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Stereochemistry is a way torepresent the three-dimension atoms in the molecule,thus affecting the chemical and physical properties of the chemical substance.

The stereochemistry of any chemical compound is studied with the help of its isomers:

• Structure isomer
• Stereoisomers

According to the Cahn-Ingold-Prelog rules, the upcoming nucleophile cyanide ion $\left({\text{CN}}^{\text{-}}\right)$ has the lowest priority than the leaving tosyl group $\left({}^{\text{-}}\text{OTos}\right)$.

When the reaction proceeds by reacting with sodium cyanide, the product formed by inversion in configuration, the priority of the substituents also changes, and the configuration remains the same, only that is S configuration.

The product formed has the lowest priority group hydrogen below the plane, so when the molecule is rotated according to the priority given to the substituent, it's anticlockwise hence s configuration.

Stereochemistry of 2-isocyno-1-methoxypropane