Problem 57 - Chemistry: The Central Science in SI Units, Global Edition

Question

(a) Draw Lewis structures for chloromethane \(\left(\mathrm{CH}_{3} \mathrm{Cl}\right),\) chloroethene \(\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{Cl}\right)\), and chloroethyne \(\left(\mathrm{C}_{2} \mathrm{HCl}\right) .(\mathbf{b})\) What is the hybridization of the carbon atoms in each molecule? (c) Predict which molecules, if any, are planar. (d) How many \(\sigma\) and \(\pi\) bonds are there in each molecule?

Step-by-Step Solution

Answer

The Lewis structures, hybridizations, planar nature, and σ/π bonds for the given molecules are as follows: **Chloromethane (CH3Cl):** Lewis structure: ``` H | H - C - Cl | H ``` Hybridization: sp³, Planarity: Not planar, σ bonds: 4, π bonds: 0 **Chloroethene (C2H3Cl):** Lewis structure: ``` H2C = CH - Cl ``` Hybridization: sp², Planarity: Planar, σ bonds: 3, π bonds: 1 **Chloroethyne (C2HCl):** Lewis structure: ``` HC ≡ C - Cl ``` Hybridization: sp, Planarity: Planar, σ bonds: 3, π bonds: 2

Step 1: Draw Lewis Structures

First, we'll draw the Lewis structures for each molecule. Recall that Lewis structures represent the arrangement of valence electrons around atoms in a molecule. In each structure, make sure that every atom has a full octet (except hydrogen, which can have a maximum of 2 electrons). **Chloromethane (CH3Cl):** The Lewis structure for chloromethane looks like this: ``` H | H - C - Cl | H ``` **Chloroethene (C2H3Cl):** The Lewis structure for chloroethene looks like this: ``` H2C = CH - Cl ``` **Chloroethyne (C2HCl):** The Lewis structure for chloroethyne looks like this: ``` HC ≡ C - Cl ```

Step 2: Determine Carbon Hybridization

Next, we'll find the hybridization of the carbon atoms in each molecule. Hybridization is the process of combining atomic orbitals from the same atom to create hybrid orbitals for chemical bonding. The hybridization of an atom can be determined by the following formula: Hybridization = Number of atoms bonded to the central atom + lone pairs on the central atom **Chloromethane (CH3Cl):** In chloromethane, the carbon atom forms four single bonds to three hydrogen atoms and one chlorine atom. Hence, the hybridization is sp³ since there are four orbitals interacting (one s orbital, three p orbitals). **Chloroethene (C2H3Cl):** In chloroethene, each carbon atom is double bonded to the other carbon and single bonded to either a chlorine or a hydrogen atom. Thus, it has a trigonal planar geometry and the hybridization is sp² (one s orbital, two p orbitals). **Chloroethyne (C2HCl):** In chloroethyne, one carbon atom is triple bonded to the other carbon and single bonded to a hydrogen or chlorine atom. This leads to a linear molecular geometry, so the hybridization is sp (one s orbital, one p orbital).

Step 3: Predict Planar Molecules

Now, we'll predict which molecules are planar. Planarity is determined by the molecule's geometry and the hybridization of its central atom. **Chloromethane (CH3Cl):** Chloromethane has tetrahedral geometry due to its sp³ hybridization. Tetrahedral molecules are not planar; therefore, chloromethane is not a planar molecule. **Chloroethene (C2H3Cl):** Chloroethene has trigonal planar geometry because of its sp² hybridization. Trigonal planar molecules are planar, so chloroethene is a planar molecule. **Chloroethyne (C2HCl):** Chloroethyne is linear due to its sp hybridization. Linear molecules are planar, so chloroethyne is a planar molecule.

