| Pages:
1
2
3 |
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
DCM vs ether
Quote: Originally posted by Magpie  | My 1960's college organic chemistry manual never specified DCM as solvent in any of the procedures although it was mentioned in a list of solvents.
Chloroform and carbon tetrachloride were occaisionally used as well as, of course, diethyl ether.
Now, during my recent refresher course the lab manual relies very heavily on the use of DCM, and rarely specifies chloroform, carbon tet, or ether.
Surely this must be due to the lower toxicity of DCM vs chloroform and carbon tet. And compared to ether there is the big advantage DCM's
non-flammability.
What I'm wondering is: If lower boiling point and somewhat higher solubility in water aren't an issue, is there any reason that DCM can't be
generally substituted for these other chlorinated methanes?
For a specific example take the case of the extraction of phenol from water. |
Many, if not most, of the synthesis procedures found in Vogel, OrgSyn, Gattermann, etc, specify ether for the workup. I dislike the use of ether
because of its flammability and lingering stench. If at all possible I would much rather use DCM (dichloromethane). Although this question has been
discussed tangentially in other threads I wanted to ask this question formally to get your opinions for discussion.
So, barring a good reason to not use DCM (eg, reactivity, azeotrope formation, etc) can it be assumed that where ether is specified DCM can be
substituted?
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
Loptr
International Hazard
Posts: 1348
Registered: 20-5-2014
Location: USA
Member Is Offline
Mood: Grateful
|
|
I can't speak for the generality of DCM, but one issue I have had with DCM is emulsions, which I don't remember with ether. It might just be me,
though. 
I also imagine that DCM could more easily be deprotonated than Et2O.
[Edited on 24-3-2015 by Loptr]
|
|
|
careysub
International Hazard
Posts: 1339
Registered: 4-8-2014
Location: Coastal Sage Scrub Biome
Member Is Offline
Mood: Lowest quantum state
|
|
This interests me as well - DCM is safer and cheaply and easily available where I live (California) in paint stripper.
Similarly, I have resolved to try isopropyl alcohol instead of methanol or ethanol in most procedures (if their only role is as a solvent) since IPA
is so cheap and easy to get.
In Williamson's "Microscale and Macroscale Organic Chemistry" (second edition) published in 1994 he used methyl tert-butyl ether (MBTE) as the all
purpose replacement for diethyl ether citing its cheapness and ready availability since it was widely used in gasoline/petrol. 20 years on it is not
cheap nor readily available, having been effectively banned from gasoline in 2004-2005.
|
|
|
Chemosynthesis
International Hazard
Posts: 1071
Registered: 26-9-2013
Member Is Offline
Mood: No Mood
|
|
I have considered this very question before, and seen the substitution used on a few different reactions without apparent issue so far. Interested in
seeing anyone come up with exceptions.
|
|
|
Loptr
International Hazard
Posts: 1348
Registered: 20-5-2014
Location: USA
Member Is Offline
Mood: Grateful
|
|
I just recently purchased a gallon of DCM in hopes of it being my general purpose non-polar solvent, even though it is slightly polar.
According to this http://web.pdx.edu/~wamserc/Research/jo100276m.pdf, pyridine and its derivatives react with dichloromethane under ambient conditions to form
methylenebispyridinium dichloride compounds. I know that diethyl ether is used to dissolve the pyridine salt as part of extraction, so it DCM might
not be a good candidate in conjunction with pyridine. Of course, there are other methods, but this is one I have run across in a quick search for
incompatibilities.
* I am just trying to find a set of general example where DCM might not be a good idea. I am in no way an authority on this, and might be wrong.
EDIT: I think it also goes without saying that being it's a chlorinated solvent, you don't dry with alkali metals as you would with diethyl ether. I
have read the usage of sodium with chlorinated solvents has led to explosions. I am not in the industry, but is something that has stuck in my head
since college.
