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Hydroxy(phenyl)methanesulfonic acid, or benzaldehyde adduct is the product of reacting benzaldehyde with sodium metabisulfite ($\ce{NaS2O5}$) solution. It is obviously a well-known and often used reaction, and yet, I'm not able to find much info on the adduct, or the reaction itself.

One thing I managed to dig out is its corresponding PubChem entry.

I want to find out the exact mechanism of the reaction with an equation, a bunch of videos of someone performing it will be nice too...

But most importantly - the hydroxy(phenyl)methanesulfonic acid: What is its color, texture and so on? What is its crystalline structure? Is it supposed to smell of benzaldehyde, how soluble it is in different solvents, how should it be stored, is it harmful or dangerous, does it decompose in some conditions, melting point, and so on.

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  • $\begingroup$ looking for Mandelic acid? $\endgroup$ – MaxW Jul 26 '18 at 9:36
  • $\begingroup$ No. I have updated with a pic of the reaction. The adduct at step 1 seems to be the same though. $\endgroup$ – Nigel Jul 26 '18 at 11:19
  • $\begingroup$ To clarify: the benzaldehyde adduct at step one of the mandelic acid preparation in that Wikipedia article is the Hydroxy(Phenyl)Methanesulfonic Acid that I'm interested in(or at least I think so) $\endgroup$ – Nigel Jul 26 '18 at 11:50
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Hydroxy(phenyl)methanesulfonate is a compound of a class commonly called "bisulfite addition products" or "bisulfite adducts", since bisulfites are commonly used for the reaction you have mentioned. But there's an equilibrium in aqueous environment between metabisulfites and bisulfites $$\ce{S2O5^{2-} + H2O \rightleftharpoons 2HSO3^{-} }$$ in favor of the bisulfite, this is why metabisulfites can be used as well, probably.

The bisulfite anion is proposed to have two tautomers:

HOS(O)O⁻ ⇌ HS(O)(O)O⁻

The form at the left has a lone electron pair at the sulfur atom, and can act as a nucleophile.

The carbonyl/aldehyde group $\ce{>C=O}$ has a partial positive charge at the carbon atom.

The reaction in now no mystery, it's a nucleophilic addition to the carbonyl group.

If I may cite a crude reaction mechanism from Clayden/Greeves/Warren's Organic Chemistry, short chapter Bisulfite addition compounds: reaction scheme w/ mechanism

And from the March's Advanced Organic Chemistry:

The reaction is reversible (by treatment of the addition product with either acid or base) and is useful for the purification of the starting compounds, since the addition products are soluble in water and many of the impurities are not.

If you want more details (kinetics, etc.), there is freely available couple of scientific articles, Sulphur isotope effects in the bisulphite addition reaction of aldehydes and ketones

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  • $\begingroup$ Thank you, really detailed and straight-forward. But how about the adduct's properties, is it storable for example? $\endgroup$ – Nigel Jul 26 '18 at 16:53
  • $\begingroup$ @Nigel This can be the issue. I have added short citation from March's book, which might help. $\endgroup$ – mykhal Jul 26 '18 at 18:04

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