Overview
This lecture explains how to recognize and draw different representations of organic compounds, focusing on carbon skeletons, Lewis (L) structures, condensed structures, and skeletal structures.
Recognizing Organic Structures
- Organic compounds are often represented using carbon skeletons, emphasizing carbon atoms and carbon-carbon bonds.
- Hydrogen atoms attached to carbons are typically omitted in skeletal structures.
- Recognizing various structure formats is essential: expanded/Lewis structures, condensed structures, and skeletal structures.
Lewis (L) Structures
- L structures, also called complete or expanded structures, show all atoms and all bonds explicitly.
- Useful for understanding full bonding but inefficient for large molecules due to excessive drawing.
- The least electronegative atoms are placed in the center.
Condensed Structures
- Condensed structures omit bonds between carbon and hydrogen to simplify diagrams.
- Carbon-carbon bonds may be shown or omitted.
- Repeated units are abbreviated with parentheses and subscripts (e.g., (CH₂)₅ for five CH₂ groups).
- Attachments to heteroatoms (e.g., nitrogen, oxygen) are shown as needed.
Skeletal Structures
- Skeletal structures use lines to represent carbon-carbon bonds; endpoints and intersections indicate carbon atoms.
- Hydrogen atoms attached to carbons are not shown, assuming each carbon achieves an octet.
- Heteroatoms and functional groups are explicitly drawn.
- Lone pairs are usually omitted but should be assumed based on atom type; formal charges must always be shown where applicable.
Formal Charge
- Formal charge = valence electrons − lone pairs − ½(bonding electrons).
- Always indicate formal charges, especially for atoms with nonzero charges (e.g., N⁺ or O⁻).
- Omitting lone pairs is acceptable unless specifically asked, but not omitting formal charges.
Translating Between Representations
- Be able to convert between L structures, condensed structures, and skeletal structures.
- Recognize when to omit or include hydrogens, lone pairs, and charges.
Key Terms & Definitions
- Carbon Skeleton — A representation showing only carbon atoms and the bonds between them.
- Lewis (L) Structure — Expanded structure displaying all atoms and bonds.
- Condensed Structure — Simplified structure omitting bonds between carbon and hydrogen, sometimes using grouping.
- Skeletal Structure — Line-based drawing omitting hydrogens on carbons, showing heteroatoms and charges.
- Formal Charge — Charge calculated by valence electrons minus lone pairs minus half of bonding electrons.
- Heteroatom — An atom in an organic molecule that is not carbon or hydrogen (e.g., N, O, S).
Action Items / Next Steps
- Practice drawing carbon skeletons, L structures, condensed, and skeletal structures for simple alkanes.
- Create flashcards with structure types on one side and examples on the other.
- Review formal charge calculations for nitrogen and oxygen in different bonding scenarios.