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Melgar
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Interesting benzyl chloride synth I discovered
I discovered this reaction a while ago, and don't think it's been documented, so here goes.
Try this: bubble chlorine gas into a flask of toluene until it's bright yellow-green and more or less saturated. Now add a catalytic amount of
lithium bromide. That's it. It seems this reaction needs some UV light to initiate it, but a fluorescent light works fine. Once bubbles start to
appear, move the flask away from the light. The bubbling should become more vigorous, and the color should change from yellow to clear. Once all the
chlorine is used up, the bubbling should stop, and there should be some salt at the bottom.
The bubbles are HCl gas, so presumably you would need two Cl atoms for each toluene molecule that gets chlorinated. The toluene can be boiled off and
collected and reused.
It might be possible to continue bubbling chlorine into the solution in order to chlorinate more of the toluene, or chlorinate it further. I never
tried this though.
I think sodium or potassium bromide should work, but I only had lithium bromide, so that's what I used. Perhaps someone else can test sodium or
potassium bromide? I don't have access to much in the way of chemicals or facilities at the moment.
I think it works because bromine is a lot more susceptible to free radical formation than chlorine is. The bromine forms radicals, brominates the
toluene, then the chlorine replaces it, forming another radical, which brominates another molecule of toluene, and so on. The reaction eventually
dies out as the bromine radicals find each other, but UV light splits them apart again. Anyway, it sure beats leaving a flask of toluene and a
chlorine generator out in the sun all day. You can have your reaction complete in under a half hour.
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psychokinetic
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Wouldn't the halogen radicals be attracted to the ring more than the methyl group?
Though, as it seems it's the same basic reaction as the chlorine generator way you described, I'm quite likely wrong.
“If Edison had a needle to find in a haystack, he would proceed at once with the diligence of the bee to examine straw after straw until he found
the object of his search.
I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.”
-Tesla
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not_important
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The halogen radicals preferentially attach the benzyl hydrogens, ring substitution is electrophilic and is added by a number of metal salts or other
Lewis acids. This is why industrial production uses the vapour phase, it greatly reduces the Lewis acid added ring chlorination.
Missing from this report seems to be any establishing of the product as benzyl chloride. And, yes, it's pretty similar to the standard method except
that the amount of chlorination is limited by the amount of chlorine dissolved in the toluene, which is likely rather low.
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497
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| Quote: | | And, yes, it's pretty similar to the standard method except that the amount of chlorination is limited by the amount of chlorine dissolved in the
toluene, which is likely rather low. |
I think he meant the advantage was that it didn't need continuous UV illumination. He mentions continuing to add Cl2 after the beginning of the
reaction. Yes a quantified yield would be nice..
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Melgar
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Err. I meant it might be possible to get other chlorides like benzal chloride. It is definitely possible to dissolve more chlorine in the
partially-chlorinated toluene, then use UV light to start the reaction off again. The amount of bromide determines how much free radical formation
needs to be done by the UV light. Even a very small amount of bromide GREATLY reduces the UV needed though. More bromide allows the reaction to be
self-sustaining in ambient light.
As far as what it is, I'd recognize that nasty smell anywhere. It is hard to figure out what yield is, especially when reaction vessels are test
tubes, but you can see the chlorine bubbles shrinking as they dissolve in the toluene, and the reaction releases a surprisingly large amount of HCl
gas when it's bubbling at top speed. I don't have the equipment to do an accurate assessment though.
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zed
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Guys like Vogel, suggest weighing the whole shebang.....Glassware included. But, the standard method usually involves gassing the solution until a
certain weight gain is attained. When you've logged enough weight gain, you are done.
But, those instructions are for macroscale, and you are working microscale. Not so easy to log weight gain, when the whole project only weighs a few
grams.
Personally, I think I would prefer adding the LiBr first, AND THEN bubbling Cl2 gas into the toluene. Mixing the reactants together, and then adding
the catalyst, seems a little perilous to me. Especially on a larger scale.
I like the idea a lot, it could make the chlorination a lot easier to run.
