Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Messi on October 05, 2012, 12:38:41 PM
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Hi guys,
How do I know if all the solvent is gone from my rotovap?
Also, I was wondering, how do I know how much drying agent (MgSO4) I need to add to my sample at the end?
Last technique question :), how much of my extraction solution do I add to my solution. For example, when I'm doing an extraction and it says wash with water. How do I know how much water to add? I usually add ~10mL to anything but I honestly have no idea how much is deemed "correct"!
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You know when nothing more distills over.
Drying agent, add a couple of grams, it really depends on the scale you are working at.
Extraction, depends on how much solvent and material you have how much water you add. 10 mL seems ok, but not if you have 100mL solvent.
Remember the partition coefficients.
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In each case, you use as much as you need. ;D Sorry, but that's the best I can do with limited information. But let me try to give you some useful areas to consider:
Your solvent is gone from the rotovap when its all gone. How much did you put into your reaction? How much total volume did you lose in workup? You can use those values to ballpark how much the rotovap is to remove. Only you know these things. And you're expected to keep track of them, for steps such as these, and general laboratory hygiene. Consider: suppose you do this process 20 times a week, or you are promoted, and you and two lab techs do this 50 times a week -- how much solvent does the lab manager need to buy? How much solvent waste do you need to contract for disposal? I know, you're still in school, but you should start keeping track of what you're doing.
You need enough drying agent to remove all the moisture. Its hard to know exactly how much there is, but if you've evaporated much of the solvent, you can only have as much moisture as is miscible in the remaining volume of solvent.
If you have to wash a product, you have to use enough water. How much is enough depends on how much there is. Only you know how much you added or produced. You probably want to wash 3 times, with 1/3 the volume each time, for best extraction.
Generally, you want to be generous at each step, doing slightly more, so your ballpark figure can be as loose as you need to be to save time. And of course, only you know if any of these procedures is detrimental to your product, in which case, it is better to do less.
Clear as mud now, right?
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Crystal clear Arkon, wish I had thought of that ???
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I type slow. ;)
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Lol but not slower than me! Or do you write too much? >:D
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In each case, you use as much as you need. ;D Sorry, but that's the best I can do with limited information. But let me try to give you some useful areas to consider:
Your solvent is gone from the rotovap when its all gone. How much did you put into your reaction? How much total volume did you lose in workup? You can use those values to ballpark how much the rotovap is to remove. Only you know these things. And you're expected to keep track of them, for steps such as these, and general laboratory hygiene. Consider: suppose you do this process 20 times a week, or you are promoted, and you and two lab techs do this 50 times a week -- how much solvent does the lab manager need to buy? How much solvent waste do you need to contract for disposal? I know, you're still in school, but you should start keeping track of what you're doing.
You need enough drying agent to remove all the moisture. Its hard to know exactly how much there is, but if you've evaporated much of the solvent, you can only have as much moisture as is miscible in the remaining volume of solvent.
If you have to wash a product, you have to use enough water. How much is enough depends on how much there is. Only you know how much you added or produced. You probably want to wash 3 times, with 1/3 the volume each time, for best extraction.
Generally, you want to be generous at each step, doing slightly more, so your ballpark figure can be as loose as you need to be to save time. And of course, only you know if any of these procedures is detrimental to your product, in which case, it is better to do less.
Clear as mud now, right?
I was hoping for an easy formula to follow :( but it seems that this doesn't seem to be the case :(
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I weigh my flask, and put it back onto the rotary evaporator, and weigh it again a few minutes later. If the solvent is all gone, the mass will be about the same.
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I weigh my flask, and put it back onto the rotary evaporator, and weigh it again a few minutes later. If the solvent is all gone, the mass will be about the same.
Nice! I like this approach Babcock. My supervisor who is post-doc never does that though and just rotavaps his product and looks at it then removes it at will!
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I used to use the Nasal spectrometer, I hope you have one in your lab.
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No, we don't. What are its advantages?
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It does not require high magnetic fields and liquid helium. It is cheap and dependable, cleaning several times a day can be a disadvantage, but the sensitivity is excellent. Blockages can occur but they are easily removed by poking them out with a finger.
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What is a Nasal spectrometer.... I just googled it but didn't find anything ???
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What is a Nasal spectrometer.... I just googled it but didn't find anything ???
