Sciencemadness Discussion Board

Drying organic compounds

biomechem - 23-10-2016 at 11:31

I wonder if there is any determinant of choosing the method of drying a particular compound.

First of all I would like to ask about molecular sieves usage.
1. When it comes to organic liquids can I just add molecular sieves to a flask (assuming that it is not strongly acidic or basic)?

2. What about substances that have low boiling point - could heating and then adding MS work?
On the other hand some of functional groups tend to oxidize so heating may destroy the compound.

3. Does the vacuum distillation provides dry product?

I think that theorizing without an example is worthless so I finally found a compound that has low boiling point and one of its functional group can oxidise. The substance is Vanillin, how would you dessicate it?

unionised - 23-10-2016 at 11:38

Vanilin boils at nearly 300C. It would be easy to dry in a desiccator.

biomechem - 23-10-2016 at 11:55

According to dessicators is there a difference what drying substance will be used? For example the same results will be obtained with H2SO4 and NaOH?

What about liquids e.g. benzaldehyde? Molecular sieves are the option?

Finally, although I still do not know if vacuum distillation provides dry product, these compounds oxidize to carboxylic acids so the distillation before synthesis would improve yield, provide purer product etc.
Is it necessary to dry compounds, which I mentioned before, afterwards or using it ASAP after distillation is better?

biomechem - 28-10-2016 at 12:49

IDK why I cannot get answer but I have some thoughts about my problem. On one hand aldehydes and bases react together in Cannizzaro reaction but on the other hand NaOH, KOH etc. are rather not volatile. That is why, in my opinion, the base shouldn't contact with aldehyde.
Moreover I found that aldehydes are dried with MgSO4 or Na2SO4. In this case drying agents are added direct into an aldehyde and the method is successful only for liquids, am I right?
Finally is it possible to dry these substances just with vacuum pump and will it be worse option than using desiccator?

Please give me any source that explains what is the rule in choosing drying method. I haven't even mentioned all of the methods as azeotropic distillation etc.. The multiplicity of methods and no idea what is the rule of choosing the one makes me concerned.
I know that ethanol can be easily dried by 3A molecular sieves, CaO, MgSO4 (and Na2SO4 is good to pre-drying but it has a lot of disadvantages). Sodium metal and benzophenone is used to dry THF, acids can destroy molecular sieves etc. but when it comes to non-solvents especially with unstable -CHO group I am completely confused.

DraconicAcid - 28-10-2016 at 12:53

One word of advice- no matter how high the melting point is, don't dry them in an oven except at low temperatures (50 oC). Hydrated inorganic compounds may need to be heated past 100 oC to dry properly, but organic stuff will sublime and make a horrific mess.

biomechem - 28-10-2016 at 13:02

Quote: Originally posted by DraconicAcid  
One word of advice- no matter how high the melting point is, don't dry them in an oven except at low temperatures (50 oC). Hydrated inorganic compounds may need to be heated past 100 oC to dry properly, but organic stuff will sublime and make a horrific mess.


With all due respect to you why did you mention using oven when every method I consider seems to be more appropriate?

DraconicAcid - 28-10-2016 at 13:08

Quote: Originally posted by biomechem  
With all due respect to you why did you mention using oven when every method I consider seems to be more appropriate?

Because I'm tired and only skimmed the thread.....

The comment about vanillin having a high boiling point reminded me of when I had some students dry their purified vanillin samples in an oven, and foolishly turned it up to 80 or 90 oC, because that was below the melting point.

[Edited on 28-10-2016 by DraconicAcid]

aga - 28-10-2016 at 13:30

Shame on you Draco !

You could have answered the OP's question after a mere few hundreds of hours of exhaustive research and experimentation !

Oh me oh my.

What shall become of us ...

@biochem : What have you tried ?
What were your results ?
Where is the write-up of your own experimentation including starting weights, hydration, procedure, techniques employed, final weights, melting points etc etc etc.

You present questions: where is the Data from Your experiments with which the people here can help you ?

This an Amateur Chemistry Forum after all, mmmkay.

We all kinda Do stuff and then ask questions about Why it fucked up.

(the smarter ones just play Golf instead, because they just know)


[Edited on 28-10-2016 by aga]

Maroboduus - 28-10-2016 at 13:45

Quote: Originally posted by DraconicAcid  
One word of advice- no matter how high the melting point is, don't dry them in an oven except at low temperatures (50 oC). Hydrated inorganic compounds may need to be heated past 100 oC to dry properly, but organic stuff will sublime and make a horrific mess.


There are an awful lot of organic compounds out there, and no hard and fast rules that can be applied across the board. Draconic acid is right in most cases about heating organics, but if, for instance, you have lyseric acid monohydrate and want it dry I believe you'll need temperatures well over 50C even with a very hard vacuum.

For aldehydes Sodium Sulphate,, Magnesium Sulphate, and Calcium Sulphate can all be used.

Magnesium and Sodium Sulphates are rapid, but not tremendously high in intensity, so they don't dry the material are rigorously as Calcium Sulphate.
In addition magnesium Sulphate is slightly acidic, and may for that reason be a problem with some acid-sensitive compounds.

Calcium Sulphate is a very complete drying agent, but has a low capacity and is slower in it's action.

I don't know if sieves or Potassium Sulphate are suitable for aldehydes, but I'm too lazy to look this easily Googled information up for myself.

unionised - 28-10-2016 at 15:50

Quote: Originally posted by DraconicAcid  
Quote: Originally posted by biomechem  
With all due respect to you why did you mention using oven when every method I consider seems to be more appropriate?

Because I'm tired and only skimmed the thread.....

The comment about vanillin having a high boiling point reminded me of when I had some students dry their purified vanillin samples in an oven, and foolishly turned it up to 80 or 90 oC, because that was below the melting point.

[Edited on 28-10-2016 by DraconicAcid]

I imagine it smelled lovely...

Maroboduus - 28-10-2016 at 15:57

Quote: Originally posted by unionised  
Quote: Originally posted by DraconicAcid  
Quote: Originally posted by biomechem  
With all due respect to you why did you mention using oven when every method I consider seems to be more appropriate?

Because I'm tired and only skimmed the thread.....

The comment about vanillin having a high boiling point reminded me of when I had some students dry their purified vanillin samples in an oven, and foolishly turned it up to 80 or 90 oC, because that was below the melting point.

[Edited on 28-10-2016 by DraconicAcid]

I imagine it smelled lovely...


And the lab was invaded by a large, unruly crowd seeking freshly baked cookies

DraconicAcid - 28-10-2016 at 15:58

They were also drying cinnamic acid, so, no, it didn't smell lovely.

biomechem - 29-10-2016 at 07:49

To be honest I don't have even this substances, but the question pesters me. Only aldehyde I had chance to use was a formaldehyde, which as far as I know polymerizes when concentrated so I do not think anybody does it in a classic way.
According to benzaldehyde I have seen a description of preparation a pharmaceutical compound and it said that benzaldehyde should be vacuum distilled before rxn, that is why I asked about necessity of drying it after distillation. In one word is vacuum distillation a good way to dry compounds?
I also had chance to use vacuum pump to dry some organic compounds but IDK if it provides dry enough products.

unionised - 29-10-2016 at 08:09

Quote: Originally posted by biomechem  
In one word is vacuum distillation a good way to dry compounds?

In one word, "usually".