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Melgar
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Quote: Originally posted by zed  | Guys like Vogel, suggest weighing the whole shebang.....Glassware included. But, the standard method usually involves gassing the solution until a
certain weight gain is attained. When you've logged enough weight gain, you are done.
But, those instructions are for macroscale, and you are working microscale. Not so easy to log weight gain, when the whole project only weighs a few
grams.
Personally, I think I would prefer adding the LiBr first, AND THEN bubbling Cl2 gas into the toluene. Mixing the reactants together, and then adding
the catalyst, seems a little perilous to me. Especially on a larger scale.
I like the idea a lot, it could make the chlorination a lot easier to run. |
Well, weighing it can be difficult for a bunch of reasons, not the least of which is the fact that you're constantly losing gases. HCl does dissolve
in toluene and slowly evaporates from it.
The reason I mentioned adding the LiBr after saturating the toluene, is because it makes the reaction a lot cooler-looking and more dramatic -- you
can easily see that there's a reaction happening what with all the bubbles and the color change. Admittedly, once you've got the reaction down this
is no longer a concern, so the LiBr could indeed be added at the beginning.
Incidentally, if someone had access to elemental bromine, (something I've never been able to generate with any sort of purity) that could be used
instead, but this reaction will generate bromine in situ from a bromide salt, producing LiCl as a byproduct. Or NaCl, or KCl, if using a sodium or
potassium salt. I don't think iodine would work though, since iodine radicals probably wouldn't be reactive enough to pull a hydrogen off of toluene,
but I could easily be wrong.
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Panache
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nice post, however as NI said verifying you have a significant amount of benzyl chloride is essential even though you can smell the product it doesn't
give one any indication of how successful the synth was, other products may simply not be odorous or weakly so, and benzyl chloride is quite distinct.
What made you do the things you did, some detail as to your musings prior to trying it would be of interest, but best of all distill off the chloride
and give a yield of some sort.
But again, nice job!
the libr is simply acting as a halogen carrier, nice to know it works so well though.
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Melgar
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Well, I was familiar with the reaction between toluene and sulfuryl chloride, where it's initiated by a little organic peroxide. Unfortunately,
sulfuryl chloride is kind of a bitch to make, so I tried just dissolving chlorine in toluene and initiating that with organic peroxide. That actually
did work, but once the reaction finished you couldn't just dissolve more chlorine in it and have it start again. The key to this chain reaction
seemed to be free-radical formation by a weaker radical than chlorine, so I thought it might be useful to try bromine, which I knew could generate
radicals more easily than chlorine when exposed to light. Not only can bromine radicals be easily generated by exposure to light, but bromine can be
generated from bromide salts in a chlorine solution. Initially, I thought the displacement of the bromide ions by chlorine would generate the free
radicals that would start the reaction, but it turned out to just add orange elemental bromine to the solution. Of course, my disappointment didn't
last long, since the anticipated reaction started as soon as I stepped outside.
Anyway, part of my reason for posting this was because I moved recently and all my chemistry equipment is packed up and not with me right now, so I
was hoping maybe someone else might take interest in this reaction. I know plenty of people here have expressed interest in synthesizing benzyl
chloride, so I was pretty sure someone would be interested, especially since this one seems a lot easier than any other I know of. I do know for sure
that if you bubble chlorine into toluene with some bromine/bromide in it, most of the gas coming out of it will be HCl, not Cl2, so it's definitely
getting chlorinated. I'm hoping that this reaction, like the sulfuryl chloride/organic peroxide synth, chlorinates the methyl group exclusively and
not the benzene ring. Since the bromine radical isn't as reactive, I'm hoping it only reacts with the methyl group before being displaced by
chlorine, but I can't verify that this is the case.
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Melgar
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Huh. Apparently bromine is so damn easy to make form radicals, that it can be done with an incandescent light bulb. And then, if there's chlorine
around, it'll substitute the bromine, thus ensuring that only the methyl group's hydrogens get chlorinated. So according to my theory, this method
should generate fairly pure benzyl chloride. Check this out:
http://dx.doi.org/10.1016%2Fj.tetlet.2006.07.109
I'm a grad student at the moment, but in mechanical, not chemical, engineering. Still, I ought to take this up with one of the chemical engineering
professors at my university. It certainly seems to have potential value.
