I'm looking for a bit of help converting p-bromoaniline to bromobenzene.
I can do the deazotisation to afford a halo-bromobenzene but I want direct conversion to the mono-bromobenzene.
Any ideas? It's been a long time since I've done any proper chemistry and my head hurts...
Regards,
Jim.panziandi - 13-9-2008 at 12:12
Two ways jump to mind.
First would be to form the diazonium compound and reduce it with hypophosphorous acid which will replace the nitrogen with a hydrogen yielding
bromobenzene directly.
Second method would be to form the diazonium compound and treat it with potassium iodide to yield the 1-bromo-4-iodobenzene then treat with correct
amount of magnesium to yield the 4-bromophenylmagnesium iodide which could be treated with water to yield bromobenzene (this method is likely to be
contaminated with iodobenzene and dibromobiphenyl from a coupling)Omniata - 13-9-2008 at 12:30
Many thanks...
Any ideas on where I can purchase sodium or calcium hypophosphite in the UK??
Regards,
Jim.panziandi - 13-9-2008 at 12:40
Nope sorry never seen it for sale! Hypophosphorous acid was for sale on ebay some time ago IIRC.kclo4 - 13-9-2008 at 14:07
Hypophosphorous acid looks really difficult to get, it is a DEA list 1 chemical and its useful.
I don't think it could be made very easily either, Seems like you'd need to mess around with phosphorous or phosphine.
hyposulfurous acid is unstable, but maybe it could be formed in situation and preform similarly? I have read it is a reducing agent, but I don't know
if it would work.panziandi - 13-9-2008 at 14:13
Well I suppose if you dont have white P or plentiful supply of zinc phosphide your pretty stuck....! Perhaps the diazonium group could be displaced
for H by something else? I know metals and acid reduce it to the hydrazine group IIRC.kclo4 - 13-9-2008 at 14:22
Could you explain what you mean a little better?
It sounds like your talking about something similar to a Clemmensen reduction.panziandi - 13-9-2008 at 14:32
I was basically saying that hypophosphrous acid is a reducing agent and is able to replace the diazo group with a hydrogen. Well I know taht the
diazonium group can be reduced to yield hydrazine derivatives using sulphites, I know metals and acid eg Zn/HCl can reduce them too, maybe to
hydrazines (i dont have any books at hand). So I'm saying perhaps another reducing agent can be used instead of hypophosphorous acid (but obviously
not sulphites and not Zn/HCl).
I'm googling possible alternatives now... It's a shame H3PO2 is controlled.kclo4 - 13-9-2008 at 14:41
Oh haha, I get it now, I thought you were saying Zn/HCl would work, and I was pretty sure it wouldn't.
What is the name of the reaction for reducing a Nitro into an Amine?not_important - 13-9-2008 at 15:02
Old school diazo reductions involved dropping the diazo salt into boiling alcohol; some ArOH and ArOR may also be formed. Don't have a specific prep
right at hand, but an overview is here:
This link says that the diazo group can be replaced by a "number of reducing agents" including hypophosphous acid and primary alcohols.Omniata - 14-9-2008 at 08:11
Any more ideas???
Could you use Chromium (II) Acetate to debrominate the bromoaniline, then perform the deazotisation with sodium bromide to afford bromobenzene?
Or better still for increasing reactivity use sodium iodide to create iodobenzene??
[Edited on 14-9-2008 by Omniata]
[Edited on 14-9-2008 by Omniata]panziandi - 14-9-2008 at 11:18
I don't know of chromium (II) acetate working on aromatic halides... any one have any knowledge of this?Sauron - 14-9-2008 at 20:22
Back to basics. Just hydrolyze the diazo group with acid and you replace -NH2 with -H and you have your bromobenzene.
Better read Vogel again. You will find as an example, preparation of benzene from aniline in the chapter on diazonium salts.
The presence of the p-Br substituent will not hinder this.
This is much more direct than removing the bromo group then putting it back on via diazotization.
[Edited on 15-9-2008 by Sauron]Omniata - 15-9-2008 at 09:29
Needs to be a reducing acid though, your adding to a +ve centre so H+ won't attach and the -ve ion will.