[Edited on 24-3-2015 by Loptr]
[Edited on 24-3-2015 by Loptr]
[Edited on 25-3-2015 by Loptr]
|
|
|
Amos
International Hazard
Posts: 1406
Registered: 25-3-2014
Location: Yes
Member Is Offline
Mood: No
|
|
Is there any reason that hexane or heptane wouldn't be suitable for many of the procedures using DCM or ether as a solvent? One of the other is often
found alongside ether in engine starter fluid. Forgive my ignorance if there's a blatantly obvious reason why it wouldn't work; I haven't really seen
many compounds explicitly described as being soluble in alkanes.
[Edited on 3-25-2015 by Amos]
|
|
|
DraconicAcid
International Hazard
Posts: 4283
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
Long ago, when I worked with funky organometallic compounds, DCM was our standard solvent due to its polarity. Ether was used to precipitate out the
compounds from solution.
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.
|
|
|
Chemosynthesis
International Hazard
Posts: 1071
Registered: 26-9-2013
Member Is Offline
Mood: No Mood
|
|
| Quote: Originally posted by Loptr | According to this http://web.pdx.edu/~wamserc/Research/jo100276m.pdf, pyridine and its derivatives react with dichloromethane under ambient conditions to form
methylenebispyridinium dichloride compounds. I know that diethyl ether is used to dissolve the pyridine salt as part of extraction, so it DCM might
not be a good candidate in conjunction with pyridine. Of course, there are other methods, but this is one I have run across in a quick search for
incompatibilities.
* I am just trying to find a set of general example where DCM might not be a good idea. I am in no way an authority on this, and might be wrong.
|
I didn't realize that. Room temperature with pyridines is a new one for me. The link didn't work for me, but I might have the citation here: J. Org.
Chem., 2010, 75 (12), pp 4292–4295
|
|
|
Loptr
International Hazard
Posts: 1348
Registered: 20-5-2014
Location: USA
Member Is Offline
Mood: Grateful
|
|
| Quote: Originally posted by Chemosynthesis | | Quote: Originally posted by Loptr | According to this http://web.pdx.edu/~wamserc/Research/jo100276m.pdf, pyridine and its derivatives react with dichloromethane under ambient conditions to form
methylenebispyridinium dichloride compounds. I know that diethyl ether is used to dissolve the pyridine salt as part of extraction, so it DCM might
not be a good candidate in conjunction with pyridine. Of course, there are other methods, but this is one I have run across in a quick search for
incompatibilities.
* I am just trying to find a set of general example where DCM might not be a good idea. I am in no way an authority on this, and might be wrong.
|
I didn't realize that. Room temperature with pyridines is a new one for me. The link didn't work for me, but I might have the citation here: J. Org.
Chem., 2010, 75 (12), pp 4292–4295 |
http://web.pdx.edu/~wamserc/Research/jo100276m.pdf
There was an issue with a comma included in the url. The link had pictures showing the formed products comparing side by side solutions.
|
|
|
Loptr
International Hazard
Posts: 1348
Registered: 20-5-2014
Location: USA
Member Is Offline
Mood: Grateful
|
|
| Quote: Originally posted by DraconicAcid | | Long ago, when I worked with funky organometallic compounds, DCM was our standard solvent due to its polarity. Ether was used to precipitate out the
compounds from solution. |
I have read of DCM being used as a cosolvent in organometallic additions to help with chelation control. Any chance this being one of the reasons? In
the particular instance I refer to, they were using temperatures around -78C to keep the DCM from becoming deprotonated. LDA to form
dichloromethyllithium is one such example of it being deprotonated.
[Edited on 25-3-2015 by Loptr]
[Edited on 25-3-2015 by Loptr]
|
|
|
subsecret
Hazard to Others
Posts: 424
Registered: 8-6-2013
Location: NW SC, USA
Member Is Offline
Mood: Human Sadness - Julian Casablancas & the Voidz
|
|
| Quote: Originally posted by Amos | Is there any reason that hexane or heptane wouldn't be suitable for many of the procedures using DCM or ether as a solvent? One of the other is often
found alongside ether in engine starter fluid. Forgive my ignorance if there's a blatantly obvious reason why it wouldn't work; I haven't really seen
many compounds explicitly described as being soluble in alkanes.