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Anders Hoveland
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Wow, I am fairly familiar with chemistry, but this seems unusual.
Are you saying that Bromine will react with toluene, if initiated by sunlight?
I would think the radicals would be consumed faster than they are made.
Let me think, SO2Cl2 and toluene, I would not think that would be reactive with toluene. What are the details of this reaction?
PhCH3 + Cl. --> PhCH2. + HCl
PhCH2. + Cl2 --> PhCH2Cl + Cl. --> PhCHCl. + HCl
PhCH2CHCl. + Cl2 --> PhCHCl2 + Cl. --> PhCCl3 + HCl
This reaction does not seem to generate excess Cl. radicals.
How is it self sustaining?
Also, in case some of you did not know, the nitronium cation preferentially attacks the ring because the positive charge is initially transfered to
the ring, creating a sort of charged radical, then the NO2 reacts with it. Otherwise, obviously the methyl group would be more reactive.
[Edited on 28-6-2010 by Anders Hoveland]
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psychokinetic
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I believe he said the sunlight (UV) would radicalise the bromine, not initiate the reaction with toluene.
/I'm starting to feel like I'm feeding a new kind of troll.
[Edited on 28-6-2010 by psychokinetic]
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I was a sorry witness of such doings, knowing that a little theory and calculation would have saved him ninety per cent of his labor.”
-Tesla
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mnick12
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Well if I understand it correctly it is pretty simple. It is a free-radical halgenation. When any halogen (except iodine) is hit with UV it splits to
the reactive radical, which will attack one of the bezylic hydrogens forming HX. The newly formed benzyl radical is highly reactive so it to combines
with the halogen forming the benzyl halide. Also the reaction is not only initiated by UV but also sustained by it.
Anyways I think I got that right, but if anything is wrong let me know.
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not_important
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This is covered in Len's fine writeup here http://www.sciencemadness.org/talk/viewthread.php?tid=10490
X2 + hv => 2 X ·
X· + H-C(stuff) => XH + ·C(stuff)
X2 + ·C(stuff) => X· + X-C(stuff)
For X2 == Cl2 you need blue light or shorter wavelengths, for Br2 green and even yellow light will generate radicals. The radical chain propagates
for quite a few cycles, interaction with the walls, collision with another radical, and various other things eventually terminate a chain. So some
ongoing level of new radical generation is needed, generally sort sort of illumination; incandescent lamps, work, especially quartz-halogen, but the
generate a lot of heat relative the the number of effective photons. Fluorescent, mercury, and metal-halid lamps all worh, clear bulb Hg lamps are the
cheapest high intensity source.
For liquid phase halogenations of aromatics in more than a small volume, visible light is better than UV as the aromatic (toluene) absorbs the UV
strongly enough that much of the volume is "shaded" and the UV photons wasted. Again, see Len's prepub linked to above for some numbers.
