The order of priority of the four substituents attached to the stereocentre
in the starting material is Br > Et > Me > H as shown below. Thus, once
the molecule is
rotated so that the group of lowest priority is at the back, the priority
of the remaining substituents decreases in an anti-clockwise direction
so the absolute configuration is S. The 3D structure may be helpful
in correctly orientating the molecule.
The order of priority of the four substituents attached to the stereocentre in the product is I > Et > Me > H as shown below. Thus, once the molecule is rotated so that the group of lowest priority is at the back, the priority of the remaining substituents decreases in a clockwise direction so the absolute configuration is R. The 3D structure may be helpful in correctly orientating the molecule.
During the reaction, there has been no change in the relative priorities
of the four substituents attached to the stereocentre since the priority
order is halogen > Et > Me > H in both starting material and product. However,
the absolute configuration of the starting material and product are different
so the reaction must have gone with inversion of configuration. This is
a classic example of an SN2 reaction.
The order of priority of the four substituents attached to the stereocentre in the starting material is Cl > SMe > Me > H as shown below. Thus, once the molecule is rotated so that the group of lowest priority is at the back, the priority of the remaining substituents decreases in an anti-clockwise direction so the absolute configuration is S. The 3D structure may be helpful in correctly orientating the molecule.
The order of priority of the four substituents attached to the stereocentre in the product is SMe > OMe > Me > H as shown below. Thus, once the molecule is rotated so that the group of lowest priority is at the back, the priority of the remaining substituents decreases in an anti-clockwise direction so the absolute configuration is S. The 3D structure may be helpful in correctly orientating the molecule.
In this case, the starting material and product have the same absolute
configuration. However, the reaction has still gone with inversion of configuration
since the relative priorities of the substituents in the starting material
and product have changed. In particular, the chlorine atom had a higher
priority than the sulphur atom in the starting material, but the sulphur
atom has a higher priority than the oxygen atom in the product. To see
that the reaction has gone with inversion, imagine changing the OMe in
the product to Cl. The starting material and product would then be enantiomers
and so would have opposite absolute configurations, so the configuration
must have been inverted during the reaction.
The starting material contains two stereocentres, and the order of priority of the substituents around both of them is Br > CBrMeH > Me > H. In both cases, the group of lowest priority is already at the rear of the molecule, and the other three substituents decrease in an anti-clockwise direction so the absolute configuration at both stereocentres is S. The 3D structure may be helpful in correctly orientating the molecule.
The order of priority of the substituents around the stereocentres in the product are Br > C(SPh)MeH > Me > H and SPh > CBrMeH > Me > H. In both cases, the group of lowest priority is already at the rear of the molecule, and the other three substituents decrease in an anti-clockwise direction so the absolute configuration at both stereocentres is S. The 3D structure may be helpful in correctly orientating the molecule.
This substitution reaction proceeds with retention of configuration, though this is actually due to two consecutive inversions of configuration due to neighbouring group participation as explained in the simple answer. That the reaction has proceeded with retention of configuration can be seen either by noting that the relative priorities of the substituents attached to the stereocentres have not changed, and the stereocentres have the same absolute configurations in the starting material and product, or by replacing SPh in the product by Br, and noting that the starting material and product are then identical.
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