Ok... I know this is frowned upon. Starting a new thread on a topic that has already been discussed. I don't care...
There are no posts in this whole site that explains the process. The best I have found is a post by Nicodem that points out that it may not be working
because there is a narrow ph range in which the adduct will form.
I don't have a lot of materials to be wasting on experiments so I need someone to give me a walk through.
I have benzaldehyde that's highly contaminated with benzyl chloride that doesn't separate even when vacuum distilled. I vacuum distilled it once and
was very careful not to let it splash up into condenser and it was like I didn't even do anything.
I tried a small experiment with sodium bisulfite and I ended up breaking my 4ltr beaker. So... I've given up for a few weeks.
From all the posts I read your supposed to take a saturated solution of sodium bisulfite and add it to the benzaldehyde. I'm not sure it temperature
should be raised or if stirring is needed. Not sure what ph of the benzaldehyde should be. bfesser - 24-11-2013 at 10:37
Ok... I know this is frowned upon. Starting a new thread on a topic that has already been discussed. I don't care...
With an attitude like that, don't expect a warm welcome.
Quote:
There are no posts in this whole site that explains the process. The best I have found is a post by Nicodem that points out that it may
not be working because there is a narrow ph range in which the adduct will form.
You've contradicted
yourself; don't mention other posts without linking to them; and learn to write pH properly.
Quote:
I don't have a lot of materials to be wasting on experiments so I need someone to give me a walk through.
If you're after spoon-feeding, search elsewhere. Demanding it is just plain rude.
Quote:
From all the posts I read your supposed to take a saturated solution of sodium bisulfite and add it to the benzaldehyde.
Again, not providing links. Detritus
[edit] Please review The ScienceMadness Guidelines (link below) before posting again.
[Edited on 24.11.13 by bfesser]
Ameture trying to figure out the bisulfite adduct of benzaldehyde
nanobot-dnp - 24-11-2013 at 12:02
I'm going to give this my best shot to explain the way I will be attempting to get it to work and why. The purpose is to show what I'm thinking.
The benzaldehyde has impurities that effect the pH. The pH has to be neutralized before the bisulfite anion will be able to go in. Once it is
neutralized an excess of a saturated solution of sodium metabisulfite is added [sodium metabisulfite releases the bisulfite anion needed]. The
contents are stirred for a very long time since it takes a while for the bisulfite to cross from the water to the benzaldehyde. After the reaction is
complete the adduct filtered off and washed with a saturated solution of sodium bisulfite.
Assuming that this works... The whole purpose is to remove the benzal chloride. The correct thing to do is vacuum distill the product to get highly
pure benzaldehyde.
[Edited on 24-11-2013 by nanobot-dnp]bfesser - 24-11-2013 at 12:23
I appreciate that you've put <em>a little</em> more thought into this. Thank you.nanobot-dnp - 24-11-2013 at 12:41
That pretty much all I've got.
Goal is to get the benzaldehyde back without have to vacuum distill. At a purity high enough for most reaction.
I'm thinking destroy the adduct then simply wash with water multiple times to remove impurities.Paddywhacker - 24-11-2013 at 23:28
Metabisulphite has a low pH, but if the reaction mixture is too low a pH then you may have difficulties. The adduct can be broken by high pH and
really low pH, but if you want a ballpark then measure the pH of saturated aqueous sodium metabisulphite.
I have seen ethanol used as a co-solvent and to induce adduct precipitation, but in order for anybody to give specific help then please give the
protocols that you have already tried and the results that you have observed.madcedar - 25-11-2013 at 06:50
And then search the first post of the thread for Purification - stage 3 and then Step 3 - Purification. It's exactly
what you are after.
It worked!
nanobot-dnp - 4-12-2013 at 22:51
I did this out of the blue...
Tested a few drops of the aldehyde I have in 10ml of a saturated solution. The crystals appear almost instantly. There is a lot of sulfide odor coming
off of the solution too.
I then immediately went for the one liter beakers. Made 600ml saturated metabisulfite solution and poured 300ml of the aldehyde into a separate
beaker. I slowly dripped in the saturated solution and it immediately went cloudy to white to think to chunky to almost rock hard luckily I broke it
up in time before it solidified or something.