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Nicodem
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| Quote: Originally posted by Melgar | | I do know for sure that if you bubble chlorine into toluene with some bromine/bromide in it, most of the gas coming out of it will be HCl, not Cl2, so
it's definitely getting chlorinated. I'm hoping that this reaction, like the sulfuryl chloride/organic peroxide synth, chlorinates the methyl group
exclusively and not the benzene ring. Since the bromine radical isn't as reactive, I'm hoping it only reacts with the methyl group before being
displaced by chlorine, but I can't verify that this is the case. |
Unfortunately, your experiment involves LiBr and in situ formed LiCl which both being relatively strong acids could catalyse the electrophilic
chlorination. So without analysing the reaction products there is no way to say that such a reaction selectively gives benzyl chloride. Chlorine does
not need much of a catalysis for electrophilic attack on toluene's aromatic ring, even traces of moisture catalyse this to some extent, so I would
tend to believe that LiCl might be quite efficient as well. If the benzylic chlorination truly involves Br2/light as the radical initiator, then you
should get the same result by just adding a drop of Br2 instead of LiBr and this way improve the chances of having a more chemoselective chlorination.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
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Melgar
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| Quote: Originally posted by Nicodem | | Quote: Originally posted by Melgar | | I do know for sure that if you bubble chlorine into toluene with some bromine/bromide in it, most of the gas coming out of it will be HCl, not Cl2, so
it's definitely getting chlorinated. I'm hoping that this reaction, like the sulfuryl chloride/organic peroxide synth, chlorinates the methyl group
exclusively and not the benzene ring. Since the bromine radical isn't as reactive, I'm hoping it only reacts with the methyl group before being
displaced by chlorine, but I can't verify that this is the case. |
Unfortunately, your experiment involves LiBr and in situ formed LiCl which both being relatively strong acids could catalyse the electrophilic
chlorination. So without analysing the reaction products there is no way to say that such a reaction selectively gives benzyl chloride. Chlorine does
not need much of a catalysis for electrophilic attack on toluene's aromatic ring, even traces of moisture catalyse this to some extent, so I would
tend to believe that LiCl might be quite efficient as well. If the benzylic chlorination truly involves Br2/light as the radical initiator, then you
should get the same result by just adding a drop of Br2 instead of LiBr and this way improve the chances of having a more chemoselective chlorination.
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Unfortunately, I don't have any pure Br2, but you are certainly right that it should produce the same result. Of course, the reaction also produces
large amounts of HCl, so it seems kind of pointless to go through the trouble of acquiring Br2 to avoid the formation of acidic salts. The reaction
needs to be exposed to light in order to happen; dissolving more chlorine in it in the dark will turn it yellow, but as soon as it's exposed to light,
the bubbles start again and it turns clear. It cannot be turned yellow by bubbling chlorine through it while the solution is exposed to bright light.
I would not be surprised if the LiCl or H2O (LiBr is one of the most hygroscopic salts that exists, and there's bound to be some water in it) or
especially the large amount of HCl that's generated by the reaction, was catalyzing ring chlorination, but the rate at which this happens is certainly
much slower than the rate at which benzyl chloride forms. Ring chlorination would not be initiated photochemically, right?
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Melgar
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| Quote: Originally posted by not_important | This is covered in Len's fine writeup here http://www.sciencemadness.org/talk/viewthread.php?tid=10490
X2 + hv => 2 X ·
X· + H-C(stuff) => XH + ·C(stuff)
X2 + ·C(stuff) => X· + X-C(stuff)
For X2 == Cl2 you need blue light or shorter wavelengths, for Br2 green and even yellow light will generate radicals. The radical chain propagates
for quite a few cycles, interaction with the walls, collision with another radical, and various other things eventually terminate a chain. So some
ongoing level of new radical generation is needed, generally sort sort of illumination; incandescent lamps, work, especially quartz-halogen, but the
generate a lot of heat relative the the number of effective photons. Fluorescent, mercury, and metal-halid lamps all worh, clear bulb Hg lamps are the
cheapest high intensity source.
For liquid phase halogenations of aromatics in more than a small volume, visible light is better than UV as the aromatic (toluene) absorbs the UV
strongly enough that much of the volume is "shaded" and the UV photons wasted. Again, see Len's prepub linked to above for some numbers.
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This is all true, however the reaction I've apparently discovered barely needs any light to sustain it compared to Len's reaction. A single LED from
a flashlight can generate enough photons to sustain this reaction, if incandescent bulbs are too inefficient.
Len also apparently has access to WAY more equipment than I ever did. I'd really like to analyze the products with a spectrometer, but since I don't
have access to one, my only recourse is to try and persuade other people to do it for me.
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mnick12
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Another thing that speeds up free radical halogenations is heat, one of my books mentions the free radical chlorination of methane using UV or simply
heating the mix to 100c. Also Meglar how dow make sure you only have benzyl chloride, and not benzal chloride or benzotrichloride?
