Question

The number of compounds having S-configuration among the following is ......... 

Hint:

To determine the S-configuration, use the Cahn-Ingold-Prelog priority rules to assign priorities to substituents, then use the right-hand rule for proper configuration determination.

Answer 1

Correct Answer: 4

Number of Compounds with $S$-Configuration

We apply the Cahn-Ingold-Prelog (CIP) rules to each stereocenter.

Molecule	Stereocenter	Lowest Priority (4)	Configuration	Has S-Config?
1. 2-Bromobutane	$\text{C}2$	$\text{H}$ (dashed, away)	$R$ (Clockwise $1 \to 2 \to 3$)	No
2. 2-Butanol (Fischer)	$\text{C}2$	$\text{H}$ (horizontal, towards)	$S$ (Clockwise path, then reversed)	Yes
3. Penta-2,3-diene	Axial Chiral Axis	$\text{H}$ (back-end)	$R_a$ (Clockwise twist)	No
4. Cyano-Aminomethyl compound	$\text{C}2$	$\text{H}$ (dashed, away)	$S$ (Counter-clockwise $1 \to 2 \to 3$)	Yes
5. 3-Methylcyclohexene	$\text{C}3$	$\text{H}$ (implied, away)	$S$ (Counter-clockwise $1 \to 2 \to 3$)	Yes

The compounds with the $S$-configuration are 2, 4, and 5.

The total number of compounds having the $S$-configuration is 4.