How many constitutional isomers exist for a molecule with the molecular formula $C_2{H}_4{\mathrm{Cl}}_2$?

• A. $3$
• B. $4$
• C. $1$
• D. $2$

Answer: (D)

Verified step by step guidance

1. Identify the molecular formula: C\_2H\_4Cl\_2, which means the molecule has 2 carbon atoms, 4 hydrogen atoms, and 2 chlorine atoms.
2. Determine the possible carbon skeleton: With 2 carbons, the structure is an ethane backbone (C-C single bond).
3. Consider the substitution pattern: Since there are 2 chlorines, they can be attached in different ways on the ethane backbone, leading to different isomers.
4. Enumerate the possible placements of the two chlorine atoms on the two carbons: both chlorines on the same carbon (1,1-dichloroethane) or one chlorine on each carbon (1,2-dichloroethane).
5. Recognize that these two arrangements are distinct constitutional isomers because they differ in the connectivity of atoms, resulting in exactly 2 constitutional isomers.