There is only one compound that is named \(1,2-\) dichloroethane, but there are two distinct compounds that can be named \(1,2\) -dichloroethene. Why?

In organic chemistry, nomenclature is crucial in identifying the structure of a compound. In this case, we are given the names 1,2-dichloroethane and 1,2-dichloroethene. Here's what each part of the name means: - "Dichloro" indicates the presence of two chlorine atoms in the molecule. - The numbers "1,2" represent the carbon atoms to which the chlorine atoms are bonded. - "Ethane" indicates that the molecule is an alkane, with a single bond between the carbon atoms. - "Ethene" indicates that the molecule is an alkene, with a double bond between the carbon atoms.

1,2-dichloroethane is an alkane with two carbon atoms single-bonded to each other and two chlorine atoms bonded to the carbons. The structure can be represented as follows: H-Cl-C-C-Cl-H. Since there is only one way to arrange the atoms in this compound, there is only one compound named 1,2-dichloroethane.

1,2-dichloroethene is an alkene with two carbon atoms double-bonded to each other, while two chlorine atoms are bonded to the carbons. Due to the double bond, the two carbon atoms have a planar geometry. This results in two different spatial arrangements of the chlorine atoms, giving rise to two distinct compounds. Compound 1 (cis-1,2-dichloroethene): The chlorine atoms are on the same side of the double bond, which is called the "cis" configuration. \[ \text{H} \text{Cl} \text{C}=\text{C} \text{Cl} \text{H} \] Compound 2 (trans-1,2-dichloroethene): The chlorine atoms are on the opposite side of the double bond, which is called the "trans" configuration. \[ \text{H} \text{Cl} \text{C}=\text{C} \text{H} \text{Cl} \] Both of these compounds have the same name (1,2-dichloroethene) according to IUPAC nomenclature, but they have distinct geometries and properties due to the cis-trans isomerism.

In summary, there is only one compound named 1,2-dichloroethane since there is only one way to arrange the atoms in the molecule. However, there are two distinct compounds named 1,2-dichloroethene because, in this case, the double bond allows for two different spatial arrangements of the chlorine atoms, resulting in cis and trans isomers.