Problem 45 - 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

The three isomers of dichloroethylene are 1,1-dichloroethylene, cis-1,2-dichloroethylene, and trans-1,2-dichloroethylene. Among these, only trans-1,2-dichloroethylene has a zero dipole moment. Chloroethylene (C2H3Cl) can have two isomeric forms, vinyl chloride and chloroacetylene, and both would be expected to have dipole moments.

Step 1: Drawing Lewis structures for dichloroethylene isomers

There are three isomers of dichloroethylene. We will draw the Lewis structures for each of them while keeping in mind that there is a C=C double bond in each isomer. Isomer 1: 1,1-dichloroethylene Both chlorine atoms bonded to the same carbon atom Isomer 2: cis-1,2-dichloroethylene Two chlorine atoms bonded to different carbon atoms with H atoms on the same side Isomer 3: trans-1,2-dichloroethylene Two chlorine atoms bonded to different carbon atoms with H atoms on the opposite sides

Step 2: Determining isomers with zero dipole moment

To determine which isomers have a zero dipole moment, we need to check if the dipoles of the bonds cancel each other out. Isomer 1: 1,1-dichloroethylene There is an unequal distribution of electrons and therefore, it has a nonzero dipole moment. Isomer 2: cis-1,2-dichloroethylene Two dipoles cannot cancel each other out, as they both have the same direction. Therefore, it has a nonzero dipole moment. Isomer 3: trans-1,2-dichloroethylene Since the chlorine atoms are located on the opposite sides, the dipoles can cancel each other. This isomer has a zero dipole moment.

Step 3: Examining chloroethylene isomers

For chloroethylene (C2H3Cl), we need to find the number of isomeric forms it can have and determine if they have dipole moments. Isomer 1: Vinyl chloride The Cl atom is bonded to one carbon atom. Isomer 2: Chloroacetylene The Cl atom is bonded to the end carbon atom with the subsequent presence of a C≡C triple bond in the molecule. As there are only two isomeric forms, both of them would be expected to have a dipole moment due to the presence of the Cl atom which makes the electron distribution unequal, leading to a nonzero dipole moment.

Key Concepts

Lewis structures

Understanding the Lewis structures of molecules is fundamental in studying the nature of chemical bonding, as they visually depict the arrangement of electrons around atoms. For dichloroethylene (C_2H_2Cl_2), Lewis structures help illustrate the position of the chlorine (Cl) and hydrogen (H) atoms around the two carbons (C) connected by a double bond (C=C). The first isomer, 1,1-dichloroethylene, shows both Cl atoms attached to one carbon, witnessing a spirit of companionship in their placement. Contrastingly, the cis and trans-1,2-dichloroethylene sway the Cl atoms to separate carbon atoms, with the former sharing the same side as the H atoms, and the latter placing them on opposing terrains, like two knights on a chessboard. This visual distinction is key to predicting physical and chemical properties, which spring from these atomic orientations.

Dipole moment

Dipole moments are like compass needles pointing from the positive to the negative, indicative of polarity within a molecule. In our dichloroethylene isomers, we look for symmetry, or rather, its absence, which heralds a nonzero dipole moment. Isomers 1 and 2 don’t enjoy this symmetry; they're lopsided, so to speak, keeping their dipoles uncanceled, pointing in a definite direction. In contrast, isomer 3, the trans-1,2-dichloroethylene, finds balance, as if its Cl atoms are dancers in a perfectly choreographed ballet, allowing their dipoles to cancel out, resulting in a zero dipole moment. This is crucial when predicting physical behaviors, like the molecule’s interaction with an electric field, or its boiling point - quirks of character grounded in its molecular geometry.

Molecular isomerism

When diving into the world of molecular isomerism, we unveil the diverse personalities of molecules that share the same molecular formula. It's akin to identical twins with different interests - looks might deceive you, but they're unique in essence. Dichloroethylene fascinates us with three isomers, each a distinct compound despite having identical passports listing their elements. The 1,1-dichloroethylene favors union; its Cl atoms prefer the same carbon neighbor. But the cis and trans-1,2-dichloroethylene, while similar, opt for a different structure, a twist that gives each its individual temperament. This variety in structure allows molecules to express themselves through physical and reactive differences, making molecular isomerism a source of immense chemical richness and diversity.

Chemical bonding

Chemical bonding is akin to the relationships that hold a society together. In molecules, it's what maintains the structure and dictates the interactions. The double bond in dichloroethylene is equivalent to a strong, shared commitment between the carbon atoms; it's a union that offers stability and a platform for the rest of the atoms to relate. But not all relationships in the molecule are so steadfast. The precise arrangement of Cl and H around this central C=C bond shapes everything from reactivity to physical properties. It's this intricate and dynamic relational dance that gives each compound its unique identity and capabilities, and understanding the intimate nature of these chemical bonds is crucial to mastering the art of chemistry.