Question

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?

Short answer

There is only one compound named 1,2-dichloroethane due to the single bond between carbon atoms which allows for only one arrangement of atoms in the molecule. In contrast, there are two distinct compounds of 1,2-dichloroethene because of the double bond between the carbon atoms, which results in a planar geometry and allows for two different spatial arrangements of the chlorine atoms: cis and trans isomers.

Step-by-step solution

Understanding the basics of organic nomenclature

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.

Drawing the structure of 1,2-dichloroethane

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.

Drawing the structures of 1,2-dichloroethene

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.

Conclusion

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.

Key concepts

Nomenclature

Nomenclature in organic chemistry is like giving each compound a unique and descriptive name based on its structure. The importance of this naming system becomes apparent when distinguishing between similar compounds. Here’s how it works:

• Prefixes: In our example, "dichloro" indicates two chlorine atoms in the compound.
• Numerical identifiers: The numbers "1,2" tell us exactly where the chlorines are attached on the carbon chain. In this case, on the first and second carbon atoms.
• Suffixes: "Ethane" and "ethene" highlight whether the carbon atoms are linked by a single bond (alkane) or a double bond (alkene).

This structured naming convention helps chemists communicate clearly about the compositions and structures of molecules in a universally understood language, enabling precise identification and differentiation.

Alkenes

Alkenes are a class of hydrocarbons that contain at least one carbon-carbon double bond. This double bond is responsible for the interesting chemistry of alkenes and leads to certain characteristics:

• Unsaturated Hydrocarbons: Unlike alkanes, alkenes do not have the maximum number of hydrogen atoms per carbon atom, making them unsaturated.
• Reactivity: The presence of a double bond makes alkenes more reactive than alkanes. This is because the double bond contains pi electrons, which are more exposed and can participate in reactions such as addition.
• Planar Geometry: The double bond causes alkenes to have a planar geometry around the carbon atoms involved in the bond.

In the case of 1,2-dichloroethene, the presence of a double bond between the two carbon atoms allows for the unique configurations known as cis-trans isomerism.

Cis-Trans Isomerism

One fascinating feature of some alkenes is the ability to exist as geometric (or cis-trans) isomers. This isomerism is possible when there is restricted rotation around a carbon-carbon double bond, coupled with different groups attached to these carbons. Here are the main points about cis-trans isomerism:

• Cis Configuration: In the cis-1,2-dichloroethene, both chlorine atoms are situated on the same side of the carbon-carbon double bond.
• Trans Configuration: In trans-1,2-dichloroethene, the chlorine atoms are positioned on opposite sides of the double bond, leading to a different geometry.
• Properties: Cis-trans isomers can have different physical and chemical properties. For example, they may have different boiling points, solubilities, or reactivities.

This ability to form distinct isomers is why there are two unique compounds with the name 1,2-dichloroethene, despite having the same molecular formula.