Sciencemadness Discussion Board

how to distinguish a solution containing chloroform from one with dichloromethane?

Ubya - 28-6-2020 at 17:40

in my endless journey of trying to distill DCM from solvents, i yet again failed to get a clear result.
in the span of 2 years, i distilled 5 different brands of italian solvents. Each of them claimed to have DCM, plus other solvents like methanol, toluene etc.
i already made a few posts crying about my fails, but the summary is this:
i usually add to the solvent i'm going to distill a large amound of anhydrous calcium chloride, to remove any possible water and methanol. i then remove the solvent layer by filtration if i don't get 2 layers, or i use a sep funnel. next i perform a simple distillation, or a fractional distillation, the next thing is always the same. the first boiling point i get, even heating slowly in the span of 1 hour, i always get the first fraction around 54-55°C, slowly rising to 64-65 and in the end rising slowly to whatever is the highest bp.
dcm has a bp of 41°C, and any azeotropes should lower it, but nope for me.
this first fraction, if added to water, doesn't go to the bottom, but floats, and when a flame is near, it burns (halogenated hydrocarbons like dcm and chloroform don't burn by themselves).

i searched, i searched, i searched, i can't find an azeotrope of dcm that would give these results.
today, after the latest failed distillation, i had an idea, if these fuckers lie so easily about the content of their solvents, why should they use DCM?
i tested pretty much every fraction of every tin of solvent with the beilstein test, to see where the chlorinated solvent was.
that's the thing though, i saw a positive test, and i would assume it meant DCM, because i trusted at least that, but today, i didn't, what if it was a halocarbon, but not DCM?
i tried with the first cousin of DCM, chloroform, and would you look at that, it has an azeotrope with hexane(bp 60°C), with methanol (bp 53°C) and acetone (most of the tins had acetone even if it wasn't mentioned on the composition, bp 64).
the numbers are pretty damn close to what i get every single time.

so, to be sure of what i have, finally after 2 years and i don't know how many sleepless nights watching drops of solvent fall from a condenser, is there a test i could do to differenciate dcm from chloroform (not in their pure form, i can't simply see the density or bp)?

i'm pretty desperate, my fumehood is full of small and big bottles of different fractions of different tins, i would like to just finish this and go on with my other projects

[Edited on 29-6-2020 by Ubya]

[Edited on 29-6-2020 by Ubya]

AvBaeyer - 28-6-2020 at 20:09

Ubya,

From "A Handbook of Organic Analysis" by H.T. Clarke and J.N. Collie several tests for chloroform are noted.

"On warming with aniline and alcoholic KOH it yields phenyl isocyanide (stench)." Note it is the isocyanide not the isocyanate.

"On warming with resorcinol and dilute NaOH gives a red color."

Chloroform, bromoform, iodoform and chloral hydrate can be detected in very small amounts. To 4cc of 20% NaOH are added enough pyridine to form a 2mm layer and a drop.the liquid suspected to contain one of these compounds. The mixture is heated to boiling and allowed to stand. A deep red color in the pyridine layer is a positive test for one off the above named compounds.

There is no specific test for DCM mentioned. I have also checked several other reference books I have and find no specific tests for DCM.

AvB

Sulaiman - 28-6-2020 at 20:38

A simple test for DCM is to put it in a sealed bottle and see how much escapes within a summer's week :P

To put it another way, unless your sample bottles seal really well,
any DCM that you may have had has probably gone by now :(

Assured Fish - 28-6-2020 at 20:55

Ill second AvBaeyer's suggestion for the dichlorocarbene test.
A ptc shouldn't be nessesary if enough agitation is used to mix the layers.
https://en.m.wikipedia.org/wiki/Carbylamine_reaction

Also something about your procedure seems off to me. If I were you I would be washing the paint thinner with several portions of distilled H2O as this would remove the miscibles such as acetone and alcohols from your solution. Neither chloroform or dichloromethane are miscible with water, you could then fractionally distill the solution over calcium chloride without too many of those lower boiling point contaminants.

Ubya - 29-6-2020 at 03:13

Quote: Originally posted by Assured Fish  
Ill second AvBaeyer's suggestion for the dichlorocarbene test.
A ptc shouldn't be nessesary if enough agitation is used to mix the layers.
https://en.m.wikipedia.org/wiki/Carbylamine_reaction

Also something about your procedure seems off to me. If I were you I would be washing the paint thinner with several portions of distilled H2O as this would remove the miscibles such as acetone and alcohols from your solution. Neither chloroform or dichloromethane are miscible with water, you could then fractionally distill the solution over calcium chloride without too many of those lower boiling point contaminants.


the first time i did exactly this, the solvent tin said only toluene, dcm and methanol, so i did a few washings with water, but dcm is quite soluble, 17g/L. even then i had strange boiling points, but i also had to deal with 1 liter of contaminated water (i usually burn chlorinated solvent wastes, but water is not that flammable.
now i got many tiny fractions, i'll try to wash them and remove any acetone present.

AvB, thanks a lot for the informations, i'll try to get the right reagents and try them out

XeonTheMGPony - 29-6-2020 at 04:12

I just distilled with a rectifying column with care full monitoring of heat, then from there I did post work ups.

Which in the end needed very little.

Perhaps just start by fractional via distillation first?

EDIT: I missed the part where you said you did that so never mind!



[Edited on 29-6-2020 by XeonTheMGPony]