Which of the following structures has the `R`-configuration at the chiral centre?

Options

A.
B.
C.
D.

Answer

A

Explanation

A priotity sequence (proposed by Cahn,Ingold and Prelog) is assigned to the four ligands bonded to the chiral center. The molecule is then visualized with the ligand of lowest priority directed away form the viewer, behind the plane of the paper. The arrangement of the other ligands determines the classification: if the sequence form the highest to the second lowest priority (`1 rarr 2rarr3)` si clockwise, the moelcule is specified `R`, and if the sequence is counterclockwise, teh molecule is called `S`.
For structure `(1)`, the priotty sequence is `OH gt CH_(3)CH_(2) gt CH_(3) gt H`. In the case of `-CH_(3)` next atoms in the chain are `H,H` and `H` while for the `CH_(2)` portion of `CH_(3)CH_(2)^(-)` these are `C,H` and `H`, clearly a higher priority.

Note that an even number of switches results in no change in the configuration.
`(2)` Priority sequence is `Br gt CH_(3)CH_(2) gt CH_(3) gt H`

`(3)` Priority sequence is `OH gt CH_(3)CH_(2) gt CH_(3) gt H`

`(4)` Priority sequence is `OH gt COOH gt CH_(3) gt H`