Introduction to Organic Compounds: Structure Representation and Skeletal Formulas

Chapter 4: Introduction to Organic Compounds

Outline of Topics

• Representing the Structures of Organic Compounds

• Alkanes: The Simplest Organic Compounds

• Families of Organic Compounds—Functional Groups

• Nomenclature of Simple Alkanes

• Isomerism in Organic Compounds

Representing the Structures of Organic Compounds

Organic Compounds and Biomolecules

Organic compounds are primarily composed of carbon and hydrogen, but may also include other elements such as oxygen, nitrogen, sulfur, and phosphorus. Important biological molecules—such as proteins, carbohydrates, lipids, and DNA—are all examples of organic compounds.

Types of Structural Representations

• Molecular Formula: Indicates only the number of each type of atom in a molecule. Example: Propane: $C_3H_8$

• Condensed Structural Formula: Shows all atoms in a molecule but minimizes the number of bonds shown. Lone pairs may or may not be displayed. Example: Propane: $CH_3CH_2CH_3$

• Lewis Structure: Displays all atoms, bonds, and lone pairs, showing complete connectivity. Example:

  • Each atom is shown with its bonds to other atoms and lone pairs (if any).

Skeletal Structures

Skeletal structures are simplified representations used for organic molecules with three or more carbon atoms. They show carbon-carbon bonds as lines, omit hydrogen atoms bonded to carbon (these are implied), and display bonds to other atoms explicitly.

• Bonds to carbon: Shown as lines.

• Bonds to hydrogen: Not shown, but implied.

• Bonds to other elements: Shown using the element's symbol at the end of the bond.

• Lone pairs: Not shown in skeletal structures.

Rules for Drawing Skeletal Structures

1. Determine the number of carbons connected end to end.

2. Draw the bonds between the carbons (the carbon skeleton).

3. Draw bonds to noncarbon atoms (e.g., oxygen, nitrogen, halogens).

Examples of Structure Representations

• Molecular Formula: $C_5H_{12}$

• Condensed Formula: $CH_3CH_2CH_2CH_2CH_3$

• Skeletal Structure: A zig-zag line with five vertices, each representing a carbon atom.

Visual Comparison Table

Representation	Description	Example (Propane)
Molecular Formula	Atom counts only	$C_3H_8$
Condensed Formula	Atoms shown, minimal bonds	$CH_3CH_2CH_3$
Lewis Structure	All atoms, bonds, lone pairs	H–C–C–C–H (with all H shown)
Skeletal Structure	Lines for C–C bonds, H omitted	Three connected lines/vertices

Practice: Drawing Skeletal Structures

• For cyclopentane: Draw a five-membered ring.

• For heptane: Draw a zig-zag line with seven vertices.

• For butane: Draw a zig-zag line with four vertices.

Additional info: The provided handwritten diagrams illustrate skeletal structures for various organic compounds, including cyclic and branched alkanes. These are essential for visualizing molecular connectivity and are widely used in organic chemistry for clarity and simplicity.