nowlan
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What does refluxing do exactly
Hello,
I am trying to get my head around refluxing.
At first glance it looks like you are distilling your solution, and then having it drip back into the original flask. Not sure what the purpose is.
Would appear that it would just mix together once more.
Are you expecting a layer to form?
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Volanschemia
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Refluxing simply prevents loss of solvent by evaporation.
For example, say you had an Fischer esterification of Ethanol and Ethanoic Acid running. Fischer esterifications generally require the temperature of
the reactants to be higher than the boiling point of the alcohol. Without a reflux condenser, you would lose a very large amount of the Ethanol
through evaporation and your esterification would not go very well. A reflux condenser re-condenses the Ethanol vapours into the liquid phase and it
falls back into the reaction vessel, minimizing loss of reactant.
"The chemists are a strange class of mortals, impelled by an almost insane impulse to seek their pleasures amid smoke and
vapor, soot and flame, poisons and poverty; yet among all these evils I seem to live so sweetly that may I die if I were to change places with the
Persian king" - Johann Joachim Becher, 1635 to 1682.
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j_sum1
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It also has the benefit of holding your reaction at a more or less constant temperature -- dependent on the boiling point of the liquid phase. This
ensures repeatability amongst other things.
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Zombie
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A third point here is as the reflux liquid falls back thru the column, the rising heated vapor carries enough BTU's to re- evaporate the more volatile
compound in the droplets.
This step enriches the rising vapor to a higher mole fraction of the more volatile component.
The most important aspect of efficiency here is the reflux must fall back into the column in a controlled fashion. Simply wicking down the column
walls reduces efficiency, lengthens distillation times, and reduces the concentration of the collected product.
Controlled reflux is reflux that is directed into the vapor path as much as possible. Systems such as funnels or something as simple as a section of
glass rod (drips of the end) can be used to force the reflux into the center of the column, and increase the vapor / liquid interaction.
I have notices the majority of Lab glassware does not take this into consideration. For lab glass columns I would recommend using some stainless steel
wool packed into the top, center, bottom sections of the column, (depending on the chemicals involved of course).
This forces more interaction, and faster, pure-er separation.
If you are distilling acids I would use PTFE shavings packed together as a "wool", as long as the Temps can support it.
I have begun modifying some columns using .001mm stainless stock cut into open ended conical shapes th use as centering rings.
I will post up the results as to if or how well they work when I get to the point of actually using them.
Predictive control or designed control of any distillation column is based on Reflux control first.
All other parameters are based off of this. Basically the least understood operation in distillation (at the hobby or amateur level) is the MOST
important.
This is a short article explaining what I just stated:
http://www.mavtechglobal.com/blog/2012/06/06/from-the-trench...
This is a more in depth mathematical explanation:
http://lorien.ncl.ac.uk/ming/control/gen/column1.pdf
All of this information will help you increase efficiency, reduce distillation times, and improve the purity of your collected product.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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DraconicAcid
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It keeps it at a constant temperature without having your solvent boil away.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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blogfast25
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Zombie:
The refluxing the OP has in mind has nothing to do with reflux in distillation, where a certain amount of product (‘distillate’) is
continuously withdrawn.
Instead, here mixture of volatile reagents is constantly boiled up and constantly completely condensed back in an overhead condenser. It’s also
known as ‘total reflux’. The main purpose is not to lose any reagent (or reaction product) through evaporative losses while the reaction proceeds.
Once the reaction has been completed, heating is stopped and all material is still in the boiling flask: there's no distillate. This kind of refluxing
only requires adequate heating of the boiler and sufficient cooling of the condenser as 'control variables'.
This video explains it very well and in great detail:
https://www.youtube.com/watch?v=b6xFAEkjmGg
Very minor squabble: as the composition of the boiler changes considerably during reaction, the boiling point of the liquid (reagents/reaction
products mix) will gradually change over time, in accordance with the changing composition. But mostly that effect has little or no practical
consequences.
[Edited on 4-4-2015 by blogfast25]
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Zombie
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Oh... Ok. Thank you for that. How did you fellas know that was what the OP was referring to?
I have a similar set up using a Soxhlet extraction apparatus (condenser on top), and understand the principal.
I did not really see Nowlan's question clearly.
I suppose in a broad sense for the purpose of answering Nowlan's question... Refluxing could be defined as Prevention loss of evaporated product to
prolong heating times.
The correct answer depends on the application...
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blogfast25
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From the context. If the OP's question was related to reflux in distillation it would have been worded very differently too, as it is a more complex
concept.
'Total reflux' is one of the most used ways of heating reagents w/o evaporative losses in organic chemistry synthesis.