Step 4: Calculate the Number of σ and π Bonds

Lastly, we'll find the number of σ and π bonds in each molecule. Sigma bonds (σ) are single bonds formed from hybrid orbitals overlapping, while pi bonds (π) are formed from unhybridized p orbitals overlapping. **Chloromethane (CH3Cl):** Chloromethane has four single bonds (C-H and C-Cl). All single bonds are σ bonds, so it has 4 σ bonds and 0 π bonds. **Chloroethene (C2H3Cl):** Chloroethene has one C=C double bond and two single bonds (C-H and C-Cl). The double bond consists of one σ and one π bond, and the single bonds are σ bonds. Therefore, it has 3 σ and 1 π bond. **Chloroethyne (C2HCl):** Chloroethyne has one C≡C triple bond and two single bonds (C-H and C-Cl). The triple bond comprises of one σ and two π bonds, and the single bonds are σ bonds. So it has 3 σ and 2 π bonds.

Key Concepts

Hybridization

Hybridization is a fundamental concept in understanding chemical bonding because it explains the shapes and bond angles in molecules. When atomic orbitals mix to form new, identical hybrid orbitals, this process is known as hybridization. This occurs to maximize the number of bonds an atom can form, increasing the molecule's stability.
To determine the hybridization for a central atom, count the number of atoms it is directly bonded to plus any lone pairs it possesses.

• For chloromethane \((CH_3Cl)\), the carbon is bonded to three hydrogens and one chlorine, making the hybridization sp³.
• In chloroethene \(C_2H_3Cl\), each carbon is bonded to two other atoms and has a double bond, resulting in sp² hybridization.
• Chloroethyne \(C_2HCl\) features a carbon bonded just to hydrogen or chlorine and another carbon, forming sp hybridization due to a triple bond and linear shape.

Understanding these hybridization states helps predict molecule geometry and reactivity.

Planarity

Planarity refers to the flatness of a molecule, which is highly dependent on its geometry and the hybridization state of its central atom. It plays a key role in chemical reactions and interactions, as planar molecules can more readily participate in reactions.
For a molecule to be planar:

• It must have a linear or trigonal planar geometry.
• The atoms involved typically exhibit specific hybridizations that favor planarity.

Chloromethane, with its sp³ hybridization and tetrahedral geometry, is not planar. In contrast, chloroethene is planar because it has a trigonal planar shape due to sp² hybridization. Meanwhile, chloroethyne, with a linear arrangement from sp hybridization, is also planar. Recognizing planarity aids in foreseeing physical properties and interaction capabilities.

Sigma and Pi Bonds

Sigma (σ) and pi (π) bonds are essential in understanding different bond types in molecular chemistry. Sigma bonds are formed by the end-to-end overlap of orbitals, while pi bonds are created by the side-to-side overlap of \(p\) orbitals. These bonds greatly influence a molecule's strength and stability.

• A single bond is always a σ bond.
• A double bond consists of one σ and one π bond.
• A triple bond comprises one σ and two π bonds.

For chloromethane, there are four single \(C-H\) and \(C-Cl\) bonds, all σ bonds. Chloroethene features a \(C=C\) double bond with 1 σ and 1 π bond, in addition to \(σ\) bonds from \(C-H\) and \(C-Cl\) connections. With one \(C≡C\) triple bond, chloroethyne includes 1 σ and 2 π bonds, supplemented by σ bonds from \(C-H\) and \(C-Cl\) single bonds. Knowing the type and number of σ and π bonds helps with understanding bond strength and reactivity.

Chemical Bonding

Chemical bonding describes how atoms connect to form molecules, which essentially forms the architecture of molecules. It involves the sharing or transfer of electrons to achieve full valence shells, leading to stable structures. There are various types of chemical bonds, primarily categorized into covalent and ionic bonds.

• In covalent bonding, atoms share electrons to reach a stable octet, like in all three examples given \(CH_3Cl, C_2H_3Cl, C_2HCl\).
• Ionic bonds, in contrast, form when electrons are transferred between atoms, creating charged ions.

The understanding of chemical bonds extends to recognizing how hybridization forms stable molecules with optimal shapes. Each bond type influences a molecule’s physical properties, chemical reactivity, and likeability to certain reactions. Grasping these bonding principles lays the foundation for further studies in molecular structure and reactions.