Introduction to Organic Compounds: Structure Representation

Representing the Structures of Organic Compounds

Overview of Organic Compounds

Organic compounds are a broad class of molecules primarily composed of carbon and hydrogen, but may also include other elements such as oxygen, nitrogen, sulfur, and phosphorus. Biomolecules like proteins, carbohydrates, lipids, and DNA are all examples of organic compounds.

• Organic compounds are defined by the presence of carbon atoms bonded to hydrogen and possibly other elements.

• They form the basis of life and are found in all living organisms.

• Examples include glucose (a carbohydrate), hemoglobin (a protein), and triglycerides (lipids).

Types of Structural Representations

Organic molecules can be represented in several ways, each providing different levels of detail about the atoms and bonds present.

• Molecular formula: Shows only the number of each type of atom in the molecule. Example: $C_3H_8$ for propane.

• Condensed structural formula: Shows all atoms but as few bonds as possible. Example: $CH_3CH_2CH_3$ for propane.

• Lewis structure: Shows complete connectivity—all atoms and all bonds, including lone pairs if necessary.

Example: For propane ($C_3H_8$):

• Molecular formula: $C_3H_8$

• Condensed formula: $CH_3CH_2CH_3$

• Lewis structure: Shows each carbon and hydrogen atom with all bonds explicitly drawn.

Skeletal Structures

Skeletal structures are simplified representations used for larger organic molecules. They show only the bonds between carbon atoms as lines, omitting hydrogen atoms bonded to carbon. Atoms other than hydrogen are shown explicitly.

• Skeletal structures are "bare-bones" diagrams, ideal for compounds with three or more carbon atoms.

• Bonds to atoms other than hydrogen (e.g., oxygen, nitrogen) are shown using their chemical symbols.

• Hydrogen atoms bonded to non-carbon atoms are shown explicitly.

• Lone pairs of electrons are typically not shown.

Example: The skeletal structure of butane ($C_4H_{10}$) is a zig-zag line representing the four carbon atoms connected in a chain.

Rules for Drawing Skeletal Structures

When drawing skeletal structures, certain conventions are followed to ensure clarity and consistency.

• Bonds to carbon are shown as lines.

• Bonds between carbon and hydrogen are not shown but are implied.

• Other elements bonded to carbon are drawn at the end of the bond using their symbol (e.g., O for oxygen).

• If these atoms have hydrogens bonded to them, the hydrogens are shown.

• Lone pairs of electrons are not shown.

Steps for Drawing Skeletal Structures

To construct a skeletal structure, follow these steps:

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 (such as oxygen, nitrogen, etc.).

Example: For pentane ($C_5H_{12}$), draw five connected lines (zig-zag) for the carbon chain. For ethanol ($C_2H_5OH$), draw two connected lines for the carbon chain, and attach an "O" at the end for the hydroxyl group.

Representation Type	Example (Propane)	Features
Molecular Formula	$C_3H_8$	Shows atom counts only
Condensed Formula	$CH_3CH_2CH_3$	Shows all atoms, minimal bonds
Lewis Structure	All atoms and bonds drawn	Complete connectivity
Skeletal Structure	Three connected lines	Only carbon skeleton shown

Additional info: Skeletal structures are especially useful for visualizing large organic molecules and for quickly identifying functional groups and connectivity without the clutter of hydrogen atoms.