stoichiometric_steve
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ten practical suggestions on how to cope with bisulfite adducts.
a couple of fuckups made me think if any of you had previously had come to the same conclusion, that making and regenerating bisulfite adducts of
aldehydes are real bitches to deal with.
first: use saturated aqueous solutions of sodium bisulfite or metabisulfite, crash out a bit of the bisulfite by adding an amount of ethanol until
turbidity lasts. then add the smallest amount of water to let turbitidy disappear.
second: thorough rinsing of the adduct filter cake with alcohol or ether or the likes is absolutely necessary to remove aldehyde which might stick to
the crystals. if you don't, you will later encounter endless grief when realizing that the aldehyde-coated crystals won't dissolve in water. thats the
point where you would usually go out with a big gun and shoot somebody.
third: thourough drying of the adduct is vital.
fourth: dissolve in minimum amount of water when liberating, apply stirring and heat if necessary. don't proceed until you have a clear solution!
fifth: always use stoichiometric amounts of HCl to liberate. NaOH or KOH will stick their nasty finger up the aldehydes' ass and do the cannizzaro
dance faster than you could believe.
sixth: do it where you have real good ventilation since quite a lot of SO2 will be released. this stuff is nasty, don't breathe it, or you will
surely cough all day long.
seventh: never use metal to stir bisulfite solutions, use glass or plastic. SO2 in combination with HCl will eat metal in a flash.
eighth: extract aldehyde immediately with the lowest boiling solvent possible, most aldehydes are pretty heat-sensitive. i used toluene and saw my
pretty yellow aldehyde turn brown.
9th!: vac distill the solvent with aldehyde or use an exhaust tube leading the SO2 which dissolved in the solvent.
tenth: if your substrate allows it, vac distill it. don't use bisulfite adducts unless positively necessary. they will give you grey hair.
[Edited on 16-12-2005 by stoichiometric_steve]
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Organikum
resurrected
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Vogel recommends sodium carbonate for the decomposition of the bisulfite adduct.
/ORG
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leu
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Steam distillation of the basified adduct is quite useful if the amount of product is sufficient to minimize mechanical losses 
Chemistry is our Covalent Bond
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stoichiometric_steve
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| Quote: | Originally posted by leu
Steam distillation of the basified adduct is quite useful if the amount of product is sufficient to minimize mechanical losses |
true. but steam distillation takes forever...
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leu
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| Quote: | | true. but steam distillation takes forever... |
That depends on exactly what type of equipment is used to conduct the steam distillation  Many more calories are transferred by using a gas burner to heat a flat bottomed container of the basified solution, which shortens the
time involved greatly  Alternatively, one can use aspirator vacuum and an
external source of steam such as a wallpaper steamer to speed things up, in that case it's best to use ice cold water for cooling purposes in the
condenser and the receiver
[Edited on 4-7-2006 by leu]
[Edited on 4-7-2006 by leu]
Chemistry is our Covalent Bond
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