A device for detecting very low concentrations of volatile solvents. (http://en.wikipedia.org/wiki/Nose) :P
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What is a Nasal spectrometer.... I just googled it but didn't find anything ???
It is right in front of your face! Every lab has several of them. Unfortunately not all are new, some can be quite old, but the reliability is still excellent.
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You guys are witty! Yes, a nose is quite important ;)
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+1 for nasal spectrometer.
Here are a few more thoughts on your questions:
Regarding excess solvent, if you have access to a high vac (i.e. a pump that pulls really hard, like 0-1 torr), leave your material on there (assuming it is not volatile) after roto-vapping for a few hours and your material will be dry. This may be over the top but if you can, NMR is a surefire way to determine if there is still sample present.
Regarding drying agents and water washes, I'd highly recommend you wash your organic layer with brine (aqueous saturated sodium chloride) after the water wash. The will get rid of a lot of the water in your sample and decrease the amount of drying agent you have to use. Typically you need to add enough MgSO4 until you don't see clumps forming, but I always thought it was kinda hard to see this effect.
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You got me with the nasal spectrometer there! I wouldn't always recommend that approach though.
I would judge the rotovap by looking to see if there is any solvent still dripping off the condenser. And then if there is a control panel you can lower the pressure to make sure the solvent is off.
With the MgSO4 I was taught that if the MgSO4 is free flowing and not clumping then you have added enough. I always add extra for good measure!
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I fell into the MgSO4 trap the first time I scaled something up in the plant! I added about 5g for a 20g reaction product in the lab. In the plant 200kg requires 50kg MgSO4. I was told to go away and think again.
Actually the best way to dry something is a toluene azeotrope. Twice or three times and you are under 100 ppm water by Karl-Fischer.
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A device for detecting very low concentrations of volatile solvents. (http://en.wikipedia.org/wiki/Nose) :P
Spoilsport! >:D Delete it.
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Why does a brine wash help reduce water from the organic phase? ???
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I fell into the MgSO4 trap the first time I scaled something up in the plant! I added about 5g for a 20g reaction product in the lab. In the plant 200kg requires 50kg MgSO4. I was told to go away and think again.
What's the right answer? How much to add?
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Not much 0.1-0.2g if you have to, as I said azeotrope with toluene.
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Not much 0.1-0.2g if you have to, as I said azeotrope with toluene.
Ah! Got it.
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Why does a brine wash help reduce water from the organic phase? ???
An extraction is a separation of material based on it's relative solubility in two different solvents. Normally, these two solvents are idealized to water and octanol and it is given that since these are immiscible solvents, there is no water in the octanol and no octanol in the water.
Once you start adding other components, or using more polar organic solvents such as chloroform or ethyl acetate, you do get some forces attracting water into the organic layer, and as you'e noticed if you've tried to strip one down, it can be quite a bit of water. One answer to this is to use a solvent that is even more attractive to water than plain water, and a good cheap one is a saturated sodium chloride solution. As you can tell from osmotic pressure experiments, strongly ionic solutions will pull water molecules away from pure water. It turns out they also pull water away from organic solvents that have water entrapped either due to some limited solubility of water in the solvent or to hydration of various solutes that might be present.
Adding salt to your water also does some other very useful things in extraction, like limiting the solubility of organic materials in the aqueous layer and breaking emulsions, but for that final wash with sat'd sodium chloride to remove water before using a drying agent, this is the effect in play.
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Thanks @fledarmus!
Another explanation I've heard in the plant was that brine increases the density difference versus lighter organics; thereby aiding phase separation. Not sure if that's true. But I'd never thought a brine wash might help dry the organic! Live 'n learn! :)
Practical problem with brine washes I faced in production was too much TDS in the effluents.
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If you have too much TDS in the waste, store it and dilute it with someone else's waste. Day by day the amount will reduce itself to zero.
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If you have too much TDS in the waste, store it and dilute it with someone else's waste. Day by day the amount will reduce itself to zero.
I'll see what the EPA thinks about this...
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Nothing as long as you stay within the plant limits for waste disposal to the water treatment plant.
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Another lab technique question! :)
So I have THF and I am told to dilute my mixture with ether because I will be doing an extraction with water there after. If I have ~15mL of THF + my product how much ether should I add to dilute my mixture?
