Problem 62 - Chemistry: An Atoms-Focused Approach

Question

The replacement of a halogen atom in a CFC molecule with a hydrogen atom makes the compound more environmentally "friendly." Draw Lewis structures for the following compounds: a. \(\mathrm{CHCl}_{2} \mathrm{F}\) (Freon 21 ) b. \(\left.\mathrm{CHF}_{2} \mathrm{Cl} \text { (Freon } 22\right)\) c. \(\left.\mathrm{CH}_{2} \text { ClF (Freon } 31\right)\) d. \(\left.F_{3} C-C H B r C l \text { (Halon } 2311\right)\) \(\mathrm{e} . \mathrm{Cl}_{2} \mathrm{FC}-\mathrm{CH}_{3}(\mathrm{HCFC} 141 \mathrm{b})\)

Step-by-Step Solution

Answer

Question: Draw the Lewis structures for the following compounds: a. CHCl2F b. CHF2Cl c. CH2ClF d. F3C-CHBrCl e. Cl2FC-CH3 Answer: a. H-C-Cl-Cl-F b. H-C-F-F-Cl c. H-C-H-Cl-F d. F-C-F-F-CH-Br-Cl e. Cl-C-Cl-F-CH-H-H

Step 1: Count the valence electrons of each compound

Refer to the periodic table and count the valence electrons for each atom in the compound. Add these values together to find the total number of valence electrons in the compound. a. \(\mathrm{CHCl}_{2} \mathrm{F}\) Carbon (C): 4 valence electrons Hydrogen (H): 1 valence electron Chlorine (Cl): 7 valence electrons (x2) Fluorine (F): 7 valence electrons Total valence electrons: 26 b. \(\mathrm{CHF}_{2} \mathrm{Cl}\) Carbon (C): 4 valence electrons Hydrogen (H): 1 valence electron Fluorine (F): 7 valence electrons (x2) Chlorine (Cl): 7 valence electrons Total valence electrons: 26 c. \(\mathrm{CH}_{2} \mathrm{ClF}\) Carbon (C): 4 valence electrons Hydrogen (H): 1 valence electron (x2) Chlorine (Cl): 7 valence electrons Fluorine (F): 7 valence electrons Total valence electrons: 20 d. \(F_{3}C-CHBrCl\) Carbon (C): 4 valence electrons (x2) Fluorine (F): 7 valence electrons (x3) Hydrogen (H): 1 valence electron Bromine (Br): 7 valence electrons Chlorine (Cl): 7 valence electrons Total valence electrons: 44 e. \(\mathrm{Cl}_2\mathrm{FC-CH}_3\) Chlorine (Cl): 7 valence electrons (x2) Fluorine (F): 7 valence electrons Carbon (C): 4 valence electrons (x2) Hydrogen (H): 1 valence electron (x3) Total valence electrons: 30

Step 2: Draw a skeleton structure for each compound

Arrange the atoms with the least electronegative atom in the center and draw single bonds between the central atoms and the surrounding atoms. a. \(\mathrm{CHCl}_{2} \mathrm{F}\to\) H-C-Cl-Cl-F b. \(\mathrm{CHF}_{2} \mathrm{Cl} \to\) H-C-F-F-Cl c. \(\mathrm{CH}_{2} \mathrm{ClF}\to\) H-C-H-Cl-F d. \(F_{3} C-CH Br Cl \to \) F-C-F-F-CH-Br-Cl e. \(\mathrm{Cl}_2 \mathrm{FC}-\mathrm{CH}_3 \to \) Cl-C-Cl-F-CH-H-H

