Sciencemadness Discussion Board

halogenation of alkenes on alumnia/silica

bongaloo - 27-3-2008 at 22:26

Recently I've found interesting article describing bromination of alkenes with PBr3 on silica gel

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S...

"The reaction of several alkenes with 0.4 mol equiv. PBr3/SiO2 in dichloromethane at room temperature gave the alkyl bromides with Markovnikov regiospecificity in 50 - 100% yield and short reaction time."

I was wondering...:cool:
can I use Al2O3 (prepared by heating Al dust - does it need to be activated or can be used as is?) and aq. HCl instead of PBr3/SiO2?
can I replace DCM with any other solvent, e.g. petroleum ether?
how will this affect the yields?

[Edited on 28-3-2008 by bongaloo]

Nicodem - 28-3-2008 at 03:26

I'm puzzled by your questions. I don't understand several things you wrote, so I guess I should address then one by one:

- What does alumina (Al2O3) have to do with that method? Clearly normal alumina does not have enough free acidic OH groups to form HBr from PBr3 and the authors never made mention of alumina anyway. Did that thought of using alumina instead silica came from your side?

- Also, you seem to have misunderstood the reaction since it is not about brominating alkenes, if is a hydrobromination of alkenes, something quite different that gives a different product.

- Heating Al powder does not give you alumina. You have to actually burn aluminium to get alumina (this is very dangerous since the temperature gets to a couple of thousand Celsius, thus it is a bit hard to contain in a container). Of course, alumina can be simply bought and is extremely cheap, however there are many different types of different acidity/basicity. But again, the authors used silica, not alumina. (Of course, silica is also relatively cheap and easily available)

- Petroleum ether would also be inert to the reaction conditions, but unfortunately very few compounds are soluble in it. It is also very likely it would fuck up the reaction kinetics since the solubility of HBr in petroleum ether, is… well, is not like in CH2Cl2.

- Of course you can not use HCl(aq) instead of PBr3/SiO2. Obviously you can not get any hydrobromination using HCl.

azo - 28-3-2008 at 03:35

Thanks nicodem that explains it well.

regards azo

Klute - 28-3-2008 at 03:50

Silicagel is very easily made with HCl aq and sodium silicate solution ("waterglass"). The preparation changes the properties though, so you need to stick to strict proportions/time once you've found the ideal silica. I used home-made for TCCA/SiO2/DCM oxidations, also for coluum chromatography even though I can't get the right mesh size (too fine, i need some kind of screening method).
I suppose chloroform could be substitued with DCM if you can't find any, but you will need to try it out.

bongaloo - 28-3-2008 at 05:07

Quote:
Originally posted by Nicodem
- What does alumina (Al2O3) have to do with that method? Clearly normal alumina does not have enough free acidic OH groups to form HBr from PBr3 and the authors never made mention of alumina anyway. Did that thought of using alumina instead silica came from your side?

I've found it here, page 4:
http://classes.uleth.ca/200201/chem2600a/notesch9.pdf
I thought particle size is very important in surface-mediated reactions so alumina (I cold easily get very fine ground Al dust) seemed easier to use than milling silica from dehydrator packages.
Quote:

- Also, you seem to have misunderstood the reaction since it is not about brominating alkenes, if is a hydrobromination of alkenes, something quite different that gives a different product.

my bad:)
Quote:
- Petroleum ether would also be inert to the reaction conditions, but unfortunately very few compounds are soluble in it. It is also very likely it would fuck up the reaction kinetics since the solubility of HBr in petroleum ether, is… well, is not like in CH2Cl2.

OK, I'll stick with DCM;)
Quote:
- Of course you can not use HCl(aq) instead of PBr3/SiO2. Obviously you can not get any hydrobromination using HCl.

again my bad, I meant analogical reaction using Al2O3/HCl, something like this:
http://cat.inist.fr/?aModele=afficheN&cpsidt=4719179
will water from aqueous HCl interfere with the reaction or significantly decrease yield?

sorry about the confusion and thanks for reply:)

Nicodem - 28-3-2008 at 06:47

Quote:
Originally posted by bongaloo
again my bad, I meant analogical reaction using Al2O3/HCl, something like this:
http://cat.inist.fr/?aModele=afficheN&cpsidt=4719179
will water from aqueous HCl interfere with the reaction or significantly decrease yield?

That question is already answered in JACS paper you referred to.

If you read that paper you will see that using just any alumina is not particularly wise. The results were quite variable and highly dependent on adsorbant treatment (moisture content) and reaction conditions/substrate/source of HCl.

Some very nucleophilic alkenes can be hydrochlorinated with conc. HCl(aq) in a biphasic system in the presence of quats as phase transfer catalysts, however you do not mention what your substrate is, so nobody can give you any advice on the possibility of using aqueous HCl.

Edit 1: Took a minute more to read the paper and noticed the authors actually solved the problem of unreliability of moisture content by finding the moisture equilibration temperature where alumina or silica gave best results: "The best results were obtained when the adsorbents were equilibrated with moisture in the atmosphere at 120°C for at least 48 h prior to use."

Edit 2: By googling around I found out that my previous statement: "Clearly normal alumina does not have enough free acidic OH groups to form HBr from PBr3," is completely wrong. Apparently, the surface of most types of alumina is just as covered with acidic hydroxyl groups (hydrated) as that of silica. I apologize, but I'm pretty much clueless when it comes to materials science and surface chemistry.

[Edited on 28/3/2008 by Nicodem]