Overview
This lecture introduces foundational concepts in first-semester organic chemistry, including bonding preferences, molecular structures, types of bonds, hybridization, formal charge calculations, and identification of key functional groups.
Basic Principles of Organic Chemistry
- Organic chemistry studies compounds containing carbon atoms.
- Carbon forms four bonds; hydrogen forms one; oxygen forms two; nitrogen forms three.
- Halogens (F, Cl, Br, I) usually form one bond.
- Drawing Lewis structures is essential for visualizing molecular bonding.
Covalent and Ionic Bonds
- Covalent bonds involve electron sharing; can be polar (unequal sharing) or nonpolar (equal sharing).
- Ionic bonds occur via electron transfer between metals and nonmetals, forming cations (+) and anions (–).
- Electronegativity difference ≥ 0.5 makes a covalent bond polar.
- Hydrogen bonds occur when H is directly bonded to N, O, or F.
Hydrocarbons and Functional Groups
- Alkanes: Saturated hydrocarbons, follow CₙH₂ₙ₊₂ formula.
- Alkenes: Contain at least one C=C double bond (unsaturated).
- Alkynes: Contain at least one C≡C triple bond (unsaturated).
- Common alkane names: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane.
Bond Properties and Hybridization
- Single bonds are longest and weakest, triple bonds are shortest and strongest.
- Sigma (σ) bonds: all single bonds; Pi (π) bonds: present in double/triple bonds.
- Hybridization: Carbon with 4 groups = sp³; 3 groups = sp²; 2 groups = sp.
Formal Charge and Electronic Structure
- Formal charge = (# valence electrons) – (# bonds + dots/lone electrons).
- Positive formal charge (carbocation); negative (carbanion); unpaired electron (radical).
- Bonding electrons = electrons in bonds; Nonbonding electrons = electrons in lone pairs.
Key Functional Groups & Examples
- Alcohol: Contains –OH group (e.g., ethanol).
- Ether: Oxygen between two carbons (e.g., dimethyl ether).
- Aldehyde: Carbonyl group at chain end (e.g., ethanal).
- Ketone: Carbonyl group in chain center (e.g., propanone).
- Ester: Carbonyl adjacent to an oxygen linking to carbon (e.g., methyl ethanoate).
- Carboxylic acid: Carbonyl with hydroxyl group (e.g., pentanoic acid).
Key Terms & Definitions
- Lewis Structure — Diagram showing bonds and lone pairs in a molecule.
- Electronegativity — Atom’s tendency to attract electrons.
- Sigma (σ) bond — Single covalent bond formed by head-on orbital overlap.
- Pi (π) bond — Covalent bond formed by side-to-side orbital overlap.
- Hybridization — Mixing of atomic orbitals into new hybrid orbitals.
- Formal Charge — Charge calculated based on valence electrons, bonds, and lone pairs.
- Functional Group — Specific group of atoms responsible for characteristic reactions.
Action Items / Next Steps
- Memorize bonding preferences and typical valences of C, H, O, N, and halogens.
- Practice drawing Lewis structures for alkanes, alkenes, alkynes, alcohols, ketones, aldehydes, esters, and carboxylic acids.
- Learn and recognize functional group nomenclature up to 10-carbon alkanes.
- Review hybridization and calculate formal charges for atoms in various molecules.