Overview
This lecture introduces core concepts of first-semester organic chemistry, covering bonding preferences, Lewis structures, bond polarity, types of organic molecules, hybridization, formal charges, and functional groups.
Bonding Preferences & Lewis Structures
- Organic chemistry focuses on compounds containing carbon, which likes to form four bonds.
- Hydrogen forms 1 bond, beryllium 2, boron 3, nitrogen 3, oxygen 2, halogens (F, Cl, Br, I) generally 1.
- Lewis structures show how atoms connect; each bond = 2 electrons, lone pairs fill the octet.
- Example: water’s Lewis structure shows O with 2 bonds and 2 lone pairs.
Bond Types & Polarity
- Covalent bonds involve electron sharing (can be equal or unequal).
- Nonpolar covalent bonds share electrons equally (e.g., C–H).
- Polar covalent bonds share electrons unequally (EN difference ≥ 0.5), causing partial charges.
- Hydrogen bonds are special polar bonds with H attached to N, O, or F.
- Ionic bonds involve electron transfer between metals and nonmetals, forming ions (cations = +, anions = –).
Alkanes, Alkenes, Alkynes & Formulas
- Alkanes: saturated hydrocarbons, formula CₙH₂ₙ₊₂ (methane, ethane, propane, etc.).
- Alkenes: contain at least one double C=C bond (e.g., ethene/C₂H₄).
- Alkynes: contain at least one triple C≡C bond (e.g., ethyne/C₂H₂).
- Alkanes are more saturated (maximum H); alkenes and alkynes are unsaturated.
Bond Length, Strength & Types
- C–C single bonds are longest; triple bonds are shortest.
- Triple bonds are strongest, single bonds are weakest.
- Single bond = 1 sigma (σ); double bond = 1 sigma + 1 pi (π); triple bond = 1 sigma + 2 pi.
- Sigma bonds are stronger than pi bonds.
Hybridization
- Hybridization relates to the number of groups (atoms/lone pairs) attached to a carbon:
- 4 groups: sp³
- 3 groups: sp²
- 2 groups: sp
- Bond hybridization is a combination of the types on each atom (e.g., sp³–s for C–H).
Sigma & Pi Bonds Counting
- Every single bond is sigma; double bond = 1 sigma + 1 pi; triple = 1 sigma + 2 pi.
- Count all single bonds for sigma; double/triple bonds contribute pi bonds.
Formal Charge Calculation
- Formula: Formal charge = valence electrons – (number of bonds + number of dots/lone electrons).
- Example: Carbon with 4 bonds: 4 – 4 = 0; 3 bonds: 4 – 3 = +1; 3 bonds + 2 dots: 4 – 5 = –1.
- Cations have positive formal charge; anions negative.
Functional Groups & Structures
- Alcohols: –OH group (ethanol).
- Aldehydes: –CHO group (carbonyl + H, ends with –al).
- Ketones: Carbonyl in the middle of chain (propanone/acetone).
- Ethers: O between two carbons (dimethyl ether).
- Esters: Carbonyl adjacent to O bonded to C (methyl ethanoate).
- Carboxylic acids: Carbonyl + OH (pentanoic acid).
Key Terms & Definitions
- Lewis structure — diagram showing atoms, bonds, and lone pairs in a molecule.
- Octet rule — atoms prefer 8 electrons in valence shell for stability.
- Electronegativity (EN) — atom’s ability to attract electrons in a bond.
- Sigma (σ) bond — strongest type of covalent bond, formed by head-on orbital overlap.
- Pi (π) bond — weaker bond, formed by side-by-side orbital overlap.
- Formal charge — calculated charge on an atom within a molecule.
- Functional group — specific group of atoms giving characteristic properties to molecules.
- Hybridization — mixing of atomic orbitals forming hybrid orbitals for bonding.
Action Items / Next Steps
- Memorize alkane names and formulas up to ten carbons.
- Practice drawing Lewis structures for alkanes, alkenes, alkynes, and functional group derivatives.
- Calculate formal charges and identify functional groups in given molecules.
- Review course playlist or textbook for deeper examples on esters and nomenclature.