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. \((\mathbf{b})\) 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 thev be expected to have dipole moments?

Short answer

The three isomers of dichloroethylene are: 1,1-dichloroethylene, cis-1,2-dichloroethylene, and trans-1,2-dichloroethylene. Only the trans-1,2-dichloroethylene isomer has a zero dipole moment due to its symmetrical charge distribution. Chloroethylene has 2 isomeric forms: 1-chloroethylene and 2-chloroethylene, both of which have dipole moments due to the unsymmetrical charge distribution.

Step-by-step solution

Understanding Dichloroethylene and Isomers

To draw the Lewis structures for dichloroethylene, we need to understand that it is composed of 2 carbon, 2 hydrogen, and 2 chlorine atoms. The isomers are molecules with the same molecular formula but with different structures or spatial arrangements of atoms.

Drawing Lewis Structures for Dichloroethylene Isomers

Since all isomers have a carbon-carbon double bond, the three possible isomers for dichloroethylene can be drawn by placing chlorine atoms on different carbons in the molecule: 1. 1,1-dichloroethylene: Both chlorine atoms are attached to the first carbon atom. Cl | H - C = C - H 2. cis-1,2-dichloroethylene: The chlorine atoms are on different carbon atoms, on the same side. Cl Cl | | H - C = C - H 3. trans-1,2-dichloroethylene: The chlorine atoms are on different carbon atoms, but opposite sides. Cl H | | H - C = C - Cl

Determining Isomers with Zero Dipole Moment

In order to find which isomer has a zero dipole moment, we need to look at the molecular geometry and electronegativity differences between atoms. 1. In 1,1-dichloroethylene, the two chlorine atoms have equal electronegativity and are attached to the same carbon atom. This results in an overall non-zero dipole moment, as there is an unsymmetrical distribution of charges. 2. In cis-1,2-dichloroethylene, the two chlorine atoms are on the same side of the molecule and have similar electronegativity. Hence, there is an overall non-zero dipole moment due to the unsymmetrical distribution of charges. 3. In trans-1,2-dichloroethylene, the two chlorine atoms are on opposite sides of the molecule, making the molecule symmetric in terms of charge distribution. This results in a zero dipole moment. So, trans-1,2-dichloroethylene is the isomer with a zero dipole moment.

Counting Isomeric Forms for Chloroethylene

In chloroethylene (\(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{Cl}\)), there are 2 carbon, 3 hydrogen, and 1 chlorine atoms. Since we still have a carbon-carbon double bond, the only possible isomers would include one with the chlorine atom on the first carbon atom and one with the chlorine atom on the second carbon atom: 1. 1-chloroethylene: Chlorine is attached to the first (primary) carbon atom. Cl | H - C = C - H - H 2. 2-chloroethylene: Chlorine is attached to the second (secondary) carbon atom. H Cl | | H - C = C - H - H Chloroethylene has 2 isomeric forms.

Checking for Dipole Moments in Chloroethylene Isomers

Both 1-chloroethylene and 2-chloroethylene have unsymmetrical charge distributions due to the electronegativity difference between carbon, hydrogen, and chlorine atoms. As a result, both isomeric forms of chloroethylene would be expected to have dipole moments.

Key concepts

Isomers

Isomers are fascinating in the world of organic chemistry because they allow molecules with the same composition to exist in different forms. These different forms arise from varied arrangements of the atoms in the molecule. Even though isomers have the same molecular formula, their physical and chemical properties can be vastly different. In the case of dichloroethylene - 1,1-dichloroethylene has both chlorine atoms bonded to one carbon - cis-1,2-dichloroethylene has chlorine atoms on the same side of the carbon-carbon double bond - trans-1,2-dichloroethylene places the chlorine atoms on opposite sides.
These structural variations allow each isomer to exhibit unique chemical behaviors.

Lewis Structures

Drawing Lewis structures helps us visualize how atoms are bonded within a molecule. They display all the valence electrons around an element, providing insights into the molecule's geometry and possible isomers. For dichloroethylene, Lewis structures clearly present - Double bonds between carbon atoms - Locations of chlorine and hydrogen atoms on each carbon.
By representing shared bonds as lines and unshared electrons as dots around the atoms, Lewis structures simplify complex molecules, making them easier to understand and analyze for properties like polarity and molecular shape.

Dipole Moment

The dipole moment is a measure of the separation of charge in a molecule. It arises due to differences in electronegativity between atoms and how they are arranged spatially. A molecule with a symmetric charge distribution generally has a zero dipole moment, indicating no separation of charges.
In dichloroethylene,

• 1,1-dichloroethylene has a non-zero dipole moment due to an unequal charge distribution as both chlorines pull in the same direction.
• cis-1,2-dichloroethylene also has a non-zero dipole moment because the chlorine atoms are on the same side, creating an asymmetric charge distribution.
• trans-1,2-dichloroethylene exhibits a zero dipole moment, thanks to its symmetrical arrangement of chlorine atoms that cancel out the dipole effects.

Understanding dipole moments helps predict a molecule's interaction with electric fields and its solubility in various solvents.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It plays a crucial role in determining the properties and reactivity of the molecule.
In dichloroethylene, a carbon-carbon double bond limits the rotation of atoms, resulting in different possible geometrical configurations. For example,

• 1,1-dichloroethylene is planar with both chlorines on the same carbon, offering unique angles and bond lengths.
• cis-1,2-dichloroethylene has a bent shape due to the chlorine atoms on the same side of the double bond.
• trans-1,2-dichloroethylene is linear, as chlorines are on opposite sides, maximizing symmetry.

Recognizing molecular geometry is essential for understanding chemical reactivity, polarity, and molecular interactions.