If I were to debrominate I could then convert the aniline to iodobenzene which would be better for grignard chemistry...Sauron - 15-9-2008 at 09:39
Then the pregnant question is why are you starting from p-bromobenzene when you can easily prepare iodobenzene from aniline?
My impression is that you are not disclosing everything, so all attempts to assist you are rendered futile.Omniata - 15-9-2008 at 09:46
I'm starting from p-bromoaniline...
The required products being either:
1) bromobenzene by deazotisation and reduction of p-bromoaniline with H3PO2 which is DEA schedule one so not an option at the minute, or,
2) either "bromo" or "iodo" benzene by debromination of p-bromobenzene followed by deazotisation of the aniline formed with either potassium bromide
to afford bromobenzene, or, potassium iodide to afford iodobenzene.
[Edit] Sorry wasn't clear, the aniline would have come from the debromination of the p-bromoaniline.
[Edited on 15-9-2008 by Omniata]Sauron - 15-9-2008 at 10:11
There are several examples in Vogel of the preparation of various substituted benzenes by the alcoholysis of the diazonium salt. No DEA-listed
materials involved.
See prep of m-bromotoluene and of 1,3,5-tribromobenzene.
But you have not answered my question. Why are you proceeding from p-bromoaniline, if what you want is iodobenzene, so easily made from aniline, which
is a lot cheaper?
Hopefully this is not a pedagogic exercise, as you really ought to do your own homework.Omniata - 15-9-2008 at 10:30
I can't source aniline at the moment unfortunately and I wanted to use the p-bromoaniline that I have.
I'm a former student of chemistry at UMIST, UK, about 8 years ago. Since leaving I've done very little chemistry and now only do it as a hobby to keep
me sane.
If you did the alcholosys with MeOH there'll be quite a proportion of p-methoxy substituted bromobenzene won't there??
I've been struggling a bit to get my chemistry "hat" back on and was looking for a little help to get me going. I'm diabetic and through numerous
hypos and poor treatment by my doctors I now have an attention span that's starting to drift into the negative haha...panziandi - 15-9-2008 at 10:45
I heard on the grape vine magnesium and isopropanol can dehalogenate arenes (dunno details - only heard about it)
Perform the alcoholysis with hot ethanol?
Iodo compounds are not favoured for Grignards usually since they couple easily, you are better off trying to get the bromobenzene.
Get the diazonium compound first then go with either an alcoholysis using boiling primary alcohol e.g. ethanol OR conversion to 1-bromo-4-iodobenzene
and treat with Mg and then with water to remove the iodo (though you will have side products too).
If you have plenty of bromoaniline can I suggest you try both methods and calculate a % yield on a semi-micro scale then scale up with the better
option based on % yield?Sauron - 15-9-2008 at 11:22
If iodobenzene is what you really want (despite panziandi's remarks) then you can make it easily from benzene and iodine using fuming HNO3 - readily
prepared from KNO3 and H2SO4 - since you say you can't source aniline.
Uou could of course also simply source nitrobenzene and reduce it, or nitrate benzene to get it and reduce it. Those are very simple easy procedures.
I didn't say anything about methanol. I said ethanol and so does Vogel. Do read Vogel, and very likely Org.Syn where he usually got his preps.
[Edited on 16-9-2008 by Sauron]
[Edited on 16-9-2008 by Sauron]Sauron - 16-9-2008 at 02:32
As I have often said, about 90% of Vogel's preps in Practical Organic Chemistry are cadged without attribution from Org.Syn. quite shamelessly. To
make matters worse, Vogel's writeups are generally less details, less clear and not as well annotated and totally without literature references.
Therefore having noted the two preps in Vogel pertinent to the thread author's problem, I went looking for these in Org Syn and was not disappointed.
m-bromotoluene is prepared from the corresponding o-toluidine by diazotization followed by reduction with copper bronse. Acetaldehyde is a byproduct.
sym-Tribromobenzene is prepared from 2,4,6-tribromoaniline by diazotization in presence of H2SO4 and acidolysis of the salt with conc H2SO4.