It took just over 400ml to get the aldehyde smell to go away. I also checked the ph and it seemed to be getting acidic. Around a 4 so I started
dripping in some potassium hydroxide solution and noticed a change in consistency. not sure it if was because of the 10ml portions of water I was
using or what but I noticed something and the ph went down to neutral again. Some parts of the ph strip were red but the majority was tan.
My main concern is why did it heat up so much? even after I filtered out the crystals... Even the crystals started to heat up! and the strong odor of
sulfide was enough to make me worried. I was about to run out side with the beaker but didn't. The bulk of the crystals is dramatically more than the
liquid. I would say that after squeezing out all of the moisture that the mass of aduct to aldehyde was at least 2x in mass. It was hard to work with.
I don't even know what to do with all of it.
I guess Ill have to do the rest tomorrow... Never doing this inside again until I can build a proper fume hood.
nanobot-dnp - 8-12-2013 at 21:15
I assume the sulfide odor is the bisulfite anion. It is very strong so I think it may be escaping the solution.
Anyways I did it again but ran out of sodium metabisulfite so I have a batch of adduct that is contaminated with aldehyde and likely the undesirable
impurities that it was meant to remove.
Now I remember reading that diethyl ether will break the bond of the adduct liberating the free aldehyde. Are there any other ways to break the bond
besides destructive distillation? Would I loose any aldehyde if I did just distil the crystals?Sedit - 9-12-2013 at 18:47
Reread what people wrote. Dramatic PH change will break the Adduct.Nicodem - 10-12-2013 at 07:03
I still cannot understand the purpose of this thread. What is wrong with the literature method of forming the bisulfite adduct of benzaldehyde?
Nanobot-dnp, you never mentioned it did not work for you, and neither did you explain why you opted for some weird approaches instead. And the same
goes for the adduct decomposition method. What is the point of all these round-about approaches?
I assume the sulfide odor is the bisulfite anion. It is very strong so I think it may be escaping the solution.
The sulfide smell sounds like nonsense. I really cannot think what could you have in the crude that would be able to reduce the sulfite ion to
hydrogen sulfide. The mixture should smell of sulfur dioxide like any bisulfite solution does.
Quote:
Now I remember reading that diethyl ether will break the bond of the adduct liberating the free aldehyde. Are there any other ways to break the bond
besides destructive distillation? Would I loose any aldehyde if I did just distil the crystals?
This makes even less sense. Why would diethyl ether liberate the aldehyde from the bisulfite adduct? And where did you hear about decomposing it by
dry distillation? And more importantly, why don't you give references when you claim something so extraordinary?Random - 10-12-2013 at 10:33
Sulfide odor is probably from very small sulfide impurity in bisulfite.nanobot-dnp - 14-12-2013 at 00:00
The mixture should smell of sulfur dioxide like any bisulfite solution does.
I'm not a chemist so that is just my best guess. Thanks. Sulfide sulfite those are just the best words I can use to describe the smell. anion? in my
mind its this little sparkly think like when you poor a cup of soda.
Reread what people wrote. Dramatic PH change will break the Adduct.
I surly did try to but nothing was making sense and couldnt find the right posts. I've tried acid and base. Base seems to work better.
Would you happen to know how I could find out the solubility of sodium or potassium hydroxide in "aldehyde"?
Another thing that I noticed is that the aldehyde sinks to the bottom in water but when you add alkaline substance the aldehyde floats to the top.
-----------------------------------------------------
Edit.
I poured off a drop of the aldehyde and tested the pH. It's completely neutral.
[Edited on 14-12-2013 by nanobot-dnp]Paddywhacker - 14-12-2013 at 01:33
Of course the aldehyde is neutral, but the bisulfite is not. However that is irrelevent. The bisulfite solution is exactly the pH that it should be,
and you adding acid or base is just going to lower the yield of your adduct.
Find a 'recipe' and follow it.nanobot-dnp - 14-12-2013 at 17:03
I was talking about the aldehyde being neutral after adding basic solution to the adduct to release the aldehye. My concern was the aldehyde would
contain some base or impurities.
It is still my concern that the released aldehyde contains sodium bisulfite or metabisulfite.
Would washing the released aldehyde with h2o remove most of the impurities? such as sodium metabisulfite?
I don't want to have to distill the aldehyde.
I beleive that washing the aldehyde will remove most of the bisulfite and the aldehye is now relatively pure from oxidization by products.
