Overview
This lecture covers the various ways to represent organic molecules, emphasizes translating between these representations, and introduces key rules, types of structures, and functional groups in organic chemistry.
Ways to Represent Molecules
- Molecular formula indicates atom counts but not structure (e.g., C3H8O).
- Condensed structures show groupings and connectivity (e.g., (CH3)2CHOH).
- Partially condensed structures include some bonds and group notations.
- Lewis structures display all atoms, bonds, and lone pairs in detail.
- Zigzag (bond-line) structures simplify drawing, with each vertex representing a carbon; hydrogens on carbons are implied.
Drawing Conventions & Rules
- Carbons always form four bonds.
- Hydrogens form only one bond each.
- Oxygens typically form two bonds; may have a third in some cases.
- Halogens (F, Cl, Br, I, represented as X) form only one bond.
- Never draw more than four bonds to carbon (“Texas carbon” is invalid).
- Hydrogens bonded to heteroatoms (non-carbon, non-hydrogen) must be drawn; those on carbons are omitted in zigzag structures.
Converting Between Structures
- Each zigzag vertex = a carbon atom; hydrogens are implied to complete four bonds per carbon.
- Groups in parentheses in condensed formulas attach to the preceding atom.
- When learning, practice drawing hydrogens onto zigzag structures to confirm atom counts.
- Be able to convert structures both ways: condensed ↔ zigzag, and zigzag ↔ Lewis.
Naming and Functional Groups
- Methyl (CH3-) and ethyl (CH2CH3-) groups are common substituents.
- Alkanes: hydrocarbons with only single bonds (C–C, C–H).
- Cycloalkanes: ring forms of alkanes (e.g., cyclohexane – six-membered ring).
- Benzene: six-carbon ring with alternating double bonds, drawn with three double bonds or a circle.
- Alkyl halides: compounds with a carbon chain (R) and a halogen (X); generic formula RX.
- Alkenes: compounds with carbon-carbon double bonds; generic formula R2C=CR2.
- Alkynes: compounds with carbon-carbon triple bonds; terminal alkynes have C≡C-H, internal have C≡C-R.
Key Terms & Definitions
- Zigzag/bond-line structure — Simplified molecular drawing where each vertex is a carbon atom, hydrogens are implied.
- Functional group — Specific groups of atoms that define classes of organic molecules (e.g., alkyl halide, alkene, alkyne).
- Heteroatom — Any atom in a molecule that is not carbon or hydrogen.
- Alkyl group — A carbon/hydrogen chain or branch (e.g., methyl, ethyl).
- Texas Carbon — An invalid carbon atom with five bonds.
- Alkane — Hydrocarbon with only single bonds.
- Alkene — Hydrocarbon with at least one double bond.
- Alkyne — Hydrocarbon with at least one triple bond.
- Cycloalkane — Ring-shaped alkane.
- Alkyl halide — Compound with an alkyl group and halogen.
Action Items / Next Steps
- Practice translating between molecular, condensed, Lewis, and zigzag structures.
- Review and memorize functional groups from textbook tables.
- Use flashcards to reinforce functional group identification and nomenclature.