Organic Chemistry Study Notes: Acid-Base Equilibria, Lewis Theory, and Alkane Conformations

Course Logistics and Schedule

• Homework 2 was due on Friday of last week.

• The Nomenclature assignment is due this Friday.

  • Nomenclature is not explicitly taught during lecture hours. Students are expected to learn the systematic naming rules independently using the Canvas module and Wiley homework assignments.

  • Learning nomenclature functions like following an instruction list: given a set of rules, one can systematically generate a chemical name from a structure or draw a structure from a given name.

• Quiz 1 grades were released on Friday.

  • The Quiz 1 answer key is released on Tuesday.

  • Students who need clarification on recorded answers should email the instructor directly.

  • For detailed explanations of specific correct answers, students should email Supplemental Instructors (SIs) or attend office hours.

• Quiz 2 is scheduled for next week on Wednesday.

  • Format: 15 multiple-choice questions.

  • Regulations: Closed-book, no electronics, no notes allowed.

  • Permitted materials: Molecular model kits are allowed.

  • Practice quizzes for Quiz 2 are available on Canvas.

• Supplemental Instruction (SI) Schedule:

  • Weekly review sessions take place on Sunday, Tuesday, and Thursday (times posted in Canvas announcements).

• Molecular Model Kit Usage:

  • Kits aid in visualizing three-dimensional molecular structures and bond rotations.

  • To visualize all core concepts in Chapter 4, a minimal model requires two carbon atoms with three distinct substituents on each carbon.

Counter Ions and Spectator Ions in Acid-Base Equilibria

• Equilibrium positions can be predicted without $pK_a$ values by evaluating the relative stability and energy of bases and conjugate bases using the KARIO framework (Charge, Atom, Resonance, Induction, Orbital).

• Pure ionic species with uncompensated charges (e.g., isolated amide anions, $\text{NH}_2^-$) cannot exist in bulk isolated quantities due to severe electrostatic repulsion.

• Chemical reagents are supplied with counter ions to maintain electrical neutrality (e.g., sodium amide, potassium amide, lithium amide).

• Counter cations coordinate to negative charges, but their specific identity (whether $\text{Na}^+$, $\text{K}^+$, or $\text{Li}^+$) has a negligible effect on basicity.

• Cations act as spectator ions that do not participate in the acid-base proton transfer mechanism.

• Spectator ions are frequently omitted from chemical equations for clarity, with charge balance implicitly understood.

Brønsted-Lowry vs. Lewis Acid-Base Definitions

• Brønsted-Lowry Definitions:

  • Acid: Proton ($H^+$) donor.

  • Base: Proton ($H^+$) acceptor.

  • Conjugate Acid: The species formed after a base accepts a proton.

  • Conjugate Base: The species formed after an acid donates a proton.

• Lewis Definitions (Electron-Centric):

  • Lewis Base: Electron pair donor. Species with higher propensity to donate electron density exhibit stronger basicity.

  • Lewis Acid: Electron pair acceptor. Species with higher electron deficiency exhibit stronger Lewis acidity.

  • Carbocations: Highly electron-deficient species containing a positively charged carbon atom that serve as strong Lewis acids.

• Hierarchy and Categorization:

  • All Brønsted-Lowry acids and bases are Lewis acids and bases.

  • Not all Lewis acids and bases qualify as Brønsted-Lowry acids and bases, as Lewis reactions do not strictly require proton transfer.

• Non-Brønsted Lewis Systems:

  • Aluminum trichloride ($\text{AlCl}_3$) functions as a Lewis acid by accepting electron pairs (e.g., used to generate electron-deficient aromatic rings).

  • Dimethyl ether functions as a Lewis base in combination with $\text{AlCl}_3$.

  • Boron-nitrogen coordination complexes feature boron acting as a Lewis acid and nitrogen acting as a Lewis base without proton involvement.

• Mechanism Arrow Standards:

  • Reaction arrows for Lewis acid-base processes must always originate at the electron pair on the Lewis base and terminate at the electron-accepting site of the Lewis acid.

Evaluating Acid-Base Equilibria via KARIO

• Example 1: Comparing Delocalized Anions on Oxygen vs. Sulfur

  • Reaction Mechanism: A nitrogen lone pair attacks a proton on an adjacent molecule, displacing electron density onto an adjacent heteroatom.

  • Identification: Electron donor = Lewis base; Electron acceptor = Lewis acid.

  • Resonance Structures: Depict electron delocalization from nitrogen lone pairs adjacent to pi bonds onto terminal heteroatoms.

  • Atom Effects (KARIO):

  • Comparing charge stabilization on Oxygen versus Sulfur.

  • Periodic Table Position: Sulfur is directly below Oxygen in Period 3, placing its valence electrons in a larger electron shell.

  • Size and Polarizability: Larger atomic volume allows Sulfur to distribute negative charge density more effectively than Oxygen, despite Oxygen's higher electronegativity.

  • Relative Stability: Negative charge localized on Sulfur is lower in energy (more stable) than negative charge localized on Oxygen.

  • Equilibrium Direction: The equilibrium favors the lower-energy species, shifting the position toward the starting materials (left).

• Notation for Equilibria:

  • Equilibrium arrows with unequally sized shafts/flags explicitly denote the favored side.

  • A single unidirectional arrow signifies an irreversible reaction (commonly used in forward synthetic steps).

• Example 2: Phenol vs. Isopropanol Acidity

  • Reaction: Isopropoxide reacting with phenol versus phenoxide reacting with isopropanol.