In exceptional cases we use what could be called 'reactive distillation'. Some time ago I synthesised 2-chloropropane from isopropanol, ZnCl2 and
strong HCl. The 2-chloropropane was distilled off during the reaction, which ensures a more complete conversion:
http://www.sciencemadness.org/talk/viewthread.php?tid=30180#...
This was possible because of the low BP of 2-chloropropane with regards to the other components of the mix.
[Edited on 4-4-2015 by blogfast25]
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Magpie
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I think of it as a technique for promotion of the reaction by allowing the highest temperature possible (at ambient pressure) without loss of
solvent/reactants. It also promotes the reaction in that boiling promotes mixing.
[Edited on 4-4-2015 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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Quote: Originally posted by Magpie  |
I think of it as a technique for promotion of the reaction by allowing the highest temperature possible (at ambient pressure) without loss of
solvent/reactants. It also promotes the reaction in that boiling promotes mixing.
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Good point about the solvent, often required in OC synthesis.
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nowlan
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I was confused about why it was done. I see lots of guides talk about how it is done, but I failed to see the purpose. As I said, it looked like you
were back at step one with evaporation falling back into the original flask.
I see now that you are raising the temperature of the solution, and recovering/preserving the evaporated solvent.
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BromicAcid
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Magpie has it right on the nose, refluxing gives you the highest temperature the solvent will allow giving what could be the best rate of reaction.
Refluxing is also excellent to remove gaseous byproducts to drive a reaction forward.
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Zombie
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Since the original question has been answered, I have a follow up question...
Suppose a volatile component of a solvent was un desirable in a reaction but you needed that particular solvent for whatever reason.
Could you not use a large column, and temperature control the column to allow a gas/vapor/specific fraction, to be held in the column, and allow the
remaining components to reflux, and do their work?
Is there ever a reaction where this sort of process is required? Or am I over thinking this?
Just a thought.
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blogfast25
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Zombie:
You're overthinking but of course it can never be excluded that some particular case could benefit from that idea. Normally you'd simply remove the
undesirable solvent component by distillation of the solvent first.
As Bromic said, in some cases very volatile reaction products (gases, e.g.) can be removed, further promoting completion of reaction. Hydrogen, HCl or
CO2 are examples of fairly commonly unwanted reaction products that would escape through the condenser.
[Edited on 6-4-2015 by blogfast25]
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Fulmen
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If the reaction produces undesirable volatile byproducts Zombies method could work. But it is impractical for several reasons. First of all it would
probably be a rare occurrence, and require more specialized glassware. And getting the setup to perform could take some experimentation, even when
replicating previous work.
But from a theoretical view you are completely correct. Reflux allows reaction to run for extended times in volatile solvents at high temperature. In
addition to the usual criteria the solvent will be chosen with regard to it's boiling point to control reaction temperature. And anything more
volatile than the solvent could be removed using distillation techniques.
We're not banging rocks together here. We know how to put a man back together.
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BromicAcid
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I have seen and used the procedure you are speaking of Zombie, as I mentioned, to drive a reaction forward. But usually I would just use a fractional
takeoff head with a slow takeoff instead of a condenser held at a specific temperature. That is the sort of setup I use when doing
transesterfications between -OMe and -OEt, the MeOH keeps coming off and you keep adding in more EtOH at reflux, the slow takeoff at the fractional
head keeps bringing over EtOH with some MeOH, but the bulk of it comes off the cold finger and goes back into the flask during the reflux.
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brubei
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A nice illustration of the use of refluxing is dehydration reaction with a dean stark in toluene solvent.
https://en.wikipedia.org/wiki/Dean-Stark_apparatus
As the reaction is an equilibria, it must be displace to give the maximum of product by removing the product formed. Removing water is done by
refluxing toluene which give an azeotrope with water. Then collected in another flask, toluene and water dismix and the toluene at the top go back in
the reaction vessel.
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Zombie
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These are exactly the answers I was hoping to see.
I have a common sense or mechanical approach to problem solving, and being aware of the potential uses of equipment or potential ways of approaching a
problem is/are VERY helpful to a fella like me.
I consider this "Ammo in the can". Something I can use.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Magpie
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The Dean-Stark apparatus reminds me of a remarkable separator that I saw at a pulp mill in Cloquet, MN where I once worked. Wood chips were cooked in
a digester to make pulp for papermaking. When a cook was finished (3-4 hrs) the cook vapor was vented through a water condenser to a separator tank
where turpentine and water were automatically separated. I was just amazed at the ingenuity when I realized it operated with no moving parts or
operator attention. Here's a sketch to show you how it worked:
The single most important condition for a successful synthesis is good mixing - Nicodem
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