benzaldehyde bisulfite problem

[magstirrer]

In short: the bisulfite adduct won't form.

Details: benzaldehyde was prepared from 20g benzyl alcohol according to the Nwaukwa-Keehn article (aqueous sodium hypochlorite/acetic acid), basified, extracted with ca 300ml xylene (low on DCM), mixed fiercely with a mech stirrer with 100ml concentrated hot aqueous bisulfite (from analytical grade Na2S2O5). Result: nothing, after 3 hours at RT plus maybe 5 in the fridge. No precipitate, just the turbid organic layer and clean aqueous one.

Ideas? I found a mention of adding alcohol to solubilize benzaldehyde, most other writeups just go mix it thoroughly and the adduct forms spontaneously - does it?

[Sedrick]

[Nicodem]

Possible problems:

#1 You did not neutralise the xylene phase with the bicarbonate. You must wash it with saturated NaHCO₃ or it will contain quite some acetic acid. Bisulphite adducts only form in a narrow pH range. Any addition of acid or base prevent their formation.

#2 You used the Na₂S₂O₅ solution to soon. The metabisulphite needs some time to hydrolize into the HSO₃^-. Let it stay for an hour before use. But then again you did left it mixing for some time.

3# There is no benzaldehyde in the xylene phase. Did the reaction at least smeled of Ph-CHO?

4# Other things that I do not imagine.

[magstirrer]

Thanks for the suggestions, Nicodem.

#3 is ruled out, the flask before extraction smelled very very much of almonds. I did this reaction in the past with different workup, just salting the benzaldehyde out of post-rxn liquid and it definitely gave results (together with bisulfite adduct forming)... maybe tried to bee too smart this time with the xylene...

#1 is an interesting one, could you say anything specific about it? Obviously, the bisulfite won't form while basic, but does too acidic hurt? I'm not sure about the actual acidity of aqueous phase... it contains damn sodium acetate buffer    What should the "optimal" pH bee?

#2 is dubious for the reasons you mentioned yourself. Do you have any hard data on the hydrolysis?

I guess under #4 goes the phase miscibility problem... Barium even suggested using PTC for these adducts,

Post 429324

(Barium: "PTC for bisulfite-adducts", Methods Discourse)

[Nicodem]

Well, then I guess is #1. Bisulphite is the only ion that forms the adduct, so the presence of acetic acid is very detrimental as it protonates the basic bisulphite and also decomposes the adduct (that is, it disturbs the fragile equilibrium just like NaOH does).
But anyway, it is considered a good laboratory practice to neuralize the organic phases and, if emulsified, also wash them with brine before further use.
My experience with bisulphite additions is quite limited, but I noticed that they would not form if I did not prepare the NaHSO3 very carefuly when I used to prepare it from H2SO3 acid. If not neutralised exactly you drop out of the narrow allowed pH range. Maybe you can find more detail in practical chemistry books (like Vogel's). I only have some theory that may or may not be interesting:

from:
A guidebook to mechanism in organic chemistry.
Peter Sykes, 6th edition
pp. 213-214

Bisulphite and other anions

Another classic anion reaction is that with bisulphite ion to yield crystalline adducts. The structure of these was long a matter of dispute before it was established that they were indeed salts of sulphonic acids (33), reflecting the greater nucleophilicity of sulphur rather than oxygen in the attacking anion. The effective nucleophile is almost certainly SO₃^2- (34) rather than HSO₃^- (HO^- + HSO₃^-  <=>  H₂O + SO₃^2-), as though the latter will be present in higher relative concentration the former is a much more effective nucleophile:

The attacking anion is already present in solution as such so no base catalysis is required, and SO₃^2- is a sufficiently powerful nucleophile not to require activation (by protonation) of the carbonyl group, so no acid catalysis is required either. This nucleophile is a large one, however, and the K values for product formation are normally considerably smaller than those for cyanohydrin formation with the same carbonyl compound (cf. p.206). Preparative bisulphite compound formation is indeed confined to aldehydes, methyl ketones and some cyclic ketones. Such carbonyl compounds can be separated from mixtures and/or purified by isolation, purification, and subsequent decomposition of their bisulphite adducts.