Problem 37 - Chemistry: The Central Science

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

Dichloroethylene has three isomers: 1,1-Dichloroethylene, cis-1,2-Dichloroethylene, and trans-1,2-Dichloroethylene. The trans-1,2-Dichloroethylene has a zero dipole moment due to the bond dipoles canceling each other as they are on opposite sides of the carbon-carbon double bond. Chloroethylene has two isomers: cis-Chloroethylene and trans-Chloroethylene, both are expected to have dipole moments due to the distribution of charge in their structures and differing numbers of hydrogen and chlorine atoms.

Step 1: (a) Drawing Lewis structures of Dichloroethylene isomers

The general formula for dichloroethylene is \(\mathrm{C}_{2} \mathrm{H}_{2} \mathrm{Cl}_{2}\). To draw the isomers, let's consider all possible arrangements of hydrogen and chlorine atoms around the double bonded carbons. The double bond between carbon atoms is a requirement. 1. First isomer (1,1-Dichloroethylene): Both chlorine atoms are connected to the same carbon atom (e.g. first carbon) while both hydrogen atoms are connected to the second carbon atom. 2. Second isomer (cis-1,2-Dichloroethylene): Both chlorine atoms are connected to different carbon atoms in such a way that they are on the same side (i.e., cis configuration). 3. Third isomer (trans-1,2-Dichloroethylene): Both chlorine atoms are connected to different carbon atoms in such a way that they are on the opposite sides (i.e., trans configuration).

Step 2: (b) Zero dipole moment isomer

Molecules with symmetry have a higher chance of having zero dipole moment since the individual bond dipoles can cancel out each other due to symmetry. The trans-1,2-Dichloroethylene has both chlorine atoms on the opposite sides, making bond dipoles along the CC axis equal and opposite in direction. Therefore, they cancel out each other, making the overall dipole moment zero. So, trans-1,2-Dichloroethylene is the isomer with zero dipole moment.

Step 3: (c) Number of isomers and their dipole moments for Chloroethylene

The general formula for chloroethylene is \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{Cl}\). Since there is a carbon-carbon double bond and one chlorine atom, we can have the following two isomers: 1. First isomer (cis-Chloroethylene): The chlorine atom is connected to one carbon atom, and one hydrogen atom is connected to the same carbon atom as the chlorine atom, while the remaining two hydrogen atoms are connected to the second carbon atom. In this configuration, chlorine and hydrogen atoms are on the same side, leading to a cis configuration. 2. Second isomer (trans-Chloroethylene): The chlorine atom is connected to one carbon atom, and one hydrogen atom is connected to the same carbon atom as the chlorine atom, while the remaining two hydrogen atoms are connected to the second carbon atom. In this configuration, chlorine and hydrogen atoms are on opposite sides, leading to a trans configuration. These two isomers have a distribution of charge in their structure due to the electronegativity difference between chlorine and carbon atoms. Both cis and trans chloroethylene isomers are expected to have dipole moments as their individual bond dipoles cannot cancel out due to the presence of three hydrogen atoms and one chlorine atom.

Key Concepts

Lewis Structures

Understanding the concept of Lewis structures is essential when studying the nature of molecules, such as dichloroethylene (C_{2}H_{2}Cl_{2})). Lewis structures, or Lewis dot diagrams, display the bonding between atoms in a molecule and any lone pairs of electrons that may exist.

For dichloroethylene, visualizing its Lewis structures involves recognizing the carbon-carbon double bond, and then arranging chlorine (Cl) and hydrogen (H) atoms around this bond. There are three possible isomers, and each presents a unique Lewis structure:

• The 1,1-dichloroethylene isomer exhibits both Cl atoms bonded to the same carbon.
• In cis-1,2-dichloroethylene, Cl atoms are on the same side of the double bond.
• In trans-1,2-dichloroethylene, Cl atoms are on opposite sides of the double bond.

Each representation captures the exact arrangement of atoms and thus, plays a pivotal role in understanding the molecule's geometry and properties.

Dipole Moment

The concept of dipole moment is tied intricately to molecular structure. A dipole moment arises due to the unequal distribution of charges in a molecule, and is a vector quantity indicating both magnitude and direction.

In the context of dichloroethylene isomers, we can determine the presence of a dipole moment by examining the molecule's symmetry. For instance, trans-1,2-dichloroethylene has mirror-image symmetry, causing its bond dipoles to cancel each other out and resulting in a zero dipole moment. In contrast, the cis isomer lacks this symmetry and exhibits a net dipole moment. Knowing whether a molecule has a dipole moment helps in predicting its physical properties, such as boiling point, solubility, and interactions with other molecules.

Geometric Isomerism

Geometric isomerism is a fascinating phenomenon observed in compounds with restricted rotation about a bond, typically a double bond, as in dichloroethylene. Geometric isomers have the same molecular formula but different spatial orientations.

The 'cis' and 'trans' prefixes become crucial in distinguishing these isomers. In the case of cis-1,2-dichloroethylene, the Cl atoms are adjacent to each other, leading to higher polarity and often different chemical and physical properties compared to the trans isomer, where the Cl atoms are at opposing ends, typically resulting in a nonpolar molecule with a zero dipole moment. Understanding geometric isomerism is essential as isomers can exhibit vastly different behaviors, which is important in fields such as medicinal chemistry and materials science.