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not_important
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Something to consider is that AFAIK the solubility of Cl2 in toluene is 1-2 moles/liter, and a mole of toluene is very roughly 100 cc, so you aren't
going to halogenate more that 10% to 20% of the toluene (which does reduce the chances of making the di or tri chlorides)
If you were to take say 20 cc of toluene and run this procedure on it, then stir it with NaHCO3 solution for awhile, then warm to say 40 C and
dropwise add KMnO4 solution with stirring until the pink colour persists, separate the aqueous and toluene layers, wash the toluene with a little cold
water. Toluene that was halogenated on the methyl, assuming it's mostly monochlorinated, will have been hydrolysed to benzyl alcohol and then
oxidised to benzoic acid, which sticks with the alkaline aqueous layer. Distill the toluene layer, chlorotoluenes have boiling points around 160 so
it's pretty easy to distill just toluene. The amount of toluene collected gives you an estimate of how much was halogenated, the distillation residue
with be chlorotoluenes and and likely any benzotrichloride that was formed. You can verify the presense of halogen in that fraction using a copper
wire flame test; benzotrichloride could be checked for by adding some to alcoholic AgNO3 and heating and possibly letting sit for several days. The
benzoic acid can be recovered from the aqueous solution by evaporation and treatment with HCl.
You could just fractionate the entire reaction mass, if you have a decent rig. Benzyl chloride boils about 18 C above the chlorotoluenes, so it has
to have reasonable fractionation capabilities.
A GC plus known samples of the possible halogenated products might do a better job of detailing results than an IR, it can be difficult to discern low
levels of other compounds if their spectra overlap too much.
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Nicodem
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I did a simple experiment regarding the possible role of bromides as (pre)catalysts for benzylic chlorination of toluene. For simplicity I used
trichloroisocyanuric acid (TCCA) instead of chlorine. TCCA can be weighted and does not need to be generated externally like chlorine. Besides it is
just as efficient at chlorinating the benzylic radicals so it should not make much difference. Instead of LiBr I used benzyltriethylammonium bromide
(TEBAB) as the source for the bromide ion, because its cation should not be able to coordinate with TCCA (which would increase its electrophilicity
and promote the undesired ring chlorination). I prepared a 0.5M solution of TCCA in toluene/ethyl acetate (1:1): 0.5M TCCA in PhMe/EA (thus a large
excess of the substrate, while EA is only added to increase TCCA solubility). The reaction mixtures and conditions tested were:
A) negative control for radical reaction conditions:
4 mL 0.5M TCCA in PhMe/EA on direct sunlight (benzylic chlorination of toluene with TCCA is known by using benzoyl peroxide as radical
initiator and heating, JOC, 35, 719–722, so it should work fine with sunlight as well, though not necessarily at room temperature)
B) negative control for test A:
4 mL 0.5M TCCA in PhMe/EA in the dark (electrophilic chlorination of toluene with TCCA does not proceed at any useful rate at room temperature
and without acid catalysis, but a control is nevertheless useful)
C) trial reaction:
4 mL 0.5M TCCA in PhMe/EA + 27 mg TEBAB (5 mol%) on direct sunlight (the mixture was shaken for 5 min prior to light exposure, but not all
TEBAB dissolved)
D) positive control for radical reaction conditions:
4 mL 0.5M TCCA in PhMe/EA + 37 mg t-butanol (25 mol%) on direct sunlight (t-BuOCl which forms via O-chlorination of t-BuOH is known to be very
easily homoliticaly cleaved to the corresponding Cl* and t-BuO* radicals by either heat or light, so we can expect t-BuOH to efficiently catalyze the
benzylic chlorination, hence considered a positive control)
E) negative control for test C:
4 mL 0.5M TCCA in PhMe/EA + 27 mg TEBAB (5 mol%) in the dark (to verify the need for light in reaction C)
These reactions mixtures were prepared in tapered glass vials and set either on direct sunlight or complete dark. Unfortunately TEBAB did not all
dissolve and thus the reaction mixtures C and E remained heterogeneous. The reaction mixtures C and D soon begun to heat up when placed on sunlight
while the reaction A remained at room temperature judging by touch. After 7 minutes cyanuric acid begun to precipitate from C and D practically at the
same time and both become hot (estimated to 60-70 °C). At about 9 minutes the reaction mixture A suddenly heated up with such speed that the cap was
blown off and 1/3 of the mixture boiled over. At 15 min all the reaction mixtures cooled back to room temperature, cyanuric acid precipitated, and
thus the reactions were considered finished. Meanwhile the reactions B and E which were all the time in dark did not show any sign of reaction (no
heating and no cyanuric acid precipitate).
