Where's the aldehyde?
I understand this is a topic that has been discussed at great length, but after trawling the FSE results, as well as Google, for several days, I'm
still at a loss for a solution to my specific issue. My problem is not forming or breaking the adduct, but reclaiming the aldehyde after dissolution
in aqueous base.
TL;DR - prepared the benzaldehyde/sulfite adduct, broke (likely) the adduct with base or acid, aldehyde fraction did not separate. Could not reclaim
with toluene.
Several times, I've made roughly 100ml of crude benzaldehyde via the nitric acid & benzyl alcohol oxidation. After isolating the
benzaldehyde-laden portion, I've washed it twice with saturated sodium carbonate and sodium chloride solutions just as a crude means of cleaning it up
a bit.
I then prepared a saturated solution of 400ml dh2o & sodium metabisulfite. After combining the crude benzaldeyhyde and sulfite solution, I stirred
vigourously for about 15 minutes. The cream coloured adduct sludge was filtered and dried, and the remaining liquid portion had little-to-no aldehyde
aroma, telling me the adduct formation was successful.
My plan was then to break the adduct and use the recovered benzaldehyde as-is to try some small further operations, firstly converting into
cinnamaldehyde. (For practice stuff, it's just more fun working with stuff that makes the house smell good - :-) )
I then prepared a basic dh20/sodium carbonate (pH roughly 11-12) solution of 500 ml and dissolved the adduct with stirring. Another time, I used HCL
and brought it to about 2-3 pH. Both times the adduct dissolved nearly completely, and a strong smell of benzaldehyde precipitated.
Herein lies my problem, and I'm sure it's something embarassingly simple. I was assuming that since A: benzaldehyde is poorly soluble in water, and B:
the pH was likely sufficient to break the adduct, that a separate layer would form eventually which I could pull off with a separatory funnel. This
has not happened. With acid or base, a small sludge of bisulfite would float to the top, a few small crystalline structures fell to the bottom, and a
strong aroma of benzaldehyde was apparent. The entire solution remained uniform however.
I do not have the means to vacuum distill (I'm not looking for lab-grade purity, just reasonably so.) and am mainly just wondering how best to reclaim
the aldehyde. I tried mixing with 100ml toluene and evaporating the toluene off after separating, but nothing remained.
My questions are:
1: Did I actually break the adduct, and if so...
2: Why did the aldehyde not separate from the water solution in situ, and...
3: Why did the toluene evaporation not leave an aldehyde remnant?
Thank you in advance for any advice.
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