You guys are the best! :)
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I would be careful here. That's the lazy man's way of doing it. Evaporate the THF then add ether to the residue and extract. That way you will avoid losses due to THF being miscible with water.
If you want to do it the other way add as much ether as THF and extract at least twice.
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I weigh my flask, and put it back onto the rotary evaporator, and weigh it again a few minutes later. If the solvent is all gone, the mass will be about the same.
I second this method. I used to do a little more also, depending on the solvent. If you are using a very volatile solvent, raise you rotovap out of the water bath and feel the flack. If it is becoming cold, solvent is still evaporating. If the flask is as warm as the bath, you are near the end. This will have more meaning if you are evaporating chromatography fractions, especially those that will crystalize. If all of the solvent evaporates, you may experience some bumping.
My rule of thumb is never ever smell your flask. Although most chemicals are not that toxic, as Disco has proven, but I think it is a bad habit.
Water wash, depends. If sample is water soluble, as little as possible, if trying to wash out something with significant organic solubility, use more water.
Drying and assuming brine wash. With a reasonably dry solvent, add some drying agent, swirl, observe, caking, add more. If flows freely, stop. This should give you an idea for different solvents. You don't want to leave your sample on the drying agent either.
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Evaporate the THF then add ether to the residue and extract.
Rotavap?
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Yes, rotavap.
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Just did that. Thanks sir! Let's see how my reaction looks like now!
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Good luck. If it goes wrong it was not me :o
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Good luck. If it goes wrong it was not me :o
Reaction worked very well :) Had an increase of yield and reaction had minimal impurity :)
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Ha ha must have been my suggestion that did it >:D
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Another Lab technique question! :)
So, I made a compound and I believe it is pure... as I separated with a column and the NMR looked nice. From what I understand, pure compounds usually tend to crystallize, no? So how can I induce crystallization of my product to make sure it is pure? Can I leave it on the counter overnight and hope it crystallizes!?
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You can certainly leave it standing. You could also dissolve it in a solvent and let it slowly evaporate, this may produce a few seed crystals for you to use for a proper crystallisation.
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First, are you sure your compound is supposed to be a solid at room temperature? You wouldn't be the first person to try to crystallize a liquid...
There are all sorts of ways to try to induce crystallization. Usually, once you can get crystals to begin forming, the oil will solidify very quickly. All you need is a nucleation site to start the crystallization, if your compound is pure.
- Add a seed crystal. Of course, that means you have to have made the compound before, or have some commercially available.
- scrape a small spatula through your compound against the glass. This may give a place for your solid to nucleate and start crystallizing.
- hold a small piece of ice, or even better, dry ice against the glass. This may quick freeze a tiny portion of the material, and give you a nucleation site to crystallize the rest.
- take a tiny amount up on a small spatula and cool it rapidly by waving it through the air, or in extreme cases, resting the back of the spatula on a piece of dry ice. If the clear amount turns white or opaque, stir it back into your sample. This may give you enough seed crystals to start.
- if worse comes to worst, take a drop of your liquid, dissolve it into a tiny amount of chloroform or dichloromethane, and add a larger portion of hexane. If you get a flocculant, filter it through a very small filter and scrape the filter over your product to get the tiny seeds to drop in. You can also strip the flocculant down and add scrapings of the residue back to your compound.
- if all else fails, add a small amount of a solvent you know the compound is completely insoluble in, and triturate it with a glass rod until it begins to crystallize. For most of the compounds I worked with, hexane was a good choice for this. If it doesn't crystallize after about five minutes of trituration, pour off the hexane, add a fresh lot of hexane, and continue triturating.
That's my fairly standard list of ways to try to force crystallization. If none of those work, there are some other possibilities based on the structure of the compound you are working with, but they are pretty specific to the situation.
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First, are you sure your compound is supposed to be a solid at room temperature? You wouldn't be the first person to try to crystallize a liquid...
Yes, I'm sure. I'm looking at it now and it's actually a solid.
- Add a seed crystal. Of course, that means you have to have made the compound before, or have some commercially available.
Unfortunately this isn't possible :(
- scrape a small spatula through your compound against the glass. This may give a place for your solid to nucleate and start crystallizing.