I have combined these into a single pdf for posting as attachment in one go.
Question: as you are in UK why do you care that hypophosphorous acid is a DEA listed reagent? The DEA's writ does not run in the UK, and the acid is
available from EU suppliers. That acid is proscribed in USA because of its use in meth cookery, like red P and I2, but neither of those is a problem
to buy in UK. So I doubt hypophosphorous acid is either. Just do not order from a US company!
Though hypophosphorous acid is commonly used for the reduction of diazonium salts to aromatics, many other compounds from where the hydrogen atom is
easily abstracted also work. There are methods developed using various alcohols (like already mentioned), formamide, DMF, THF and other reducents.
Just search the literature or read a book/review on diazonium salts. You should be able to find plenty of alternatives to hypophosphorous acid. Given
that ethanol often works relatively well, I see no problems in using it given its availability.panziandi - 16-9-2008 at 04:21
You should be able to get hold of hypophosphorous acid in the UK if you shop around. It is not restricted and I doubt it would be a hassel, but
alcoholysis is much easier and likely to be a much cheaper option.Sauron - 16-9-2008 at 05:32
It was not my intention to push this guy back toward hypophosphorous acid. Rather, I was just wondering aloud at the illogic of his statements that he
is in the UK and that he can't get something because of its DEA status. A non sequitur.
Reading a good review on diazonium chemistry and following up with its cited references would be a very wise move.not_important - 16-9-2008 at 06:26
Note that both of the OrgSyn examples given are alcohol-based reductions, one using a copper promoter and the other just hot acidic solution.
The method can produce varying amounts of the corresponding ether of the aromatic substrate, although that is favoured by low temperatures; overly
water-diluted solutions will also give some of the corresponding phenol. Adjustment of the conditions, including choice of alcohol, can minimise
those problems, the reference I gave notes that with your starting material
Quote:
The diazonium sulphates from p-chloro- and p-bromo-aniline furnish only p-chloro- and p-bromo-benzene with ethyl alcohol, but with methyl alcohol they
give rise mainly to the methyl ethers of p-chloro- and p-bromo-phenol
For simple compounds you can likely find detailed preparations in older org-lab methods and "practical" org-chem books.panziandi - 16-9-2008 at 07:58
kclo4 was the poster who originally mentioned the DEA schedule status which i think the author of this thread took up thinking perhaps it meant it was
not possible to buy in this country as it may be scheduled here too, which is not the case.
alcohlysis with hot ethanol say rectified spirit would yield a high % of the arene with very little by product (ether and phenol)Sauron - 16-9-2008 at 08:33
In this thread people have referred erroneously to p-bromobenzene, but of course there is no such thing. Just bromobenzene. It's an unworthy
mistake.Anyway glad I could be of assistance.panziandi - 16-9-2008 at 15:18
indeed Sauron you have a point there!
Now with that p-bromoaniline you have there you could open up many possible doorways.
Have you considered making these too:
4-bromophenol and methylating to 1-bromo-4-methoxybenzene (p-bromoanisole), or methylate the diazonium with methanol - can be used for Grignards too!
4-bromobenzonitrile - that bi-functional arene could be quite usefull!
1-chloro-4-bromobenzene and also 1-bromo-4-fluorobenzene possibly interesting chlorophenyl and fluorophenyl Grignards
You can make 4-bromothiophenol by thiolysis with a thiol
Diazoniums are very useful!panziandi - 22-9-2008 at 13:17
I have just found an interesting paper:
Sasaki, M. et al. Bull. Chem. Soc. Jpn., 65, 1739-1740, 1992
Metallic Nickel-Catalyzed Reduction of Aryl Halides with Zinc Powder and Ethanol
Reports PhI 90% conversion to benzene in 24hrs, PhBr 47% conversion to benzene in 36hrs.
_ _ _
Now I'm thinking perhaps converion of the aniline to the iodo compound with potassium iodide then a reduction of teh compound using the method
outlined in in the ref. Conversion of the iodo would be more favoured than the bromo. Obviously it will not be a straight forward conversion but could
be useful for dehalogenation of a variety of halogenated arenes.
