Three Ways of Drawing Organic Molecules: Structural, Condensed, and Skeletal Formulas

Three Ways of Drawing Organic Molecules

Introduction

Organic molecules can be represented in several ways to convey their structure and connectivity. The three most common representations are structural formulas, condensed formulas, and skeletal (bond-line) formulas. Each method provides a different level of detail and is used for different purposes in organic chemistry.

Structural Formula

• Definition: A detailed drawing showing every atom and bond in the molecule.

• Features:

  • All atoms (including hydrogens) and all bonds are explicitly shown.

  • Useful for beginners and for understanding the full connectivity of atoms.

• Example: For ethanol, the structural formula is: H  H |    | H–C–C–O–H |    | H   H

Condensed Formula

• Definition: A more compact way of writing organic molecules, grouping atoms together to show connectivity without drawing all bonds.

• Features:

  • Hydrogen atoms are listed next to the atom they are attached to.

  • Parentheses may be used to indicate branching or repeating groups.

• Example: Ethanol can be written as CH3CH2OH.

Skeletal (Bond-Line) Formula

• Definition: The fastest and most common way to draw complex organic molecules, especially for larger structures.

• Features:

  • Lines represent carbon-carbon bonds; the ends and bends ("corners") of lines represent carbon atoms.

  • Hydrogen atoms attached to carbons are usually omitted for simplicity.

  • Atoms other than carbon and hydrogen (heteroatoms) such as O, N, S are shown explicitly.

  • Hydrogens attached to heteroatoms are shown.

• Example: The skeletal formula for ethanol is a simple zig-zag line ending with an OH group.

Rules for Drawing Skeletal Formulas

• C–C bonds are shown as lines.

• Each corner or end of a line represents a carbon atom (unless another atom is shown).

• Hydrogen atoms attached to carbon are not shown; it is assumed that each carbon has enough hydrogens to complete four bonds.

• Atoms other than carbon and hydrogen (e.g., O, N, S) are always shown.

• Exception: Hydrogens attached to heteroatoms (like O or N) are shown.

Example: Determining the Number of Hydrogens

To determine the number of hydrogen atoms attached to each carbon in a skeletal structure:

• Count the number of bonds to each carbon (from lines or connections).

• Subtract this number from four (since carbon forms four bonds).

• The difference is the number of hydrogens attached to that carbon.

Example: If a carbon in a skeletal structure has two lines (bonds) attached, it must have two hydrogens (4 - 2 = 2).

Practice Problems

• Practice 1: Draw a skeletal formula for the molecule CH3CH(CH3)CH2CH2OH.

• Practice 2: Convert a given skeletal formula into condensed and structural formulas.

Summary Table: Comparison of Formula Types

Formula Type	Features	Example (Ethanol)
Structural Formula	All atoms and bonds shown	H–C–C–O–H (with all H's and bonds drawn)
Condensed Formula	Atoms grouped, bonds not shown	CH3CH2OH
Skeletal Formula	Lines for C–C bonds, corners/endpoints = C, H's on C omitted	Zig-zag line ending with OH

Key Points

• Structural formulas are detailed but time-consuming for large molecules.

• Condensed formulas are compact and useful for simple molecules.

• Skeletal formulas are the standard for complex organic structures due to their simplicity and clarity.