Overview
This lecture covers key concepts and questions related to homologous series, types of carbon atoms, alicyclic vs. aromatic compounds, unsaturated hydrocarbons, double bond equivalents, sigma bonds, and degrees of unsaturation.
Homologous Series
- Homologous series have the same general formula with each successive member differing by a CH₂ group.
- Example: Alkanes have the formula CₙH₂ₙ₊₂; alkenes have CₙH₂ₙ.
Types of Carbon and Hydrogen Atoms
- Secondary hydrogen is attached to a secondary carbon (a carbon bonded to two other carbons).
- In given structures, secondary carbons are identified by their attachment to two other carbons.
- Tertiary and quaternary carbons are bonded to three and four other carbons, respectively; primary is bonded to one.
- The number of secondary hydrogens equals the number of hydrogens attached to secondary carbons.
Alicyclic and Aromatic Compounds
- Alicyclic compounds are cyclic but not aromatic (do not have delocalized pi electrons).
- Aromatic compounds are distinguished by their ring structure and resonance stability.
Unsaturated Hydrocarbons
- Unsaturated hydrocarbons contain double or triple bonds between carbon atoms.
- Only hydrocarbons with C=C or C≡C bonds and no heteroatoms are considered unsaturated hydrocarbons.
Double Bond Equivalent (DBE)
- DBE indicates the number of pi bonds and rings in a molecule.
- Calculation: DBE = C + 1 - (H/2) - (X/2) + (N/2), where C=carbons, H=hydrogens, X=halogens, N=nitrogens.
- Example: For C₄H₆, DBE = 4 + 1 - (6/2) = 2.
Sigma Bonds
- Sigma bonds are single covalent bonds.
- When counting sigma bonds, include all bonds to hydrogens in the structure.
- Example: After adding missing hydrogens, a structure may have more sigma bonds than initially apparent.
Degree of Unsaturation
- Calculated as the total number of pi bonds plus the number of rings in the structure.
- When given a molecular formula, use the DBE formula.
Key Terms & Definitions
- Homologous series — Compounds with the same general formula differing by CH₂ units.
- Secondary carbon — A carbon atom bonded to two other carbon atoms.
- Alicyclic compound — Cyclic compounds that are not aromatic.
- Aromatic compound — Cyclic compounds with delocalized pi electrons and resonance stability.
- Unsaturated hydrocarbon — Hydrocarbon with double or triple C–C bonds.
- Double bond equivalent (DBE) — Number of pi bonds and rings in a structure.
- Sigma bond — A single covalent bond formed by head-on orbital overlap.
- Degree of unsaturation — Total number of pi bonds plus rings in a compound.
Action Items / Next Steps
- Practice identifying primary, secondary, tertiary, and quaternary carbons in various structures.
- Use the DBE formula to calculate degrees of unsaturation for different molecular formulas.
- Review formulas and definitions for quick recall before exams.