Problem 43 - Chemistry The Central Science In Si Units

Question

Dichloroethylene \(\left(\mathrm{C}_{2} \mathrm{H}_{2} \mathrm{Cl}_{2}\right)\) has three forms (isomers), each of which is a different substance. (a) Draw Lewis structures of the three isomers, all of which have a carbon-carbon double bond. (b) Which of these isomers has a zero dipole moment? (c) How many isomeric forms can chloroethylene, \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{Cl}\), have? Would they be expected to have dipole moments?

Step-by-Step Solution

Answer

(a) Three isomers for dichloroethylene. (b) Trans-isomer has zero dipole moment. (c) Two isomers for chloroethylene, both with dipole moments.

Step 1: Understand the Molecule Composition

The formula given is dichloroethylene, \(\mathrm{C}_2 \mathrm{H}_2 \mathrm{Cl}_2\), which implies two carbon atoms, two hydrogen atoms, and two chlorine atoms. Each structure needs a C=C double bond given in the problem statement.

Step 2: Draw Lewis Structures for (a)

For dichloroethylene, distribute the two chlorine atoms across the two carbon atoms to form three distinct isomers. 1. Cis-isomer: Place both Cl atoms on the same side of the C=C bond. 2. Trans-isomer: Arrange Cl atoms across the double bond on opposite sides. 3. 1,1-dichloro isomer: Put both Cl atoms on the same carbon.

Step 3: Determine Dipole Moment for Isomers (b)

A molecule's dipole moment is zero if its charge distribution is symmetrical. The trans-isomer of dichloroethylene has a symmetrical structure, making it nonpolar with a zero dipole moment.

Step 4: Examine Chloroethylene Formulation for (c)

The formula for chloroethylene is \(\mathrm{C}_2 \mathrm{H}_3 \mathrm{Cl}\). Isomeric forms depend on how the Cl and H atoms are arranged:1. The Cl atom can be on one carbon, and hydrogen atoms fill the remaining positions.2. Asymmetrical distribution of hydrogen and chlorine makes any isomer polar, thus having a dipole moment.

Step 5: Conclude Number of Isomers for Chloroethylene

Chloroethylene can have two isomers based on the positioning of the Cl atom: one where the Cl atom is bonded to one carbon, and the second where it is bonded to the other carbon atom. Both will have dipole moments due to the asymmetrical distribution of charge.

Key Concepts

Lewis Structures

Lewis structures are a way to represent the arrangement of atoms within a molecule. They show which atoms are bonded together and the lone pairs of electrons that may exist in the molecule. In the case of dichloroethylene (\(\mathrm{C}_{2}\mathrm{H}_{2}\mathrm{Cl}_{2}\)), the key feature is the carbon-carbon double bond (C=C), which affects how the other atoms are arranged.To draw the Lewis structures for the isomers:

• Cis-isomer: Both chlorine atoms are on the same side of the C=C bond.
• Trans-isomer: Chlorine atoms are placed on opposite sides of the C=C bond, leading to a balanced distribution.
• 1,1-dichloro isomer: Both chlorine atoms are on the same carbon atom, creating a different configuration.

These different arrangements result in varying geometric shapes and chemical properties.

Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges within a molecule. It's a vector quantity, represented by an arrow pointing from the positive to the negative charge. Molecules with symmetrical charge distribution often have a net dipole moment of zero. In dichloroethylene: - The **cis-isomer** has an asymmetrical charge distribution, resulting in a dipole moment. - The **trans-isomer** is symmetrical across the C=C bond, canceling out dipole effects and resulting in a zero dipole moment. - The **1,1-dichloro isomer** is also asymmetrical, thus having a positive dipole moment. Understanding dipole moments helps in predicting molecular interactions like solubility and boiling points.

Cis-Trans Isomerism

Cis-trans isomerism is a type of stereoisomerism where the isomers differ in the position of substituents relative to a reference plane. It's commonly seen in alkenes due to the presence of a C=C double bond, which restricts rotation and allows for different spatial arrangements. - **Cis Isomer:** Substituents are on the same side of the double bond. This can lead to a polar molecule with a non-zero dipole moment. - **Trans Isomer:** Substituents are on opposite sides, often resulting in a more symmetrical structure with no net dipole moment. Cis-trans isomerism can affect a molecule's physical properties, such as its melting and boiling points, and can even influence chemical reactivity.

Chloroethylene Isomers

Chloroethylene (\(\mathrm{C}_{2}\mathrm{H}_{3}\mathrm{Cl}\)) can form isomers depending on the placement of the chlorine atom relative to the double-bonded carbon atoms. Even with a simpler formula than dichloroethylene, it displays unique isomeric forms.Potential isomers are:

• **1-Chloroethylene:** Where the Cl atom is bonded to one carbon while the other carbon is bonded to three hydrogen atoms.
• **2-Chloroethylene:** Where the Cl atom is bonded to the second carbon, and hydrogen atoms take the remaining positions.

Both configurations create asymmetrical molecules with non-zero dipole moments because of the polar bond between carbon and chlorine. These isomers can exhibit different chemical behaviors, impacting how they interact with other molecules.