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thunderfvck
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^
Yeah?
Forgive me as I haven't read the entire post with my full attention, but wouldn't you need a catalyst? It's cramming time for my
oragnic II test this Friday, and to halogenate benzene you need a catalyst, FeCl3 or whichever (in the limited perspective of my course on the
varities of catalysts). Toluene with its methyl group only slightly activates the ring to ortho/para substitution....Anyways, maybe I'm missing
something that you've already mentioned...
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madscientist
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The UV light catalyzes the reaction of halogens with toluene we're discussing. It breaks the halogen-halogen bond, creating very reactive free
radicals. Heat can sometimes do the trick. Alternative catalysts for this reaction are organic peroxides.
I weep at the sight of flaming acetic anhydride.
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Organikum
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Thunderfuck:
vulture wanted a sidechainhalogenation no ringhalogenation
Madscientist:
Maybe youre basically right, dont know, but some iodination takes place for sure.
vulture:
You shall not direct your hot air gun at maximum heat directly onto the flask but you may use a lower power setting and tangential input of the hot
air of course. So the air goes round and round between Al-foil and flask, let a small slit between foil and flask for it to escape there and you will
be fine.
You can even heat a mason jar this way.
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frogfot
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| Quote: | Originally posted by acx01b
what is the role of formaldehyde ???
wow ok its double addition ???
1st benzyl alcohol formed ?
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ZnCl2 suppose to make nucleophile from formaldehyde, which will in turn attack benzene.. so, yeah alcohol is formed. I guess HCl substitutes hydroxyl
(this is probably catalysed by Zn2+ or H+)
Samosa, that synth seemed to be easy to perform with regard to reagents, but yeah, now it seems a bit to dangerous..
Maby by changing the acid and salt catalyst one would avoid formation of chloromethyl.
[Edited on 13-5-2004 by frogfot]
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vulture
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I already pointed out that it isn't possible to make benzyliodide from I2 + toluene and Organikum already pointed out why.
Is it too hard to read a page of replies? Let's not go around in circles please.
I'll have to look into this hotair bath, thanks organikum.
Ofcourse, if a catalyst would allow direct reaction between I2 and toluene, I'm all ears.
Furthermore, bombarding the reaction mixture with UV isn't as easy as it seems. Borosilicate glass stops most UV radiation, except the wavelength
that is capable of splitting chlorine, but not bromine.
A NS fittable mercury vapor lamp is, well, just bloody expensive! It'll cost me three arms and two legs, which I don't have unfortunately.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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Organikum
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The catalyst of choice for sidechainhalogenations would be red phosphorus. This forms PX3 or PX5 (X=Cl or Br) in situ with the halogen (reaction
conditions should be somehow anhydrous here) and halogenates the sidechain fast and selective. Nice here: One PX3 molecule halogenates three toluene
molecules at once. Light pushes the reaction foreward but is not essential. Light and phosphorus allow the halogenation to proceed at lower
temperatures whats nice regarding the exothermic character of it. But dont get too low as this would cause ringchlorination.
vulture:
A usual 500W halogen lamp - the cheap ones yes, provides enough light for a bromination in borosilcate glassware - no problem.
But a catalytic amount of red phosphorus makes things really easy.
For those who say now:
No phosphorus!
Dont they sell they matchboxes at yours? A catalytic amount is not much. More phosphorus makes the reaction faster but for to get it going tiny
amounts suffice.
[Edited on 13-5-2004 by Organikum]
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vulture
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Hmm, so in fact, using PX3 or POX3 as halogenating agents should even be superior to using the halogen itself?
I think I'm going to have to have a chat with a chem supplier...
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Organikum
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Superior? Dont know. The amounts needed would be insane.
This a CATALYST.
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vulture
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What I'm trying to say is that I want to use PX3 as the halogenating agent, instead of using elemental halogens, would that work?
Don't worry about the amounts needed.
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Organikum
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I guess it should.
PCl5 is a wellknown chlorination agent similar or better as thionylchloride as I remember. But its a waste for such a simple task as the halogenation
of toluene.
Buy benzylchloride directly and save the PCl5 for somthing serious.
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FritzHaber
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okay, let me extract the essential
1) the reaction PhCH<sub>3</sub> + X<sub>2</sub> ---> PhCH<sub>2</sub>-X + HX
is a radical substitution rxn; it isn't an electrophile aromatic substitution or something similar.
as quoted, this reaction needs to be initialised by breaking the X-X (X = halogene) bond to form free radicals, beeing carried out by strong heating,
UV-irradiation or by an radical initiator like organic peroxides or diazo-compounds
2) Iodination is, as posted not really possible to do via simple halogenation -- given the poor stability of the C-I bond and the ease, which HI
reacts with the aralkyliodide (in fact, this reaction grounds the P/I<sub>2</sub> reduction of ephedrine to methamphetamine).
