Brøn stud
Harmless
Posts: 2
Registered: 19-2-2013
Member Is Offline
Mood: No Mood
|
|
Separation of Ketoximes and ketones
(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
Super Moderator
Posts: 4230
Registered: 28-12-2004
Member Is Offline
Mood: No Mood
|
|
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).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
|
|
|
zed
International Hazard
Posts: 2283
Registered: 6-9-2008
Location: Great State of Jefferson, City of Portland
Member Is Offline
Mood: Semi-repentant Sith Lord
|
|
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
Harmless
Posts: 8
Registered: 19-2-2013
Member Is Offline
Mood: No Mood
|
|
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
Harmless
Posts: 8
Registered: 19-2-2013
Member Is Offline
Mood: No Mood
|
|
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)
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
