CycloRook
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Refluxing temperature question.
If you have a reflux reaction and for example your solvent is Chloroform which boils at 60C, your product that you are forming decomposes at 150C, you
boil the apparatus so reflux is vigorous. Will your apparatus ever climb over 60C ?
I think I know the answer to this question but I wanted to be sure.
If I set my hotplate to 60C it would take over an hour to reach reflux so I want to turn up the heat but don't want to destroy my product.
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Dr.Bob
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You cannot heat a solution above it BP unless sealed (that only works in high pressure tube or apparatus, otherwise things break in a catastrophic
way). So the temp won;t go above 60 no matter how hard you try, but the pointy of reflux is to only barely heat the reaction to the reflux point,
otherwise your condenser will be overtaxed and solvent will escape and it will boil dry, at which the temperature can go very high.
I often set the bath temp only slightly above (or even at or slightly below) the BP, so that I don't need a water condenser, and then can just use a
air condenser or vigreaux instead. A degree or two below BP will not usually slow a reaction down much, and often I need to reflux things overnight,
so making sure they don't boil dry is paramount, especially as water pressures can change or drop.
For CHCl3, I would use a water or oil bath, rather than going directly on a hotplate, as most hotplates are just not precise enough to control to 60C,
but the thermal mass of a liquid will allow you to regulate the temp better.
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Tsjerk
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My mom used to tell me boiling eggs on high fire would give hard boiled eggs, and low fire soft boiled eggs. It doesn't work that way because boiling
is boiling. Unless you boil to dryness, then the smell will tell you a couple of rooms away the eggs definitely became more than 100 degrees.
[Edited on 2-2-2022 by Tsjerk]
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CycloRook
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Quote: Originally posted by Dr.Bob  | You cannot heat a solution above it BP unless sealed (that only works in high pressure tube or apparatus, otherwise things break in a catastrophic
way). So the temp won;t go above 60 no matter how hard you try, but the pointy of reflux is to only barely heat the reaction to the reflux point,
otherwise your condenser will be overtaxed and solvent will escape and it will boil dry, at which the temperature can go very high.
I often set the bath temp only slightly above (or even at or slightly below) the BP, so that I don't need a water condenser, and then can just use a
air condenser or vigreaux instead. A degree or two below BP will not usually slow a reaction down much, and often I need to reflux things overnight,
so making sure they don't boil dry is paramount, especially as water pressures can change or drop.
For CHCl3, I would use a water or oil bath, rather than going directly on a hotplate, as most hotplates are just not precise enough to control to 60C,
but the thermal mass of a liquid will allow you to regulate the temp better. |
This is what I thought. I'm
about to try bromination of different organics using Bromosuccinimide. These reactions usually say to reflux for 3 hours. I have done one bromination
so far but I have not done the workup yet because I have no indication of any reaction taking place. Flask did not turn orange or anything. The
Bromosuccinimide looks like it make have been debrominated to succinimide because there are glass like crystals in the RBF. I will reflux for 3 hours
again just to make sure. Does anyone have a reliable indicator for reaction progression ?
I don't want to do workup for no reason.
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Dr.Bob
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TLC (or HPLC, etc) is the best way to monitor a reaction. Color changes and other tricks are usually only known after the reaction is done, and
are far less reliable. Many reactions are OK to run longer rather than shorter times. But you need some way to monitor the reaction progress.
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CycloRook
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| Quote: Originally posted by Dr.Bob | | TLC (or HPLC, etc) is the best way to monitor a reaction. Color changes and other tricks are usually only known after the reaction is done, and
are far less reliable. Many reactions are OK to run longer rather than shorter times. But you need some way to monitor the reaction progress.
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well I am here to learn. Do you know a super simple method I could use ?
I checked wiki and it has a dot test on paper.
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Texium
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TLC is the way. Spot the starting material in one lane, the reaction mixture in another, and a co-spot of the two in the middle. If the reaction is
complete, you shouldn't see any starting material show up in the reaction mixture lane. If you're working with aromatic compounds and you buy
fluorescent TLC plates, you can visualize them with a UV mineral light. If your compounds aren't UV active or you want to use cheaper or homemade
plates, simpler methods involve staining the plate with iodine vapor or potassium permanganate solution.
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Texium
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Thread Moved
2-2-2022 at 14:01 |
CycloRook
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| Quote: Originally posted by Texium | | TLC is the way. Spot the starting material in one lane, the reaction mixture in another, and a co-spot of the two in the middle. If the reaction is
complete, you shouldn't see any starting material show up in the reaction mixture lane. If you're working with aromatic compounds and you buy
fluorescent TLC plates, you can visualize them with a UV mineral light. If your compounds aren't UV active or you want to use cheaper or homemade
plates, simpler methods involve staining the plate with iodine vapor or potassium permanganate solution. |
So I filtered my solids.
The liquid I have not distilled yet. Based off of succinimide vs bromosuccinimide solubility in water it looks to me like the majority is unreacted
bromosuccinimide.
Bromosuccinimide is 14g per liter h2o approximately where as succinimide is 330g per liter h2o. If I had succinimide I would be able to get it all to
dissolve in water.
So I guess I'll either put everything back together and reflux again or distill my chloroform and hope my product is left over.
I did see that bromosuccinimide uses a catalyst to get the reaction going like hydrochloric acid or benzoyl peroxide.
What would make more sense to you guys ?
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Texium
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What you're trying to do doesn't make any sense. The sensible thing
would be for you to stop guessing and actually make an effort to follow the advice that you were given. TLC is a simple and reliable method. If you're
unwilling to make the small investment to acquire the materials for it, you're just wasting everyone's time (including your own).
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CharlieA
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| Quote: Originally posted by Tsjerk | My mom used to tell me boiling eggs on high fire would give hard boiled eggs, and low fire soft boiled eggs. It doesn't work that way because boiling
is boiling. Unless you boil to dryness, then the smell will tell you a couple of rooms away the eggs definitely became more than 100 degrees.
[Edited on 2-2-2022 by Tsjerk] |
Excellent analogy! I enjoyed it very much.
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