Notes on Molecular Representations and Resonance

Page 1: Introduction

• Course: Auburn Sciences and Mathematics CHEM2070

• Instructor: Dr. Gordon

Page 3: Importance of Molecular Representations

• Variety of Representations: Different ways to draw molecules serve various purposes.

  • Information Needed: Understanding what information is necessary to describe a molecule.

  • Most Informative Representations: Identifying which representations convey the most information.

Page 4: Limitations of Lewis Structures

• Impractical for Large Molecules: Lewis structures are not suitable for large compounds like Amoxicillin.

• Condensed Formulas: Provide minimal information about molecular shape.

• Bond-Line Structures:

  • Benchmark representations for organic compounds.

  • Essential for success in organic chemistry courses.

Page 5-6: Reading Bond-Line Structures

• Carbon Representation: Each corner or endpoint in a bond-line structure represents a carbon atom.

• Counting Atoms: Ability to determine the number of carbons and hydrogens present in a structure.

Page 7-8: Drawing Bond-Line Structures

• Conversion Skills: Ability to draw bond-line structures from Lewis or condensed structures is crucial.

Page 9: Advantages of Bond-Line Structures

• Visualizing Reactions: Easier to see bonds made or broken in chemical reactions.

• Comparison: Understanding the differences between condensed formulas and bond-line structures.

Page 10-11: Functional Groups

• Definition: Specific groups of atoms that determine the characteristics of organic compounds.

• Examples:

  • Alkyl halides, ketones, aldehydes, alcohols, carboxylic acids, etc.

  • The "R" in functional groups refers to the remainder of the compound, typically carbon and hydrogen.

Page 13: Carbon Atoms with Formal Charges

• Bonding Patterns:

  • Carbon typically forms four bonds when neutral.

  • Carbocations have three bonds and one empty orbital.

  • Carbanions have five bonds.

Page 14-15: 3D Representation of Molecules

• 3D Space: Molecules occupy three-dimensional space, which is challenging to represent in two dimensions.

• Pi Bonds and Formal Charges: Often more spread out than bond-line structures imply.

Page 16-19: Resonance

• Resonance Structures:

  • Pi electrons can exist on both sides of a carbon atom.

  • Curved arrows are used to describe electron delocalization.

• Formal Charges: Important to consider when deriving resonance structures.

Page 20-27: Patterns of Resonance

• Five General Patterns:

  1. Allylic lone pair

  2. Allylic carbocation

  3. Lone pair adjacent to a carbocation

  4. Pi bond between atoms with different electronegativities

  5. Conjugated pi bonds in a ring

• Practice: Recognizing these patterns is essential for mastering resonance.

Page 29-32: Resonance Structure Contributions

• Hybrid Structure: Represents a blend of all resonance structures.

• Stability Rules:

  • Structures with fewer formal charges are more significant.

  • Negative charges on more electronegative atoms increase significance.

Page 33-34: Localized vs. Delocalized Lone Pairs

• Delocalization: Not all lone pairs are delocalized just because they are near pi bonds.

• General Rule: If an atom has both a pi bond and a lone pair, they typically do not participate in resonance.

Conclusion

• Mastery of molecular representations and resonance is crucial for success in organic chemistry. Regular practice and understanding of the concepts will facilitate learning and application in