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Author: Subject: Converting the configuration of (R)-2-butonal
bjon-07
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[*] posted on 16-11-2004 at 21:20
Converting the configuration of (R)-2-butonal


How do I convert (r)-2-butonal to (s)-s-butonal using toluenesulfonyl chloride tosyl chloride (tsCL) as a replacment leaving group sense -OH is such a poor leaving group.

I can normal preform this kind of reaction, if the nuclelophil is different than the orginal leaving group. I got to the point where i have (R)-2-butyl tosylate.

The leaving is TsO- I want to convert this into HO-. How should I do this? Should I add an protic acid, or Na OH.


BTW why do SN2 reaction invert the configuration. I am having a hard time visualling why they invert configuration.


Thank for your help.
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Geomancer
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[*] posted on 17-11-2004 at 11:30


If you want to invert 2-butanol (I assume that's what you're reffering to) using Walden inversion (I think that's what it's called), you have two problems. First, OH is a lousy leaving group. Second, even if you could get it to be replaced by another OH, there would be nothing to stop the reaction from happening again (hence racemizing the compound). The solution, of course, is to first replace the OH with a better leaving group, and then swap out that group for the new OH. You already have been given enough of a hint to figure this out. You know TosCl is involved. How does this react with alcohols? How does that help you?

Note that inversion only happens in some nucleophilic replacements: SN2 reactions. SN1's will proceed with racemization. You should be able to find pictures (and even animations) on the net and in the library, and my words can't compete with those.

[Edit: Oops, I see you mentioned you were only refering to SN2 reactions. Sorry.]

[Edited on 17-11-2004 by Geomancer]
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solo
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[*] posted on 19-11-2004 at 13:17
Ref:clarification of 2-butanol isomer


(r-2-butanol)...........................(s-butanol)

..............................................................................OH
..............................................................................|
r-CH3-CH2-CH-CH3------> CH3-CH2-CH2-C--H
......................|........................................................|
......................OH..................................................H

Is this what you're asking? As opposed to the isomer of r-2-butanol----------->s-2-butanol,
.......................OH
.........................|
S-CH3-CH2-CH-CH3 (s-2-butanol)

the answer would be different for the conversion ......solo




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