Problem 8 - Chemistry and Chemical Reactivity

Question

Draw a Lewis structure for each of the following molecules: (a) methanol, \(\mathrm{CH}_{3} \mathrm{OH}\) (b) vinyl chloride, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCl}\), the molecule from which PVC plastics are made (c) acrylonitrile, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCN}\), the molecule from which materials such as Orlon are made (IMAGE NOT COPY)

Step-by-Step Solution

Answer

Draw each molecule's Lewis structure ensuring all atoms satisfy their valence shell requirements.

Step 1: Determine Total Valence Electrons

Count the total number of valence electrons for each molecule. For methanol (\(\mathrm{CH}_3\mathrm{OH}\)), carbon has 4, each hydrogen has 1, and oxygen has 6, giving a total of 14 valence electrons. For vinyl chloride (\(\mathrm{H}_2\mathrm{C}=\mathrm{CHCl}\)), carbon has 4, hydrogen has 1, and chlorine has 7 valence electrons, totaling 18. In acrylonitrile (\(\mathrm{H}_2\mathrm{C}=\mathrm{CHCN}\)), accounting for two carbon atoms, hydrogen, and nitrogen (which has 5 valence electrons), results in a total of 16 valence electrons.

Step 2: Sketch Initial Structure

Draw the skeletal structure of each molecule. Connect the atoms to the central atom (usually the least electronegative except hydrogen) with single bonds. For methanol, place carbon as the central atom connected to three hydrogens and one oxygen, which is itself connected to another hydrogen. For vinyl chloride, make a chain C=C-Cl with each carbon bonded to a hydrogen. For acrylonitrile, make a chain C=C-CN with each carbon also bonded to a hydrogen.

Step 3: Distribute Remaining Electrons

Distribute the remaining valence electrons as lone pairs, starting with the outer atoms, to satisfy the octet rule. For methanol, after forming the initial structure, place the remaining electrons around the oxygen to complete its octet. In vinyl chloride, distribute the remaining electrons around chlorine to complete its octet. For acrylonitrile, place any remaining electrons around the nitrogen to fulfill its octet.

Step 4: Form Double/Triple Bonds if Necessary

Check if all atoms besides hydrogen have full octets. Adjust the bonds if necessary. For methanol and vinyl chloride, the structures already align with the octet rule. In acrylonitrile, form a triple bond between carbon and nitrogen to complete their octets.

Step 5: Verify the Structure

Check the Lewis structure for each molecule to ensure all atoms have the correct number of electrons, achieving a stable configuration. All atoms except hydrogen should have a complete octet, and ensure that the total number of used electrons matches the total calculated valence electrons.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. Understanding valence electrons is essential when drawing Lewis structures. Each element in the periodic table has a specific number of valence electrons.

• Carbon has 4 valence electrons.
• Hydrogen has 1 valence electron.
• Oxygen has 6 valence electrons.
• Chlorine has 7 valence electrons.
• Nitrogen has 5 valence electrons.

When you calculate the total number of valence electrons for a molecule, it's necessary to add the valence electrons of each atom within that molecule. For instance, in methanol ( \( \mathrm{CH}_3\mathrm{OH} \) ), add up the valence electrons of one carbon, one oxygen, and four hydrogen atoms to get a total of 14 valence electrons.

Octet Rule

The octet rule is a fundamental concept in chemistry that guides the formation of stable molecules. It states that atoms tend to bond in such a way that they have eight electrons in their valence shell. This rule helps predict the bonding behavior of atoms in molecules.It is important to remember:

• Most atoms seek to have 8 electrons in their outer shell.
• Hydrogen is an exception and is content with 2 electrons.
• Elements like carbon, nitrogen, and oxygen commonly follow the octet rule.

When drawing Lewis structures, we distribute electrons around atoms to satisfy the octet rule. If the octet isn't complete, you might need to adjust by forming double or triple bonds, as seen in molecules like acrylonitrile (\( \mathrm{H}_{2} \mathrm{C} = \mathrm{CHCN} \) ).

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Understanding the geometry helps predict properties like polarity and reactivity.When constructing a molecule:

• Remember that the arrangement of atoms around the central atom is influenced by the repulsion between electron pairs.
• For example, in methanol ( \( \mathrm{CH}_3\mathrm{OH} \) ), the molecule forms a tetrahedral shape around the carbon atom.
• Vinyl chloride (\( \mathrm{H}_2\mathrm{C} = \mathrm{CHCl} \) ) results in a planar structure due to the double bond, which restricts rotation and maintains a flat geometry.

Knowledge about geometry is pivotal for predicting how molecules interact and behave, especially in reactions and when forming polymers like PVC built from vinyl chloride.

Chemical Bonding

Chemical bonding involves the attractive forces that hold atoms together in molecules. There are several types of chemical bonds, including covalent, ionic, and metallic bonds. For organic molecules, like those in the given exercise, covalent bonding is most common.Important points about chemical bonding:

• Covalent bonds occur when atoms share electron pairs.
• Single, double, or triple bonds convey different strengths and lengths of bond.
• Methanol (\( \mathrm{CH}_3\mathrm{OH} \) ) includes single covalent bonds, whereas acrylonitrile (\( \mathrm{H}_{2} \mathrm{C} = \mathrm{CHCN} \) ) features a triple bond between carbon and nitrogen, demonstrating varied bond types.

These bonds determine the structure and properties of a molecule, influencing everything from its boiling and melting points to its chemical reactivity.