sonogashira - 27-9-2006 at 03:41
Is there any way to replace a bromide with a CH3, Et etc. group?
For instance bromopropane--------->butane (though obviously i would not try to make butane!)
I can only think of going via the grignard or organolithium, but i was wondering if there were some other simple routes for this conversion..
Any ideas?
[Edited on 27-9-2006 by sonogashira]
Nerro - 27-9-2006 at 04:42
Might AlBr3 or FeBr3 do the job?
I imagine H3CBr + AlBr3 <==> H3C+ + AlBr4- followed by H3C+ + H3CBr -> C2H6 + Br2 + AlBr3
I've never tried but I imagine someone has, maybe I'm just giving a standard example of poor thermodynamics here 
In that case I'd love to hear.
sonogashira - 27-9-2006 at 09:18
Thanks for the response Nerro. I may be wrong but the reaction you propose doesn't seem too likely to me.. (coupling of two cations, is that what
you're suggesting??) Perhaps i've misunderstood your suggestion?
If it did work, it seems that there would be little control over the products that one would obtain...
Nerro - 27-9-2006 at 10:16
Well it's an -let's say- adaptation of the Friedel-Crafts Alkylation. The reaction is utterly unlikely but since you're looking for an alternative to
the Grignard reaction there's not much else I can think of.
I meant the coupling of the cation and the neutral particle. Like I said it's entirely unlikely to work.
Grignard with Mg seems sensible enough, Mg costs next to nothing and the reaction is quite easy to perform. We did it multiple times in our freshman
year.
[Edited on Wed/Sep/2006 by Nerro]
sonogashira - 27-9-2006 at 10:31
Yeah the grignard route is easy enough, i was just wondering whether there is some funky chemical that would do the transformation... it seems not!
Nerro - 27-9-2006 at 10:41
Well it's probably possible to come up with something that can catalyse the reaction R-Br + R'-Br -> R-R' + Br2 but afaik it hasn't been made yet.
Somehow I wouldn't be entirely surprised if nature had already come up with some cool enzyme for this that we just didn't know about yet.
matei - 27-9-2006 at 11:10
There is a very simple reaction (Wurtz, 1855) for obtaining symmetric hydrocarbons from halogenoalkanes:
2 R-Br + 2 Na = R-R + 2 NaBr
Nerro - 27-9-2006 at 12:21
How does that work?
2CH3Br + 2Na -> 2NaBr + 2CH3.
2CH3. -> C2H6
As in, radical reaction?
matei - 1-10-2006 at 02:45
I don't know exactly how that works. It may be radicalic as you said or carbanions may be the intermeidates.
Nicodem - 1-10-2006 at 03:36
The Grignard reagents (RMgX) don't couple with simple alkyl bromides just like that. They only do so with "activated" alkyl bromides (for example,
benzyl or allyl bromides). Similarly, with alkyl lithiums one generally does not always obtain the coupling product with alkyl bromides, but the
metathesis product instead ( R'-Li + R-Br <=> R'-Br + R-Li ). I seem to remember that Cu(I) organometalic compounds can be used in such
couplings and in certain cases they can be formed in situ from a Grignard or organolitium reagent. But I'm not curious enough to check out
if I'm correct.
Sandmeyer - 1-10-2006 at 14:54
Wurtz gives quite bad yield and mixture of products unless only one type of alkylhalide is used, it works somehow OK for ring construction otherwise
it gives low yields, side reactions and it has very limited functional group tolerance. I would prefer to react alkylhalide with an alkylzinc reagent
(made from alkylhalide) and induce coupling with Pd or Nickel-catalysis in excellent yield and functional group tolerance. See: http://pubs3.acs.org/acs/journals/supporting_information.pag... supporting info is free of charge. I haven't read the above reference, it may not
be good, it was just picked out of literally thousands on this topic. I remember that I have once posted a very interesting review by Paul Knochel (of
course!) on the use of alkylzinc reagents in the synthesis of complex, highly functionalized carbogens, UTFSE for username Sandmeyer and search term
"Knochel".
[Edited on 2-10-2006 by Sandmeyer]