German - 4-8-2023 at 21:39
I know I retain stereo-chemistry going from alcohol to tosylate. And I know I retain stereo-chemistry going from PRIMARY alcohol to alkyl halide. But
do I retain stereo chemistry going from a primary alcohol to the tosylate then to the alkyl halide? Since the tosylate attaches at the oxygen I feel
that when the iodine replaces the oxygen that an inversion occurs???? Or no? Could someone please help me here. Thank you (I'm aware this is a dumb
question I should know).
[Edited on 5-8-2023 by German]
SyntheticFunk - 6-8-2023 at 21:42
It’s an inversion.
When substituting a functional group, it will either racemize due to a carbocation intermediate, or invert due to following an SN2 mechanism.
Primary carbocations (as one would expect for an SN1 mechanism) are quite unstable. Therefore, it is more likely that the reaction will proceed via an
SN2 mechanism and invert the stereochemistry.
The reason tosylate retains the stereochemistry Of the alcohol is because the alcohol isn’t being substituted by OTs. Rather, it is acting as a
nucleophile and attacking TsCl
Texium - 7-8-2023 at 05:28
SF is correct, but more importantly, there’s no stereochemistry to speak of if you’re working with a primary alcohol! Unless you have an
asymmetric secondary alcohol, you don’t have a stereocenter.
SyntheticFunk - 7-8-2023 at 11:41
Good point Texium, I had overlooked the fact that **the carbon of** primary alcohols can’t be chiral.
Edit: technically this is not 100% true either, as one could possible have a compound like fluorochlorobromomethanol, which would be chiral, though
unlikely to exist.
[Edited on 7-8-2023 by SyntheticFunk]
DraconicAcid - 7-8-2023 at 12:28
Well, they can be, but the stereocentre will be in another part of the molecule, and won't be affected by the reaction of the alcohol.
For example, (R)-2-methyl-1-butanol will give (R)-1-bromo-2-methylbutane.