Skeletal Structures for Organic Molecules

Introduction

• Purpose: Simplify drawing of organic molecules using skeletal structures.
• Organic Compounds: Contain carbon and hydrogen, may also include oxygen, nitrogen, sulfur, and phosphorus.

Carbon in Organic Molecules

• Why Carbon?: Carbon forms 4 bonds due to 4 valence electrons in sp3 state.
• Complex Structures: Carbon’s bonding capacity leads to complex molecules.
• Drawing Complexity: Simplification needed for larger molecules.

Representing Molecules

• Butane Example: C4H10
  • Condensed structural formula: Group-wise representation.
  • Lewis Structure: Shows how atoms are bonded.
• Scaling Issue: Larger molecules like octane and decane take longer to draw.
• Solution: Skeletal structures simplify representation.

Skeletal Structures

• Principles:
  • Do not show hydrogen atoms attached to carbon.
  • Only show bonds between atoms (except C-H bonds).
  • Non-carbon atoms (oxygen, nitrogen, sulfur) are shown.
• Butane in Skeletal Form:
  • Show 4 carbon atoms and bonds between them.
  • Use lines to represent bonds.
  • Important: Count corners and ends as carbon atoms.

Counting Hidden Hydrogen Atoms

• Rule of Thumb:
  • Each carbon should have 4 bonds.
  • Subtract visible bonds from 4 to determine hidden hydrogens.
• Example:
  • Carbon with 1 bond: 4 - 1 = 3 hydrogen atoms.
  • Carbon with 2 bonds: 4 - 2 = 2 hydrogen atoms.

Drawing Examples

• Pentane: 5 carbons, use zigzag lines.
  • Always count and number carbons to avoid mistakes.
• 3D Structure Representation
  • Bond angles and tetrahedral shapes.
  • Use wedge and dash to show bonds coming in and out of the page.

Complex Representations

• Cyclic Structures
  • Example: Cyclohexane
  • Number and connect atoms in a ring.
  • Simplification saves time and avoids messiness.
• Double Bonds
  • Example: Trans-2-butene
  • Show 4 carbons in a chain, add a double bond between specific carbons.
• Triple Bonds
  • Example: 2-butyne
  • Keep bonds linear to represent 180-degree bond angles.

Practice and Application

• Reverse Engineering: Convert skeletal structures back to molecular formulas.
• Common Mistakes
  • Miscounting carbons and hydrogens.
  • Incorrectly drawing angles and bonds.

Working with Heteroatoms

• Ethanol Example: CH3CH2OH
  • Draw carbon skeleton first.
  • Include bonds to non-carbon atoms, show attached hydrogens.
• Butanamide Example: CH3CH2CH2(C=O)NH2
  • Draw carbon chain and add double bonds and heteroatoms as needed.

Resources

• Practice Quizzes: Available for further practice.
  • Online quizzes for skeletal structure practice.