[Edited on 3-25-2015 by Amos] |
Petroleum ether, toluene, or xylene could be used if hydrocarbons work well for extractions. According to wikipedia, carboxylic acids can easily be
extracted with benzene. Bromine and iodine can easily be extracted with these hydrocarbons (which might be a good way to extract iodine from tincture
of iodine, after it is acidified and H2O2 added).
But I think we are missing something here, because these solvents are never mentioned for the most common extractions.
Fear is what you get when caution wasn't enough.
|
|
|
Loptr
International Hazard
Posts: 1348
Registered: 20-5-2014
Location: USA
Member Is Offline
Mood: Grateful
|
|
I am considering purchasing P2O5 for use in drying the dichloromethane. Would this be an optimal choice?
I haven't attempted to dry dichloromethane before, however, I have ran across the use of calcium hydride several times so far, which is not easily
acquired, for me at least.
http://chemwiki.ucdavis.edu/Reference/Lab_Techniques/Distill...
EDIT: Also, drying it with a more general desiccant, such as calcium chloride might do the trick, and then distill it. I am planning on storing the
DCM with 3A molecular sieves. 4A molecular sieves won't exclude any methanol that has been added as a stabilizer, and might cause issues with pressure
build up.
[Edited on 25-3-2015 by Loptr]
[Edited on 25-3-2015 by Loptr]
|
|
|
Loptr
International Hazard
Posts: 1348
Registered: 20-5-2014
Location: USA
Member Is Offline
Mood: Grateful
|
|
Here is an old post from back in 2008 that is a translated page showing azeotropes of DCM--might be useful. (way before I got here)
http://www.sciencemadness.org/talk/viewthread.php?tid=10306&...
Here is the thread link: http://www.sciencemadness.org/talk/viewthread.php?tid=10306
* I attached the image to this thread because it wouldn't let me link the image into this post for some reason... redundancy... :-/
[Edited on 25-3-2015 by Loptr]
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
| Quote: Originally posted by subsecret |
Petroleum ether, toluene, or xylene could be used if hydrocarbons work well for extractions. According to wikipedia, carboxylic acids can easily be
extracted with benzene. Bromine and iodine can easily be extracted with these hydrocarbons (which might be a good way to extract iodine from tincture
of iodine, after it is acidified and H2O2 added).
But I think we are missing something here, because these solvents are never mentioned for the most common extractions. |
I wonder if what we are missing is the somewhat polar nature of both DCM and ether.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
DraconicAcid
International Hazard
Posts: 4283
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
| Quote: Originally posted by Loptr | | I am considering purchasing P2O5 for use in drying the dichloromethane. Would this be an optimal choice? |
That's what we always used back in the day. Reflux over P2O5, then distil.
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.
|
|
|
DraconicAcid
International Hazard
Posts: 4283
Registered: 1-2-2013
Location: The tiniest college campus ever....
Member Is Offline
Mood: Semi-victorious.
|
|
| Quote: Originally posted by Loptr | | Quote: Originally posted by DraconicAcid | | Long ago, when I worked with funky organometallic compounds, DCM was our standard solvent due to its polarity. Ether was used to precipitate out the
compounds from solution. |
I have read of DCM being used as a cosolvent in organometallic additions to help with chelation control. Any chance this being one of the reasons? In
the particular instance I refer to, they were using temperatures around -78C to keep the DCM from becoming deprotonated. LDA to form
dichloromethyllithium is one such example of it being deprotonated.
|
No, we just used it because the compounds we were using were very soluble in it, and it didn't react with the stuff we were using.
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.
|
|
|
deltaH
Dangerous source of unreferenced speculation
Posts: 1663
Registered: 30-9-2013
Location: South Africa
Member Is Offline
Mood: Heavily protonated
|
|
I second that, I've found DCM highly useful in dissolving stubborn organic compounds. It is an exceptional solvent IMHO.
|
|
|
careysub
International Hazard
Posts: 1339
Registered: 4-8-2014
Location: Coastal Sage Scrub Biome
Member Is Offline
Mood: Lowest quantum state
|
|
| Quote: Originally posted by Amos | Is there any reason that hexane or heptane wouldn't be suitable for many of the procedures using DCM or ether as a solvent? One of the other is often
found alongside ether in engine starter fluid. Forgive my ignorance if there's a blatantly obvious reason why it wouldn't work; I haven't really seen
many compounds explicitly described as being soluble in alkanes.
