91Ó°ÊÓ

Any atom in a molecule that bears different substituents is called a stereogenic center. In this atom, the interchange of any two constituents leads to the formation of stereoisomers.

For an organic molecule, the stereocenter may be a carbon, oxygen, or sulfur atom.

The number of stereoisomers of a compound can be found by the formula $2^{\mathrm{n}}$ , where n is the number of stereogenic centers of the compound.

For carbon to be a stereogenic center, it has to be hybridized. Therefore, double-bonded and triple bonded carbon cannot be stereogenic centres.

In order to find the number of chiral carbon centers in a molecule, the substituents of tetrahedral carbon are analysed.

If all four substituents are different, it is a stereogenic carbon. The stereochemistry at a stereogenic center is indicated using dashed and wedge bonds.

a. Alcohols and acid chloride react to form esters. The given alcohol contains one stereogenic center. During this reaction, the stereochemistry of the reactant is maintained in the product.

Reaction of acid chlorides with alcohols

b. Sodium cyanide can replace halides in an SN2 mechanism. The reactant halide has one stereogenic centre. During the substitution reaction, the stereochemistry of halide is reversed.

Substitution of halide with cyanide

c. Alcohols and acids react to form esters. The stereochemistry of chiral carbon in reactant acid is maintained in the product ester.

Esterification reaction

d. acid chlorides react with 1 equivalent of amine to form amides, and further reaction with amine causes the formation of quarternium salts. Both the reactions maintain the stereochemistry of the reactant.

Reaction of acid chlorides with amines