> Problem 20 seems confusing inthat I do not understand why bonds
> are not broken at the central atom on both reactions. Arn't they both
> sn2?
> Hope.
Both of these syntheses are via Sn2 mechanisms, but they yield enantiomers
because of which constituent is used as a nucleophile. In the first
example, the 1-phenyl-isopropanol (I think) has its hydroxyl group
stripped of its hydrogen. This produces a potential nucleophile. When
C2H5Br is added, an Sn2 reaction takes place with Br as the leaving group
and there is no stereochemical change because no new stereocenters were
created and the old one (C2 in propanol chain) was unaltered.
In the second synthesis, there is a tosylate group attached to the
initail reagent. I guess this would be called 1-phenylisopropyltosylate.
(???) Anywho, the tosylate is a good leaving group so ethanol
(actually ethoxide) is used as the nucleophile. When the Sn2 reaction
takes place, the stereocenter is inverted, producing the enantiomer of the
previous reaction.
Hope, I hope this clears this up for you and gives you some hope, Hope. :)
Andy Glandon
President
Department of Redundancy Department