Problem 33 - Chemistry An Atoms First Approach

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?

Step-by-Step Solution

Answer

The difference in the number of unique compounds between 1,2-dichloroethane and 1,2-dichloroethene is due to the free rotation around the carbon-carbon bond. In 1,2-dichloroethane, with a single bond, free rotation allows for different orientations to be interchangeable, resulting in only one unique compound. In 1,2-dichloroethene, the double bond restricts rotation, giving rise to two distinct chemical compounds with different spatial orientations of the Cl atoms (cis and trans isomers).

Step 1: Understand the compound structures

First, let's draw the structures of 1,2-dichloroethane and 1,2-dichloroethene. 1,2-dichloroethane: H2C-CH2 with two Cl atoms attached to the carbons. The structure can be represented as: CH2Cl-CH2Cl 1,2-dichloroethene: H2C=CH, with two Cl atoms attached to the carbons. There are two different ways to represent the 1,2-dichloroethene, based on the orientation of the double bond: \(cis\)-1,2-dichloroethene: ClH2C-CHCl \(trans\)-1,2-dichloroethene: ClH2C-CHCl with the Cl atoms on the same side and on opposite sides of the double bond, respectively.

Step 2: Compare isomers in 1,2-dichloroethane and 1,2-dichloroethene

Now, let's compare isomers for these two compounds to understand the reason behind the number of distinct compounds. In 1,2-dichloroethane, the carbon atoms are single bonded, which allows free rotation around the bond. Therefore, any possible orientation of chlorine atoms will be interchangeable, and there is only one unique compound named 1,2-dichloroethane. On the other hand, in 1,2-dichloroethene, the carbon atoms are double bonded, which prevents free rotation around the bond. As a result, two distinct chemical compounds are formed: cis-1,2-dichloroethene and trans-1,2-dichloroethene, with the Cl atoms in different spatial orientations.

Step 3: Explain the reason behind the difference in the number of unique compounds

The reason for the difference in the number of unique compounds between 1,2-dichloroethane and 1,2-dichloroethene lies in the free rotation around the carbon-carbon bond. In 1,2-dichloroethane, with a single bond, free rotation allows for different orientations to be interchangeable, resulting in only one unique compound. In 1,2-dichloroethene, the double bond restricts rotation, giving rise to two distinct chemical compounds with different spatial orientations of the Cl atoms (cis and trans isomers).