impossible to do so, using solely adduct techniques?

Also I've heard of the use of NaI in forming adducts that are restored to the ketone/aldehyde (actually only heard of this being used on ketones, but never specifically seen an example of its use on aldehydes. Does it work with both?) and does KI form a similar adduct, or is it specifically with NaI, I only seem to see NaI mentioned in the literature. Not sure if this is because NaI gives one more iodine per quid spent vs the heavier potassium cation taking up money, so labs buy NaI instead, or if it is because the adduct formation is specific to NaI,

Which is it? and what solvents are these adducts soluble/insoluble in and how readily?

Also is it possible to vacuum distill either bisulfite or NaI (or KI) (I only have KI) adducts?

Lastly, does NaBr form a similar adduct? and what is the mechanism of forming the NaI (or other group I metal halide adducts in terms of the molecular mechanisms taking place in adduction and reformation of the parent carbonyl compounds?)
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[*] posted on 25-7-2017 at 07:31

In my experience, bisulfite adducts tend to be soluble in water and dilute bisulfite solutions, and insoluble in strong to saturated bisulfite solutions as well as organic solvents. Aldehydes are of course more reactive and thus form the adducts more readily than ketones, with a few exceptions like acetone. For example, benzaldehyde and 2,5-dimethoxybenzaldehyde form the insoluble adducts almost immediately upon addition as an alcoholic solution to saturated sodium metabisulfite, but pulegone took a couple days of standing in saturated metabisulfite for any solids to form from the oil.

I haven't experimented extensively with different conditions regenerating the carbon compound, but addition of excess 5% NaOH to an aqueous solution of the adduct worked satisfactorily. If the process is treated like a regular acid-base solvent extraction, I think the product is reasonably pure to obviate vacuum distillation. This was the case at least with dimethoxybenzaldehyde; the melting point was right on and there was only one spot by TLC (1:1 ethyl acetate:ligroin). A THF solution of the crude aldehyde was added to saturated sodium metabisulfite. Some adduct precipitated immediately, but the mixture was allowed to stand overnight to ensure completion of the reaction. The adduct was filtered and washed with THF, air dried, and dissolved in water. To the aqueous solution was added 5% NaOH until solids ceased to appear. These were filtered, washed with cold water and allowed to dry. Yield wasn't great but purity was satisfactory.

Edit: poor yield may have been due to the reaction/reaction conditions and not necessarily due to purification method.

[Edited on 25-7-2017 by Crowfjord]
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