HPLC analysis was performed on A, C, D and E reaction mixtures (MeCN/H2O (70:30 v/v), Nucleosil C18 reverse phase column, UV detection at 254 nm).
Chromatograms A, C and D were practically identical: essentially two peaks, one smaller at 5.30 min and one more intense at 6.12 min. Some minor peaks
could only be observed at retention times shorter than 4 min, but in that region only relatively polar compounds show up. All these reaction mixtures
had the characteristic benzyl chloride smell. Reaction mixture E showed only one peak at 6.12 min even after 1:20 h of standing in the dark. Reaction
B was thus not even analysed as it showed no sign of reaction either. HPLC analysis of pure toluene and benzyl chloride standards at the same
conditions gave retention times of 6.12 min and 5.32 min respectively. Reaction mixture C will be analysed with 1H NMR when time will allow in order
to additionally confirm the identity of the product and absence of ring chlorinated products (these could have the same retention time on HPLC).
Conclusions:
TEBAB and t-BuOH appear to catalyze the benzylic chlorination with TCCA already at room temperature. In the absence of catalyst the reaction mixture
goes to a runaway mode which is an indication that the reaction rate increases more rapidly at higher temperatures than the catalyzed reactions. In
all cases the only product detected chromatographically is benzyl chloride. In the absence of light no reaction occurs either in the presence or
absence of TEBAB. Extreme care should be taken when designing a preparatory version of any of these reactions, particularly of the noncatalysed one
(TCCA should be added in small portions to an excess of toluene!).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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DJF90
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Nice experiment Nicodem!
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Sedit
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Thank you Nicodem very nice.
So in its essence Meglar suspicions have been confirmed correct and the addition of a Bromine ion does indeed promote the chlorination of toluene
under better conditions then one normally deals with in such a reaction. I don't know if you remember sometime back myself having and issue with
runaways using hypochlorite as the chlorine source and this lead to runaways that made me deam the reaction worthless from my point of view. But with
this new infomation and the ability to avoid that runaway when a certine temperature is reached means I may once again have a go at reproducing a
variation of the conditions you just laid out but using a bromine salt as the halogen carrier instead out of accessablity.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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Magpie
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Thank you, Nicodem, for such a nice and well thought out experiment. This sets a standard for us all. A proper experiment with controls and results
is just so much more meaningful than endless speculation. 
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Anders Hoveland
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Let me get this straight: heating to 100degC will generate chlorine radicals? (mnick12) Can you please give a reference?
"benzotrichloride could be checked for by adding some to alcoholic AgNO3 and heating and possibly letting sit for several days."
(not_important)
I thought that only organic bromine and organic iodine reacted in a precipitation reaction, not chloride. However, 3 chlorines on the same carbon is
fairly electron withdrawing, so perhaps an electron could be donated to a chlorine through one of the double bonds/ delocalized bonding on the
benzene, somewhat similar to hydrolysis of 1,1,1 trichlo acetone.
"Chlorine does not need much of a catalysis for electrophilic attack on toluene's aromatic ring, even traces of moisture catalyse this to some extent"
(Nicodem) I thought chlorine was soluble in toluene, are you saying adding a drop of water to the mix will induce a reaction? Is the reaction rate
very slow?
This is a fascinating reaction, though I am not sure it is exceptionally useful.
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rrkss
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Thanks Melgar for sharing, now I have another project to consider for my grignard reagent work that I am currently doing. Since this reaction makes
benzyl chloride accessable to me, I am going to give it a shot.
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