romanceliu - 22-9-2008 at 20:23
now i synthesis for 6-Bromo-1-hexanol from 1,6-Hexanediol .
the mixture 1,6-Hexanediol and 48%Hydrobromic acid dissolved in the Toluene . then raised to refulx ,reaction time is about 24 hours . the remove by
vacume , the product 6-Bromo-1-hexanol get by high vacume and high temperature.
but the yeild is low , about 45%. the purity is 97% and impurity is 2%. when i recycle , i find a lot of poly in the bottle ,
can u gei me some advices , what do u do when i want to improve yeild and purity ?
the reaction time is long . what cut time ?
when i want to scan up , i find a lot of poly in the reactor ,
can you gei me some advice ?
thanks ,
romanceliu
[Edited on 23-9-2008 by romanceliu]
Nicodem - 22-9-2008 at 23:10
Please write a proper account of the experimental procedure. People here do not read minds.
What function the toluene has in your reaction? And what do you mean by "dissolved in Toluene"? The 48% HBr(aq) does not dissolve in toluene - you
should have a biphasic mixture. How much 1,6-dibromohexane do you isolate? How did you confirm the identity of the product (is that just GC area
purity of whatever you got, or you actually characterized that product, or at least verified against an authentic standard?).
Polymeric crap is normal in such a reaction.
Sauron - 23-9-2008 at 14:20
The preparation of a bromohydrin like your target is best done not with hydrobromic acid but with 1 molar equivalent of a better brominating agent.
There is an embarrasment of choices of such reagents, many of which have been discussed here before. PBr3 in presence of a tert. amine is one. Gold's
Reagent (cyanuric chloride/DMF 1:2) in DCM with KBr is another. See my thread on mild and efficient halogenating agents. Ph3P/CBr4 is another very
good system.
Yields are high, conditions mild. Contamination by dibromide should be nonexistant.
Barium - 23-9-2008 at 16:19
It's sooo encouraging to see that the chem industry is operated by competent people. 
romanceliu - 23-9-2008 at 21:58
thanks , some advice, i try them ,
Sauron - 24-9-2008 at 02:32
The use of triphenylphosphine and carbon tetrachloride to chlorinate sensitive alcohols is illustrated in Org.Syn. for geranyl chloride. Same
procedure goes for bromides with CBr4. Reagents must be perfectly dry. Ph3P is air sensitive so do it under argon.
Note that Org.Syn. has two procedures in seperate articles for geranyl chloride so make sure you get the right one for Ph3P.
For the TCT/DMF/KBr prep of alkyl bromides, including bromohydrins, Lydia deLuca in Organic Letters, I will post the paper asap.
The reference 12 in the Org.Syn. paper is the pertinent one for Ph3P/CBr4.
A rapid, mild procedure for the preparation of alkyl chlorides and bromides
J. Hooz, S. S. H. Gilani
Can. J. Chem. 46(1): 86-87 (1968) | doi:10.1139/CJC-46-1-86
[Edited on 24-9-2008 by Sauron]
Attachment: CV6P0634.pdf (184kB)
This file has been downloaded 1394 times
Sauron - 24-9-2008 at 13:20
Here's the cited paper.
[Edited on 25-9-2008 by Sauron]
Attachment: cjc.pdf (129kB)
This file has been downloaded 913 times
laoqianxiu - 29-10-2008 at 22:37
the function of the Toluene is the extractant of targeting product
DNA - 30-10-2008 at 00:31
NBS in DMF works also really well to selectively bromate compounds in my experience.
Nicodem - 3-11-2008 at 00:37
Romanceliu, as difficult as he may be to understand, is not talking about electrophilic brominations. Instead he is doing a nucleophilic bromination
of a diol to a alpha,omega-bromoalcohol by substituting only one of the -OH groups with -Br. Therefore he can not use NBS, except if he first reduces
NBS with Ph3P like in the Appel reaction, but that would have no point since even simple&cheap HBr seem to work.
Sauron - 3-11-2008 at 03:23
Yes, Nicodem, but, but. He wants to improve yield. TCT/DMF/KBr in DCM gives near quantitative yields in 15-30 min at r.t., is mild, and is easy to
hold to monobromination of a diol. If he wants to employ cheap brute force reagents like HBr he will have to trade off yield for stoichiometric
control. Unless he wants to put a PG on the other hydroxyl, then he can use HBr in excess, and afterwards deprotect to obtain the bromohydrin.
And it is cheap. Conc HBr isn't free'