Question

Draw the Lewis structure for each organic compound from its condensed structural formula. a. \(\mathrm{C}_{3} \mathrm{H}_{8}\) b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) c. \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) d. \(\mathrm{CH}_{3} \mathrm{COOH}\) e. \(\mathrm{CH}_{3} \mathrm{CHO}\)

Short answer

a. A straight chain of three C atoms with H atoms single-bonded to fulfill the valence of C; b. O atom between two CH3 groups; c. Central C atom double-bonded to an O atom and single-bonded to two CH3 groups; d. CH3 group bonded to a C atom which is double-bonded to an O atom and single-bonded to an OH group; e. CH3 group bonded to a C atom which is double bonded to an O atom.

Step-by-step solution

Identify Valence Electrons

Count the valence electrons for each atom in the molecule. Carbon (C) has 4 valence electrons, hydrogen (H) has 1, and oxygen (O) has 6.

Sketch the Skeleton Structure

Arrange the atoms to show which atoms are connected to which and connect them with single bonds.

Complete the Octet for Non-Hydrogen Atoms

Around each carbon and oxygen atom, draw enough electrons to complete the octet rule. Remember that hydrogen can only have two electrons.

Place Remaining Electrons on Terminal Atoms

After filling the octet of the non-hydrogen atoms, place the remaining electrons on the terminal atoms (usually hydrogen).

Draw the Lewis Structures

Using steps 1 to 4, draw the Lewis structures for each compound. For (a) C3H8, a chain of 3 carbons should be drawn with hydrogens that fill each carbon's valence to 4 electrons with single bonds. For (b) CH3OCH3, two CH3 groups should be bonded to an oxygen in the center. For (c) CH3COCH3, two CH3 groups should be bonded to a central carbon which also has a double bond with an oxygen (as a ketone). For (d) CH3COOH, a CH3 group should be bonded to a carbon which has a double bonded oxygen (carbonyl) and a single bonded OH (carboxyl) group. For (e) CH3CHO, a CH3 group should be bonded to a carbon with a double bonded oxygen (as an aldehyde).

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining an atom's chemical bonding behavior. These electrons are available for forming bonds with other atoms and play a key role in the creation of molecules. In organic compounds, which primarily consist of carbon (C), hydrogen (H), and often oxygen (O), the number of valence electrons is particularly important. Carbon has 4 valence electrons, hydrogen has 1, and oxygen has 6. With this knowledge, it becomes easier to predict how these atoms will bond together.

• Carbon's 4 valence electrons make it incredibly versatile, allowing it to form up to four single bonds, two double bonds, or a combination thereof.
• Hydrogen's 1 valence electron means it seeks to share just one more electron to achieve stability.
• Oxygen's 6 valence electrons drive it to form two additional bonds to fulfill its valence shell.

When drawing Lewis structures, beginning with counting valence electrons helps to determine how to connect atoms in a molecule to satisfy their bonding needs.

Octet Rule

The octet rule is a fundamental concept in chemical bonding, particularly for main group elements. It states that atoms tend to bond in such a way that they each have eight electrons in their valence shell, giving them the same electronic configuration as a noble gas. This drive for stability affects how organic compounds are structured.

Hydrogen is an exception to this rule; it follows the 'duet rule' and is stable with just two electrons. When applying the octet rule in the context of organic compounds:

• Carbon will typically form enough bonds to be surrounded by eight electrons.
• Oxygen will usually form two bonds, either as two single bonds, a double bond, or one single and one double bond, completing its octet.
• Hydrogen, with its duet rule, is satisfed forming just one bond.

The octet rule guides the arrangement of electrons in the Lewis structures and helps to predict the location of single and double bonds in molecules.

Chemical Bonding

Chemical bonding is the process by which atoms combine to form molecules through the sharing or transfer of valence electrons. The main types of chemical bonds include ionic, covalent, and metallic bonds. Organic compounds typically involve covalent bonding, where atoms share pairs of electrons. In Lewis structures of organic compounds, the bonds are depicted as lines connecting the atoms:

• A single line represents a single covalent bond (one pair of shared electrons).
• A double line indicates a double covalent bond (two pairs of shared electrons).

To correctly depict an organic compound's structure using Lewis structures, we focus on the ability of carbon to form four covalent bonds, oxygen's tendency to complete its octet, and hydrogen's need to form a single bond. By adhering to these principles and considering the number of valence electrons available, we can accurately represent the compound's covalent bonding framework and ensure that each atom achieves the proper electron configuration for stability.