Overview
This lecture introduces foundational concepts in organic chemistry, focusing on bonding, molecular structure, and functional groups, essential for first-semester college students.
Bonding Preferences of Common Elements
- Carbon forms 4 bonds; nitrogen 3; oxygen 2; hydrogen, fluorine, and other halogens typically form 1 bond.
- First-row elements (C, N, O, F) generally seek 8 electrons (octet rule).
Lewis Structures and Bond Types
- Lewis structures represent atoms’ bonding and lone pairs; bonds are lines, lone pairs are dots.
- Water (H₂O): oxygen forms 2 bonds and 2 lone pairs; hydrogens form 1 bond each.
- Methyl fluoride: carbon in the center, bonded to hydrogen and fluorine, fluorine with 3 lone pairs.
- Hydrogen bonds occur when H is bonded to N, O, or F—responsible for high boiling points in water.
- Polar bonds occur with electronegativity difference ≥ 0.5; C–F bond is polar, C–H bond is non-polar.
Covalent vs. Ionic Bonds
- Covalent bonds share electrons (equally = non-polar, unequally = polar).
- Ionic bonds transfer electrons (e.g., Na gives an electron to Cl, forming Na⁺ and Cl⁻).
Alkanes, Alkenes, Alkynes, and Structural Drawing
- Alkanes: saturated hydrocarbons (CnH₂n+2), e.g., methane, ethane, propane, butane.
- Alkenes: contain at least one C=C double bond; alkynes: contain at least one C≡C triple bond.
- Carbon-carbon single bond is longest, triple bond is shortest and strongest.
Sigma and Pi Bonds
- Single bond: 1 sigma; double bond: 1 sigma + 1 pi; triple bond: 1 sigma + 2 pi.
- Sigma bonds are stronger than pi bonds.
Bond Order and Hybridization
- Bond order: single = 1, double = 2, triple = 3.
- Hybridization determined by groups (atoms + lone pairs) on atom: 4 = sp³, 3 = sp², 2 = sp.
- Bond hybridization: determined by both atoms involved (e.g., sp³–s).
Formal Charge Calculation
- Formal charge = valence electrons – (bonds + dots/lone electrons).
- Positive (carbocation), negative (carbanion), or neutral depending on electron count.
Functional Groups and Nomenclature
- Alcohols (–OH): suffix –ol, e.g., ethanol.
- Aldehydes (–CHO): suffix –al, e.g., ethanal.
- Ketones (C=O in-chain): suffix –one, e.g., propanone.
- Ethers (C–O–C): name as alkyl groups + ether, e.g., dimethyl ether.
- Esters (R–COO–R’): name as alkyl group + alkanoate, e.g., methyl ethanoate.
- Carboxylic acids (–COOH): suffix –oic acid, e.g., pentanoic acid.
Structural Expansion Tips
- Methyl (CH₃) groups appear at structure ends; methylene (CH₂) in chain centers; CH groups branch or attach to special atoms (e.g., Br, OH).
Key Terms & Definitions
- Octet Rule — Atoms tend to have 8 electrons in their valence shell.
- Lewis Structure — Diagram showing bonds and lone pairs.
- Hydrogen Bond — Strong dipole-dipole attraction when H is bonded to N, O, or F.
- Polar Bond — Bond with significant charge separation due to electronegativity difference.
- Sigma Bond — First, strongest bond between two atoms.
- Pi Bond — Weaker bond formed above/below sigma bond in double/triple bonds.
- Hybridization — Mixing of atomic orbitals to form new, directional orbitals (sp, sp², sp³).
- Formal Charge — Charge assigned to an atom using valence bond theory.
Action Items / Next Steps
- Practice drawing Lewis structures and identifying functional groups.
- Memorize alkane names (methane to decane) and general formulas.
- Calculate formal charges and determine hybridizations in provided structures.
- Review assigned textbook readings on bonding and molecular structure as directed by your instructor.