Chemistry - STEREO ISOMERISM Question with Solution | TestHub

Pair (P): Identical

Structure 1: A sawhorse projection of a chlorodiol.

Structure 2: Another sawhorse projection of the same molecule.

Relationship: If you rotate Structure 2 by $180^{\circ }$ in the plane of the paper, it superimposes perfectly onto Structure 1.

The Cl group (bottom-left in P-2) rotates to become the Cl group (bottom-right in P-1).The CH${}_3$ group rotates to the other side similarly.The stereochemistry is preserved.

Pair (Q): Enantiomers

Structure 1: Newman projection of 2-bromo-3-chlorobutane.Front Carbon (with Cl): Cl is on the right, H on the left.Back Carbon (with Br): Br is up.

Structure 2: Newman projection of the same molecule.Front Carbon: Cl is on the left, H on the right. (Configuration is inverted compared to Structure 1).Back Carbon: Br is on the left. (Configuration is inverted compared to Structure 1).

Relationship: Both chiral centers are inverted (R,R $\leftrightarrow$ S,S or R,S $\leftrightarrow$ S,R). They are non-superimposable mirror images of each other.

Pair (R): Diastereomers

Structure 1: Sawhorse projection. Let's analyze the configuration. The substituents on the two carbons are arranged in a specific way (e.g., OH groups are syn-clinal).

Structure 2: Newman projection. By converting Structure 1 to a Newman projection or determining the R/S configuration:In Structure 1, let's say the configuration is $(2S,3S)$.In Structure 2, analysis shows the configuration at one carbon remains the same while the other is inverted (e.g., $2R,3S$).

Relationship: Stereoisomers that are not mirror images of each other (different configuration at one or more, but not all, chiral centers) are diastereomers.

Pair (S): Homologous

Structure 1: $H_{2}N-CH(C{H}_3)-COOH$. This is Alanine ($\alpha$-amino acid with 3 carbons).

Structure 2: $H_{2}N-CH(C_2{H}_5)-COOH$. This is $\alpha$-Aminobutyric acid ($\alpha$-amino acid with 4 carbons).

Relationship: They possess the same functional groups and general formula but differ by a $-C{H}_2-$ unit. Members of such a series are called homologues.