zbde00 - 18-4-2005 at 17:04
i have a substrate,CH3CH2OCH2CH2OH
i want to convert it to CH3CH2OCH2CH2Br
i have used PBr3,PPh3+CBr4,but the yield is very low.
who can suggest a good way to finish this conversiton?please post it in detail.
what i care is the yield,not the cost.
thanks in advance
[Edited on 19-4-2005 by zbde00]
Magpie - 18-4-2005 at 18:25
I believe what you are working with is a cellosolve. As the ether linkage is relatively inert you should be able to treat this as an alcohol. I
can't give you a recipe but believe you should protonate the alcohol with a strong acid so a water group can leave. Then add your halide of
choice in salt form. Be sure to vent your acid gas to a suitable absorber so you don't breathe it. I'm sure the real organic chemists of
this forum can correct me if I've missed something here. This seems like organic chemistry 101. 
I sure don't think you should have to waste any high energy reactants like PBr3!
Sergei_Eisenstein - 19-4-2005 at 09:28
thionyl bromide?
solo - 19-4-2005 at 11:11
If you want to look at it as a primary alcohol, you will find that the tertiary alcohol is the easiest to halogenate folowed by the secondary then the
hardest would be the compound in question a primary alcohol.
Thionyl chloride would do the job but as a chlorinated compound, hence not sure if they make thionylbromide, as I've herd it's a nasty acid , not that thionyl chloride is a sweetheart ,if so that would be the solution. Other
halogenating agents would be the phosphorous bromides.........solo
Magpie - 19-4-2005 at 20:41
Solo I agree that the primary alcohol is the least favored for the route I proposed. In class we brominated n-butyl alcohol by this method using an
hour of reflux. Considerable workup followed to remove water soluble byproducts. My yield was 60.6%. I don't know what yield was expected.
The product IR spectrum indicated a 97% match to the library.
Now that I have reviewed my notes more closely I see that the H2SO4 we used as acid also reacted with the ether side product making it water soluble!
Therefore, this methods may well be totally inappropriate for a cellosolve. 
[Edited on 20-4-2005 by Magpie]
sparkgap - 19-4-2005 at 22:04
I find it curious that PBr<sub>3</sub> gives low yields for this reaction. 
zbde00, did you attempt to characterize the other products resulting from your synthesis attempt? There might be a small possibility that your desired
product can be made from the other products. Otherwise, knowledge of those should give a clue as to what went wrong.
Definitely, a solution to this quandry should no involve strong acid(s), since they have an unfortunate tendency to cleave the ether linkage in your
molecule.
solo, shouldn't the fact that the substrate in question is primary mean that this would be a good candidate for bimolecular nucleophilic
substitution (SN2)? The trend you described would be applicable if the unimolecular mechanism (SN1) was in effect.
If speculation be allowed, maybe the alcohol can be tosylated before being reacted with bromide ion. If I remember my trends correctly, bromide is
quite able to displace tosylate. For best results, I would think this would be best done in a polar aprotic solvent like DMSO or DMF.
Hope this helps.
sparky (^_^)
P.S. What solvents did you use, zbde00? Maybe that could have contributed to your low yield as well!
Organic Syntheses
Kinetic - 20-4-2005 at 10:42
There is a procedure given in OrgSyn for this transformation, but it isn't particularly nice. The 2-ethoxyethanol is used as the solvent so the
yield based upon it isn't high. See beta-Ethoxyethyl bromide for the preparation. Pyridine has also been used as solvent which I suspect may lead to a higher yield.
Since money is no object, you may be more interested in a novel procedure. Tetrahedron, 58, 8689-8693 (2002) details a
general synthesis of alkyl halides from alcohols using PPh3, a tetraalkylammonium halide salt and DDQ. The general procedure is below:
4.1. Typical procedure for the conversion of 1-dodecanol to 1-bromododecane
To a flask containing a stirring mixture of DDQ (1.2 mmol) and PPh3 (1.2 mmol) in dry CH2Cl2 (10 ml), was added (n-butyl)4NBr (1.2 mmol) at room
temperature. 1-dodecanol (1 mmol) was then added to this mixture. The yellow color of the reaction mixture was immediately changed to deep red. GC
analysis showed the immediate completion of the reaction. The solvent was evaporated. Column chromatography of the crude mixture on silica-gel using
n-pentane as eluent gave 1-bromododecane in 85% yield. The product was identified by its comparison with an authentic sample.
No reaction times are given, other than 'immediate completion'. The procedure was used on 2-ethoxyethanol to give the corresponding alkyl
bromide in 98% yield (by NMR).
zbde00 - 8-5-2005 at 17:16
thank everyone!i will try all the way you suggested until i succeed.