3) the benzyl iodide can be prepared either via nucleophile substitution of BzCl or BzBr with NaI in acetone or DMF ,
or, by a nice reaction:
Ph-CH<sub>3</sub> + I-Cl --(hv)--> Bz-I + HCl
iodine monochloride, ICl is prepared by simple reaction between I<sub>2</sub> and Cl<sub>2</sub> and then purified by
fraction-distillation.
[Edited on 14-5-2004 by FritzHaber]
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Organikum
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| Quote: |
iodine monochloride, ICl is prepared by simple reaction between I2 and Cl2 and then purified by fraction-distillation
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Iodine monochloride:
- mp: 27,2°C
- bp: 97°C (decomposition)
Says the Merck Index.
How do you want to fractional distill it when it decomposes? Ok. High vacuum perhaps. But dont forget: ICl forms I2O5 in the presence of air. So
vacuum distillation under inert atmosphere? This seems not very practical to me.
At least for those who donr have a complete lab by hand.
The Finkelstein swap - BzCl or BzBr to BzI by NaI in acetone seems much more conveniant.
But vulture seems to be satisfied with any benzylhalogen - no need for exotic tricks.
The most easy one is as I believe the BzBr with red phosphorus as catalyst and some light to speed the reaction up.
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FritzHaber
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okay, reading back, the iodine monochloride isn't really an ideal tool to prepare and work with in a kitchen lab..
also, I realised that it would, unless extreme conditions, as reaction in gaseous phase in presence of radical initiators, far more likely attack the
arene ring, so let us forget about ICl
you can made benzyl chloride f.e. from benzyl alcohol, ZnCl<sub>2</sub> and HCl<sub>(g)</sub>,
benzyl bromide could be obtained by bubbling chlorine gas through sodium bromide suspension in toluene...using the UV irradiation...but isn't it
particulary for benzyl chloride much more easy to obtain it from chem supplier??
I bought some BzCl (100g) from Merck few years ago, no curious questions about purpose or similar..
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vulture
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Where's the satisfaction in buying? I have the precursors, I have enough glass, I have a vacuum pump, etc...
A total workup from OTC compounds is the biggest satisfaction you can get IMHO.
I'm just doing it for the fun of it, no particular goal.
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Organikum
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| Quote: |
you can made benzyl chloride f.e. from benzyl alcohol, ZnCl2 and HCl(g),
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You dont need any ZnCl2 here. Just boiling benzylalcohol with HCl will produce already BzCl.
But we talk toluene here.
Come on vulture! Buildt a chlorine generator and Samosa and I will wellcome you in the famous "chlorine huffer club". 
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t_Pyro
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I've got a weird idea! How about oxidizing toluene to benzoic acid, and then reacting it with PBr<sub>3</sub>? That would produce an
acyl bromide. Reducing the carbonyl carbon could be a problem. Would Zn-Hg + conc.HCl work here? If it does, you'd end up with benzyl bromide.
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vulture
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Seems like a sure way to waste PBr3 to me...
Bromination of toluene should be relatively easy compared to chlorination.
I might try this in a few months.
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FritzHaber
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| Quote: | Originally posted by t_Pyro
I've got a weird idea! How about oxidizing toluene to benzoic acid, and then reacting it with PBr<sub>3</sub>? That would produce an
acyl bromide. Reducing the carbonyl carbon could be a problem. Would Zn-Hg + conc.HCl work here? If it does, you'd end up with benzyl bromide.
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no,
no it won't. upon reduction of acyl halogenides, you usualy obtain initialy aldehyde, this is then further reduced to alcohol, in the benzyl-
case even to the starting hydrocarbon (toluene)
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t_Pyro
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Well, if the aldehyde is an intermediate product, I guess the final product will mainly be toluene. Yuck, what a waste of reagents.
Regarding the light energy that is to be provided, how dependent is it on the wavelength of the light? Does the reaction perform better at a specific
wavelength, or are shorter wavelengths better?
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vulture
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The wavelength depends on the bond in X-X, that is the halogen used.
Necessary wavelength can be calculated through E = hv and dissociation enthalpy IIRC.
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Saerynide
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How do you store I2? Wrapping the stopper with teflon tape?
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thunderfvck
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I keep mine in the freezer, I don't like the fact that it sublimes like a motherfucker.
I think most people just keep it in a cool, dry place. The usual. With a screw on cap or something so the vapors don't have a chance of escaping.
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Saerynide
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Yeah, but I dont want anything diffusing out. Can you melt teflon tape and use that to seal the glass vial? Or does teflon tape just burn?
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Organikum
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No you cant melt teflon tape. But you dont have to melt it, for it is a sufficent seal if you wrap it in some layers. Over the teflon wrap tightly
some usual PVC-tape to keep the teflon from loseneing.
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vulture
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Wetting the teflon with a bit of solvent is enough to make it stick to the joint.
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