  • Resonance Evaluation:

  • Isopropoxide anion: Negative charge localized strictly on a single oxygen atom (no resonance).

  • Phenoxide anion: Negative charge delocalized into the aromatic ring across three additional carbon atoms.

  • Stability and Equilibrium: Phenoxide is significantly lower in energy due to extensive resonance stabilization, shifting the equilibrium toward the right (products). Phenol is substantially more acidic than isopropanol.

• Charge Dominance Rule:

  • Neutral starting species are dramatically lower in energy than charged products.

  • The presence or absence of full formal charges (Charge in KARIO) takes precedence over resonance stabilization of charged intermediates.

• Exceptions to Standard Rules:

  • Strong mineral acids (e.g., $H_2SO_4$) strictly act as acids regardless of conjugate base KARIO comparisons.

  • Extremely large conjugated systems (e.g., 10 to 15 resonance forms) can provide sufficient stabilization to override standard rule hierarchies.

Alkanes and Hydrocarbons

• Hydrocarbon Definition: Organic compounds composed exclusively of carbon and hydrogen atoms ($C_x H_y$).

• Alkane Definition: Saturated hydrocarbons containing only single covalent bonds (no double or triple bonds).

• Priority in Classification: If a molecule contains any higher-priority functional group (e.g., an alkene double bond), the molecule is classified primarily by that functional group rather than as an alkane.

• Empirical Formula for Acyclic Alkanes: $C_n H_{2n+2}$, where $n$ represents the number of carbon atoms.

  • Pentane Example: Contains 5 carbons ($n = 5$) and 12 hydrogens ($2(5) + 2 = 12$), yielding $C_5 H_{12}$.

• Constitutional Isomers of Alkanes:

  • Constitutional isomers possess identical empirical formulas ($C_n H_{2n+2}$) but different atom connectivity.

  • Rearranging carbon skeletons requires breaking and forming C-C and C-H bonds, conserving the total number of hydrogen atoms across all isomers.

• Practical Applications of Alkanes:

  • Energy and Fuels: Liquid propane ($C_3 H_8$) and butane ($C_4 H_{10}$) serve as common combustible fuels.

  • Cell Biology: Membrane fatty acids contain alkane chains. The specific ratio of saturated alkane chains to unsaturated alkene chains dictates cell membrane fluidity and structural morphology.

  • Chemical Engineering: Alkanes constitute the main chemical component of crude oil. Functionalizing unreactive alkanes into complex organic molecules increases their economic value significantly.

Spatial Representations and Conformational Analysis

• Wedge-and-Dash Notation:

  • Solid lines depict bonds residing directly within the two-dimensional plane of the board/page.

  • Wedge bonds depict substituents projecting out of the plane toward the viewer.

  • Dash bonds depict substituents projecting behind the plane away from the viewer.

  • Positional Invariance: Swapping the relative vertical placement of a wedge and a dash on the same tetrahedral carbon center does not alter the absolute 3D structure.

• Sawhorse Projections:

  • An oblique, angled perspective viewing down a carbon-carbon bond axis.

  • Serves as an intermediate visual representation between wedge-dash drawings and Newman projections.

• Newman Projections:

  • Formed by viewing directly down a specific carbon-carbon single bond axis (perpendicular to the viewing plane).

  • Front Carbon: Represented by a central point or vertex where three bonds intersect.

  • Back Carbon: Represented by a large circle behind the central point.

  • Carbon-Carbon Bond Axis: The central bond between the front and back carbon is hidden from view along the line of sight.

  • Spatial Purpose: Clearly displays dihedral angles and conformational relationships between substituents on adjacent carbons.

  • Representation Conventions: Substituents coming off Newman carbons are projected flat in two dimensions without wedge or dash notation on the spokes.

Step-by-Step Construction of Newman Projections

• Observation Axis:

  • Constructing a Newman projection requires a designated viewing vector (indicated by an observer arrow or explicit bond target).

  • The carbon closest to the observer arrow head represents the front carbon; the adjacent connected carbon represents the back carbon.

• Conversion Procedure:

  1. Identify the designated front and back carbons along the target bond axis.

  2. Explicitly draw all implicit hydrogen atoms on both carbons in the line-angle/wedge-dash structure.

  3. Draw the Newman template: a central dot with three spokes for the front carbon, and a circle with three spokes for the back carbon.

  4. Orient substituents based on spatial alignment:

  • In-plane substituents point directly vertical (straight up or straight down).

  • Out-of-page (wedge) groups map to one side (right or left depending on viewing angle).

  • Into-page (dash) groups map to the opposite side.

• Example Walkthrough:

  • Front Carbon Parameters: Contains an in-plane methyl group pointing straight down, with implicit hydrogen atoms on a wedge and a dash.

  • Back Carbon Parameters: Contains an in-plane chlorine atom pointing straight up, a methyl group on a dash (projecting away), and a hydrogen atom on a wedge (projecting forward).

  • Observer Position: Viewing along the axis from left to right.

  • Resulting Newman Projection:

  • Front Carbon: One spoke pointing straight down to a $\text{CH}_3$ group; two upper spokes pointing left and right to $\text{H}$ atoms.

  • Back Carbon: One spoke pointing straight up to a $\text{Cl}$ atom; lower-left spoke pointing to a $\text{CH}_3$ group (corresponding to the dash); lower-right spoke pointing to an $\text{H}$ atom (corresponding to the wedge).