Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: zsinger on February 06, 2014, 08:44:52 PM
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They say that a typical and normal Sn1 reaction does not usually lead to a near perfect racemate with no optical activity after days of reaction time. They claim an 85-90% racemate. I would argue that it is higher. Their reasoning is that when the leaving group departs, it "temporarily blocks" one of the faces of the carbon. I find that quite unlikely. Any help/suggestions/insight is so greatly appreciated, as I could only hope to know as much as many forum members here!
Cheers
Zack
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They say that a typical and normal Sn1 reaction does not usually lead to a near perfect racemate with no optical activity after days of reaction time. They claim an 85-90% racemate. I would argue that it is higher. Their reasoning is that when the leaving group departs, it "temporarily blocks" one of the faces of the carbon. I find that quite unlikely. Any help/suggestions/insight is so greatly appreciated, as I could only hope to know as much as many forum members here!
Cheers
Zack
Just like the continuum of pure ionic to pure covalent bonds, there is a continuum between SN1 and SN2 reactions. Look into concepts like intimate ion pairs etc. As reaction vessels have finite volume (though of course many times greater than molecular size) there may be blocking to one extent or another.
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It will also depend upon the stability of the carbocation intermediate. I don't know the source of the data, but I will guess that the more commonly available chiral halides are secondary. Secondary carbocations may not separate as well as tertiary ones to allow rotation to scramble the chirality.
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Not all SN1 reactions are created equally.
There are two possible outcomes here, one leading to solvent separated ion pairs (high probability of epimerization or racemization) and the other leading to a contact ion pair, where things get strange sometimes. In contrast to the solvent separated ion pair, a contact ion pair derived from a chiral compound IS chiral.
The contact ion pair SN1 proceeds via a mechanism "in between the SN1 and SN2." The carbocation is formed via first order kinetics, however the anion remains in close proximity to the cation, and varying degrees of the substitution may take place from the opposite face, leading to stereochemical inversion.
Please see March... Advanced Organic Chemistry, Stereochemistry of SN1 Reactions for a more comprehensive explanation.