Overview
This lecture covers the main methods for representing organic molecules, introduces their structural rules, and reviews important functional groups and their classification.
Molecular Representations
- The molecular formula lists atom types and counts (e.g., C₃H₈O for isopropyl alcohol), but not structure.
- The condensed structure shows connectivity (e.g., (CH₃)₂CHOH), grouping repeated units.
- The partially condensed structure draws some bonds and lone pairs for clarity.
- The Lewis structure fully expands all atoms, bonds, and lone pairs, giving the most detail.
- The zigzag (bond-line) structure simplifies drawing by using lines and vertices for carbon atoms, omitting most hydrogens.
Rules for Drawing Structures
- Carbon atoms must have four bonds.
- Hydrogen atoms can only have one bond.
- Oxygen atoms typically form two or three bonds.
- Halogens (F, Cl, Br, I, denoted X) form only one bond.
- Carbons cannot exceed four bonds ("Texas carbon" with five bonds is not allowed).
- In zigzag structures, hydrogens bonded to carbon are implied; hydrogens on heteroatoms must be drawn.
Converting Between Structures
- Each vertex or endpoint in a zigzag represents a carbon atom.
- All implied hydrogens should be mentally counted to ensure each carbon has four bonds.
- Groups in parentheses in condensed structures attach to the carbon immediately before them.
- Practice converting structures in both directions to build proficiency.
Common Structural Groups and Naming
- Methyl group: CH₃
- Ethyl group: CH₂CH₃
- Alkanes: only C and H, all single bonds.
- Rings: Cycloalkanes have carbons arranged in a ring (e.g., cyclohexane, cyclopentane).
- Benzene: six-membered ring with alternating double bonds, often drawn with a circle to represent delocalization.
Functional Groups
- Alkyl halide: Carbon chain (R) bonded to a halogen (X), generic formula RX.
- Alkane: Only C and H with single bonds.
- Alkene: Contains C=C double bond, generic formula R₂C=CR₂.
- Alkyne: Contains C≡C triple bond, generic formula R–C≡C–R.
- Terminal alkyne: triple bond at molecule’s end (R = H).
- Internal alkyne: triple bond between two carbons (both R = alkyl).
Key Terms & Definitions
- Condensed structure — representation grouping atoms, sometimes with parentheses, to show connectivity.
- Lewis structure — full expansion of all atoms, bonds, and lone pairs.
- Zigzag/bond-line structure — lines and vertices denote carbon atoms, hydrogens on carbons are omitted.
- Methyl group — CH₃ unit.
- Ethyl group — CH₂CH₃ unit.
- Alkane — saturated hydrocarbon with only single bonds.
- Alkene — hydrocarbon with at least one double bond.
- Alkyne — hydrocarbon with at least one triple bond.
- Cycloalkane — saturated hydrocarbon ring.
- Benzene — six-membered aromatic ring with alternating double bonds.
- Functional group — atom/group imparting specific chemical properties to a molecule.
- Alkyl halide — compound with a halogen directly attached to an alkyl group.
Action Items / Next Steps
- Practice drawing molecules in all representations (condensed, Lewis, zigzag).
- Memorize key functional groups and their generic formulas.
- Use flashcards to learn group names and structures.
- Read textbook tables summarizing functional groups.