true_alchemy
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Rearrangement in sec-Alkyl halide preps
Anyone familiar with typical rearrangements when preparing halides from sec-alcohols? For example, I know that 2-pentanol gives a considerable amount
of rearrangment upon distillation with HCl and a salt but I don't remember what the biproduct is. 2,4-dimethyl-3-pentanol also rearranges if I
remember correctly. What about 3-methyl-2-butanol, 3-methyl-2-pentanol or 3,3-dimethyl-2-butanol?
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Marvin
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Maybe you are thinking of the carbocation problem.
SN1 reaction would indicate you form the carbocation of the alcohol and this reacts in a seperate step to form the alkyl halide.
+ charges tend to wander all over the molecule and sometimes this will break/form carbon bonds but the usual annoyence is just that this results in
other halides being formed. 1 and 2 propanol will both give amounts of 1 and 2 halides and since secondary carbocations are more stable we'd
expect the 2 halide to predominate. We also get this in freidel krafts reactions for example.
Some elimination products might also be expected with sulphuric acid...
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true_alchemy
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Thanks Marvin for responding. When it comes to generic terms for mechanisms I am at my knowledge limit. The term I use in my notes is
"c-shift" or "carbon shift." The chlorination of 2-pentanol w/HCl gave , I thought, about 20% of something like
2-chloro-3-methylbutane. I could be wrong on the biproduct but I thought I was sure it was a "methyl butyl?" I couldn't find my notes.
The rearrangement is generally only applicable to C5 and higher alcohols. I just made sec-butyl chloride this week w/HCl as I have done many times.
The only noticable biproducts are butenes, and a trace of what is probably sec-butyl ether. IPA acts the same.
3,3-dimethyl-2-alkanols typically shift or rearrange to the 2,3 by about 60%. I think the majority of 2,4-dimethy-3-pentanol shifts to the 2,3
also.
Anyway I tried a branched C7 sec- alcohol this week and finished just a few hours ago and found that it had completely rearranged. Too bad! There
is so much I could do if I could put a halogen wherever I wanted it. I have several other routes to try. Usually the isomers are too difficult to
separate and such chloros are often not stable enough for distillation anyway.
Of all alcohols below C5, I only know of the isobutyl to be a problem but I dont know if I actually ever tried n-propyl. For C5 and above I
don't know if I have ever had a sec- without rearrangement. Most tertiarys and many primarys work fine.
[Edited on 2-10-2004 by true_alchemy]
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