Step 3: Distribute the remaining valence electrons

Use the remaining valence electrons to satisfy the octet rule for each atom in the compound. Except for Hydrogen, which needs only two electrons to be satisfied. #a# H-C-Cl-Cl-F Remaining electrons: 26 - 8 = 18 Skeleton structure: H-C-Cl-Cl-F H-C-Cl-Cl-F (All atoms satisfy the octet rule, and H has fulfilled its 2-electron requirement) #b# H-C-F-F-Cl Remaining electrons: 26 - 8 = 18 Skeleton structure: H-C-F-F-Cl H-C-F-F-Cl (All atoms satisfy the octet rule, and H has fulfilled its 2-electron requirement) #c# H-C-H-Cl-F Remaining electrons: 20 - 6 = 14 Skeleton structure: H-C-H-Cl-F H-C-H-Cl-F (All atoms satisfy the octet rule, and all H atoms have fulfilled their 2-electron requirement) #d# F-C-F-F-CH-Br-Cl Remaining electrons: 44 - 12 = 32 Skeleton structure: F-C-F-F-CH-Br-Cl F-C-F-F-CH-Br-Cl (All atoms satisfy the octet rule, and H has fulfilled its 2-electron requirement) #e# Cl-C-Cl-F-CH-H-H Remaining electrons: 30 - 10 = 20 Skeleton structure: Cl-C-Cl-F-CH-H-H Cl-C-Cl-F-CH-H-H (All atoms satisfy the octet rule, and all H atoms have fulfilled their 2-electron requirement) Now that we have successfully drawn the Lewis structures for each compound, the student should practice on their own to become more comfortable with the process.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. They're located in the atom's outer shell and determine how atoms interact with each other to form molecules. Understanding the number of valence electrons helps us in drawing Lewis structures. Here's a simple breakdown:

• Carbon (C) has 4 valence electrons.
• Hydrogen (H) has 1 valence electron.
• Halogens like Fluorine (F) and Chlorine (Cl) have 7 valence electrons.
• Bromine (Br), another halogen, also has 7 valence electrons.

To find the total valence electrons in a molecule, sum the valence electrons of all constituent atoms. This total helps in setting up Lewis structures by determining how many electrons will be involved in bonding and lone pairs.

Octet Rule

The octet rule is a guideline in chemistry that suggests that atoms tend to form bonds until they are surrounded by eight electrons, simulating the electron configuration of noble gases. This rule is essential for drawing Lewis structures, though there are some exceptions.

• Most atoms strive for an electron configuration with eight valence electrons.
• Hydrogen is an exception, needing only two electrons.
• Some elements, like those in higher periods, can accommodate more than eight electrons.

In Lewis structures, the placement of electrons for each atom is adjusted until this rule is satisfied for stability. This is why distributing the remaining valence electrons after connecting atoms in a skeleton structure is crucial.

Halogen Replacement

Halogen replacement refers to substituting a halogen atom in a compound with another element, commonly hydrogen, to alter the compound's chemical properties. This concept is particularly relevant in designing environmentally friendly chemicals.

• Halogens (F, Cl, Br) are highly electronegative, affecting the reactivity of compounds.
• Replacing halogens with hydrogen can reduce the environmental impact.
• CFCs (Chlorofluorocarbons) replaced by HCFCs (Hydrochlorofluorocarbons) are less harmful to ozone due to fewer halogen atoms.

This process is crucial in developing alternatives to harmful compounds such as CFCs that contribute to ozone depletion.

CFC Compounds

CFC compounds, or chlorofluorocarbons, are organic compounds made up of carbon, chlorine, and fluorine. Due to their stability and non-flammability, they were widely used in refrigerators, aerosol sprays, and as solvents. However, they have significant environmental downsides.

• CFCs are known to deplete the ozone layer when released into the atmosphere.
• Alternatives such as HCFCs and HFCs have been developed to mitigate this issue.
• Halogen replacement, especially substituting with hydrogen, can reduce ozone depletion potential.

Understanding the structure and replacement of halogens in CFCs is key to creating more environmentally friendly compounds that align with global eco-sustainability efforts.

Skeleton Structure

A skeleton structure is the basic arrangement of atoms in a molecule, illustrating how they are connected without showing individual electrons. This is a crucial initial step in drawing Lewis structures.

• Begin by identifying the least electronegative atom to serve as the central atom.
• Connect surrounding atoms to the central atom using single bonds.
• The skeleton gives a roadmap for distributing valence electrons to satisfy the octet rule.

This step simplifies the process by visually establishing the relationship between atoms, setting the foundation for completing the Lewis structure with electron pairs and checking for rule compliance.