[Edited on 3-25-2015 by Amos] |
BTW heptane is available OTC in the US as the rubber cement solvent Bestine, about $16 a quart. Amazon has it, and your better art supply houses have
it also (Dick Blick, ArtSupplyWarehouse, etc.).
|
|
|
careysub
International Hazard
Posts: 1339
Registered: 4-8-2014
Location: Coastal Sage Scrub Biome
Member Is Offline
Mood: Lowest quantum state
|
|
| Quote: Originally posted by Loptr |
EDIT: Also, drying it with a more general desiccant, such as calcium chloride might do the trick, and then distill it. I am planning on storing the
DCM with 3A molecular sieves. 4A molecular sieves won't exclude any methanol that has been added as a stabilizer, and might cause issues with pressure
build up. |
I find CaCl2 listed as a dessicant for DCM in multiple places. That would be the one to start with, for cheapness.
I intend to use it to dry distilled paint stripper DCM to remove any methanol and ethanol as well.
|
|
|
gdflp
Super Moderator
Posts: 1320
Registered: 14-2-2014
Location: NY, USA
Member Is Offline
Mood: Staring at code
|
|
| Quote: Originally posted by Magpie | | Quote: Originally posted by subsecret |
Petroleum ether, toluene, or xylene could be used if hydrocarbons work well for extractions. According to wikipedia, carboxylic acids can easily be
extracted with benzene. Bromine and iodine can easily be extracted with these hydrocarbons (which might be a good way to extract iodine from tincture
of iodine, after it is acidified and H2O2 added).
But I think we are missing something here, because these solvents are never mentioned for the most common extractions. |
I wonder if what we are missing is the somewhat polar nature of both DCM and ether. |
Perhaps one of the issues arises in removing the solvent. Both DCM and ether are very volatile compounds with low boiling points. This allows for
easily removing the solvent after an extraction with a hot water bath. If the product is a liquid, it may have a boiling point similar to that of the
solvent, thus requiring a fractional distillation. If the product is a solid, it may melt lower than the boiling point of the solid thus leaving a
mess after solvent removal, or decompose below the solvent's boiling point, undesirable for obvious reasons(liquids could suffer this problem as
well). The volatility of ether and DCM will also just allow for the product to dry faster, requiring less time taking up oven space or taking up
space sitting in a dish.
|
|
|
Loptr
International Hazard
Posts: 1348
Registered: 20-5-2014
Location: USA
Member Is Offline
Mood: Grateful
|
|
| Quote: Originally posted by gdflp | | Quote: Originally posted by Magpie | | Quote: Originally posted by subsecret |
Petroleum ether, toluene, or xylene could be used if hydrocarbons work well for extractions. According to wikipedia, carboxylic acids can easily be
extracted with benzene. Bromine and iodine can easily be extracted with these hydrocarbons (which might be a good way to extract iodine from tincture
of iodine, after it is acidified and H2O2 added).
But I think we are missing something here, because these solvents are never mentioned for the most common extractions. |
I wonder if what we are missing is the somewhat polar nature of both DCM and ether. |
Perhaps one of the issues arises in removing the solvent. Both DCM and ether are very volatile compounds with low boiling points. This allows for
easily removing the solvent after an extraction with a hot water bath. If the product is a liquid, it may have a boiling point similar to that of the
solvent, thus requiring a fractional distillation. If the product is a solid, it may melt lower than the boiling point of the solid thus leaving a
mess after solvent removal, or decompose below the solvent's boiling point, undesirable for obvious reasons(liquids could suffer this problem as
well). The volatility of ether and DCM will also just allow for the product to dry faster, requiring less time taking up oven space or taking up
space sitting in a dish. |
Aren't higher boiling solvents more difficult to remove given Raoult's law, meaning you want the solvent be as volatile as is practical?
|
|
|
Magpie
lab constructor
Posts: 5939
Registered: 1-11-2003
Location: USA
Member Is Offline
Mood: Chemistry: the subtle science.
|
|
Yes, the low boiling point of DCM (40°) and ether (35°) is important for the reasons you have stated. But pet ether can also have a very low
boiling point depending on the petroleum cut selected (30°-40°, 40°-60°, etc). But it would be a non-polar solvent.
