Thankfully in this case, what I was doing didn't need precise temperature control.
I just had a solution of a compound in isopropanol, saturated at room temperature, and had treated it with aqueous sodium metabisulfite, because the compound I wanted to make, required an aldehyde as a precursor, and in the final steps of what the compound prepared is to be used for, one of the techniques to be tested for reduction of one of the functional groups; aldehydes are known to interfere with the success, or at best lower the yield if only small quantities are present.
Bisulfite/metabisulfite forms a sulfonic acid adduct with aldehydes and ketones both, and these adducts, they have a tendency towards either abysmal or nonexistent solubility in water, or organic solvents. They just don't like to dissolve in anything, period. As a general principle for the lot of them, they are all like that, so forming such an adduct and then filtration is one way either to purify aldehydes and ketones if one cannot for some reason use vacuum distillation, as they can be regenerated, albeit with a certain percentage of loss in the process.
This aldehyde, it isn't rare, expensive, precious or extremely difficult to get hold of or make, I've got plenty of it and can either buy more or make some if I needed to without too much trouble, so I'm not really fussed about loss, if I decide to try regenerating it at all. In this case, I was forming the bisulfite adduct to trap and be able to filter off any residual aldehyde present, that could then interfere with one of the two methods of reduction in the final step, which involves two reductions one after another; one using sodium borohydride to reduce a double bond, removal of the spent borohydride, as one of the two second-step reductions involves acidic conditions and the presence of a lewis acid also, so I can't afford to have any salts present that might form a nitronate which would then undergo hydrolysis to a ketone, in a Nef reaction.
As I could use such conditions, with glacial acetic acid or concentrated hydrochloric, plus ferric chloride in a catalytic quantity as the lewis acid to deliberately form the ketone in question, I could still work with that and bring about my final goal, but it'd mean more steps, more purification, filtration of a whole load of irom oxide wastes, rust, basically, and other viscous, insoluble shite, by omitting the first borohydride reduction of the double bond in the compound I'm working on, to go straight to the ketone, and proceed from there. But it's more work, more use of reagents, more time and buggering around.
The other technique I'll be testing for the second of the two reductions is catalytic, based on a copper based catalyst used alongside borohydride. No Nef reaction there,
So as the adduct is insoluble as hell I could afford to just wing it, and bastardly fucking digital hotplate and chingrish partial instructions that I am either too damn dyscalculic to get, or else, and in this instance I think it actually the case, that they are talking bollocks; so I just heated up the isopropanol, to improve solubility of my compound, and vacuum filtration to separate off any aldehyde bisulfite adduct crap, although it seems there was only a little.
Brought to the boiling point of isopropanol, dissolved near enough all of my product, leaving the adduct sitting at the bottom of the flask as a solidk filtered off, and now allowing the flask the Buchner filtration funnel was attached to to cool, so that my product crystallizes as it returns to room temperature, now, I hope, of greater purity. It'll need water washing with saturated NaCl brine several times, to remove the remains of the sodium metabisulfite,and then recrystallization a couple of times, although I'll take a melting point test in a microcapillary tube at each recrystallization, until my compound is at the right melting point, so far, so good, tiny crystals are forming in the flask the hot isopropanol solution of the compound was being sucked into by the vacuum filtration, as it cools, being supersaturated, bits are slowly precipitating out.
I'll strip off say 3/4 of the isopropanol by vacuum distillation, recover it for recycling and reuse after cleaning it thoroughly, earmark it for that specific reaction, as it's recycled stuff, and that ought to cause much of the material to crash out and crystallize, and still more, after a tiny seed crystal is added to encourage it to crystallize, then wash it well with brine, then ice cold water (this material is insoluble in water, or if soluble at all, then it isn't to any meaningful extent.
Even now, still warm, it's precipitating little fluffy needle-like crystals of the sought reaction product. My job isn't done, far from it,, but at least I'm getting there and it's going as it ought to.
