Sciencemadness Discussion Board

Separation of Ketoximes and ketones

Brøn stud - 20-2-2013 at 10:10

(43mmol, 5.0g) 3-heptanone and (47mmol, 3.28g) hydroxylamine.hcl were added to a 100mL round bottom flask, brought to 100C for 8 hours. After 8 hours the mixture was brought to room temperature and extracted with Et2O and washed with dH2O. 1H NMR and 13C NMR show both optical isomers of 3-heptanone oxime with a small fraction of starting material.

Separation Methods

There is an obvious separation method and that would be vacuum distillation. The boiling points are vastly different where a fractionated column isn't necessary. But what about separating the ketone out via sodium bisulfite adduct? Also how would this process look? I assume create a saturated solution (42g/100mL) Sodium bisulfite and add it to the resulting mixture, filter the crystals, and wash with dH2O. Now is there an appropriate solvent that I should use so that my oxime/ketone mixture is in solution prior to the addition of the bisulfite reagent? How long does this process take? Any ideas would be greatly appreciated.

Nicodem - 20-2-2013 at 11:10

3-Heptanone is not an methyl alkyl ketone and thus does not form a bisulfite adduct. Besides, the procedure for the formation of bisulfite adducts of aldehydes and methyl ketones calls for ethanol as the solvent, not water (bisulfite adducts are generally relatively soluble in water). See the general procedure, if you still need answers to your other questions, but like I said, 3-heptanone is unlikely to form a stable adduct. Besides, if it would, it would be more practical to use a wash with the bisufite solution, as you want to remove the starting material, rather than isolate it.

To remove traces of the starting ketone try this: Add some water to your product (I assume it is an oil) and rotavap this water out. Repeat this once or twice, until you steam distill away all of the ketone. You will sacrifice some of the oximes, though.

Next time, to avoid such problems, rather use a slightly higher excess of the hydroxylamine and some sodium acetate or bicarbonate as the base. Without the base, the reaction only reaches an equilibrium point, but never goes to full conversion. In the presence of a base it only takes about 10 min at 60-80 °C for most aliphatic ketones (unless hindered).

zed - 20-2-2013 at 17:47

Yup. The way I recall, Hydroxylamine HCl is reacted in solution with Sodium Acetate, to form NaCl plus Hydroxylamine Acetate. This form of Hydroxylamine reacts rapidly with ketones or aldehydes, often producing crystalline products, in very high yields.

ash - 23-2-2013 at 05:13

Quote: Originally posted by zed  
Yup. The way I recall, Hydroxylamine HCl is reacted in solution with Sodium Acetate, to form NaCl plus Hydroxylamine Acetate. This form of Hydroxylamine reacts rapidly with ketones or aldehydes, often producing crystalline products, in very high yields.


this is necessary if the ketone has aromatic part from any side.
in case of aliphatic ketones i take 1.5 eq NH2OH*HCl plus 1.4 eq NaHCO3 in methanol. stir overnight at rt and voila 100% yield)

ash - 23-2-2013 at 05:16

Quote: Originally posted by Brøn stud  

Separation Method

is easy. dissolve your mixture in methanol, add another portion of NH2OH*HCl and NaHCO3 as i described and stir overnight. then evap MeOH and extract in DCM/H2O. Drying.. evaporation of solvent.. and be happy)