The single most important condition for a successful synthesis is good mixing - Nicodem
|
|
|
gdflp
Super Moderator
Posts: 1320
Registered: 14-2-2014
Location: NY, USA
Member Is Offline
Mood: Staring at code
|
|
Yes, I think that multiple factors come into play. For example, one of my recent orgo labs(synthesis of methyl 3-nitrobenzoate) called for the
product to be extracted with and dissolved in methanol, then ligroin(similar to pet ether, higher boiling fraction in this case) added and the
methanol evaporated at which point the methyl 3-nitrobenzoate precipitated due to the insolubility in the non polar solvent and was filtered. So a
desirable solvent has to be moderately polar to have desirable solvent characteristics, but also needs to have a low boiling point to be a viable
extraction solvent.
| Quote: Originally posted by Loptr |
Aren't higher boiling solvents more difficult to remove given Raoult's law, meaning you want the solvent be as volatile as is practical?
|
Yes, that was the point that I was trying to make. Sorry if I wasn't clear.
[Edited on 3-25-2015 by gdflp]
|
|
|
Dr.Bob
International Hazard
Posts: 2668
Registered: 26-1-2011
Location: USA - NC
Member Is Offline
Mood: No Mood
|
|
DCM is often the best solvent for extractions of both non-polar and somewhat polar compounds. Ether works well with similar compounds, but is not as
good at either end of the spectrum, in general (it will not dissolve grease or really polar compounds as quickly as DCM). So DCM can be used in
place of ether, except that it can actually extract out some unwanted compounds that ether will not extract, like amines, acids, etc, which you often
want to stay in the water layer, for instance in an amide formation. Chloroform is as good or better then DCM for most cases, but is considered less
safe. CCl4 is too non-polar for most work, other then very non-polar compounds, and becoming hard to find.
Hexane, heptane and pet ether are all all great for extracting oily compounds, but are very poor solvents for most even slightly polar compounds,
which is why ethyl acetate is more often used for moderately polar compounds. Heptane is often used in process chemistry, due to higher flash point,
BP, and lower toxicity (in theory at least). And many process chemists use isopropyl acetate instead of EtOAc for safety as well.
MTBE is also used by process chemists in place of ethyl ether, higher BP and flash point, but similar in peroxide formation, so still have to watch
that. It was cheap until most fo the plants to make it closed suddenly due to being banned in gasoline.
So chemists often want the lowest BP for ease of removal, engineers want the highest BP for safety and avoidance of evaporation loses. There are
tables of the "polarity" of most solvents, but if you stick with "like dissolves like", that will tell you if a solvent will work.
So for the least polar compounds, aromatics, hexane, heptane and pet ether all work well.
For intermediate polarity, I like ethyl acetate, DCM, ether, and sometimes THF (although it is somewhat miscible with water, unless mixed with DCM or
hexane).
And for very polar extractions from water, you might need n-butanol, THF, DCM/MeOH (9:1 is common). If the extraction is from a solid, then
methanol, ethanol, or isopropanol can work well, also acetonitrile can work, not as well as most alcohols. If you need DMSO or DMF, then you are not
going to extract anything from water, but they are great polar solvents.
|
|
|
adk
Hazard to Self
Posts: 97
Registered: 28-10-2014
Location: Sydney, Australia
Member Is Offline
Mood: No Mood
|
|
I generally prefer DCM as it allows faster workup & extraction from a sep funnel. As a general rule, the organic layer is going to be on the
bottom meaning that you do not have to drain the aqueous layer every time you shake & extract.
Flammability is also a concern, plus ether is slightly soluble (edit!) in water so a certain degree.
I also use ethyl acetate quite a lot as it comes off relatively quickly and is probably much less harmful to health than DCM -and much easier to
dispose of the waste (not halogenated!).
YMMV though
[Edited on 26-3-2015 by adk]
|
|
|
| Pages:
1
2
3 |
![[*]](images/xpblue/default_icon.gif){kind=icon}