- hold a small piece of ice, or even better, dry ice against the glass. This may quick freeze a tiny portion of the material, and give you a nucleation site to crystallize the rest.
I like these two options. I'll try this. How long will crystal formation take? Also, when I give my crystals to the crystal expert, how long will it take him to elucidate my structure? :)
Also, do I need to make sure my compound is completely dry? No solvent is present such as ethyl acetate or DCM for crystal formation to occur?
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First, are you sure your compound is supposed to be a solid at room temperature? You wouldn't be the first person to try to crystallize a liquid...
Yes, I'm sure. I'm looking at it now and it's actually a solid.
Okay, now I'm confused. If you already have a solid, what are you trying to crystallize? Or are you saying that you would like to RE-crystallize your material to make it more pure? Because that is a whole 'nother thesis...
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I guess it's the lack of knowledge on my part that is confusing me. I thought a solid and a crystal were two different things? I thought you can still have a compound in a solid form that isn't pure while crystals are pure?
Also, is taking a crystal of my compound better than taking its NMR?
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You can have pure solids which are amorphous, no defined crystal form. Fortunately a lot of solids are crystalline, but exist in various forms of the crystal.
To get a crystal for x-ray you need to grow a "perfect crystal" one that has no flaws in it, no solvent inclusions, no twinning, etc. etc. It's not so easy.
X-ray gives you the spacial arrangement of the atoms in the unit cell, i.e. the solid phase structure. Combine that with NMR which gives you the solution phase structure and you get a lot of information about your compound. Because x-ray is so very time consuming NMR is usually the way to go.
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Again, explained so nicely! Thank you!
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ANOTHER Lab technique question! :)
How do I know how much solvent I should add to my reaction!?
My reaction includes 0.15g of sulfur, 1mL of an amine, 0.3mL of an alkyne and 0.2mL of an acid, 0.4mL of a strong base.
Thanks!
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Hopefully, the procedure you got from a publication will give you some indication of a concentration. If not, you sort of have to pick a concentration and hope for the best. After a while you get good at figuring out which reactions are going to go very fast and may need to be run more dilute, and which ones will be slow and will need to be concentrated. And of course, the solubility of your starting materials or products may determine how much solvent you need.
Usually I would start with ~0.1 to ~1 M of my limiting reagent in the solvent. Smaller reactions I tended to run a little more dilute, just because it is hard to measure and manipulate very small quantities of solvent.
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After a while you get good at figuring out which reactions are going to go very fast and may need to be run more dilute, and which ones will be slow and will need to be concentrated.
Why do fast reactions need to be run more dilute?
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When you dilute you slow down the reaction, that makes it easier to control.
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Thanks Borek
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ANOTHER technique question :)
Is it ok to run two SIMULTANEOUS reactions in the same fumehood!?
Two for the price of one. Save some time :)
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Run as many reactions as you want!
https://www.google.com/search?q=reaction+block&hl=en&safe=off&tbo=u&rls=com.microsoft:en-us:IE-Address&rlz=1I7ADRA_en&tbm=isch&source=univ&sa=X&ei=3FekUNvfI8Ts0QGyzYF4&ved=0CEoQsAQ&biw=1680&bih=819 (https://www.google.com/search?q=reaction+block&hl=en&safe=off&tbo=u&rls=com.microsoft:en-us:IE-Address&rlz=1I7ADRA_en&tbm=isch&source=univ&sa=X&ei=3FekUNvfI8Ts0QGyzYF4&ved=0CEoQsAQ&biw=1680&bih=819)
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Run as many reactions as you want!
Now that's what I call epic!
Too bad my school is old and everything is done with my two hands :(
Maybe Santa will find me this year though? :)
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You mean you want hands for Xmas? You'd better write to Santa now then he must have a forum somewhere on the net.
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You mean you want hands for Xmas? You'd better write to Santa now then he must have a forum somewhere on the net.
You can actually post a card marked "Santa, North Pole" and it gets delivered to him!
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I do that every year but it obviously never gets there as I never get what I put on the card :-\
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I do that every year but it obviously never gets there as I never get what I put on the card :-\
Maybe you just weren't good enough in the years past?
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That could be ???
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You can actually post a card marked "Santa, North Pole" and it gets delivered to him!
These cards are actually delivered to Rovaniemi on arctic circle. Been there two years ago.