Chapter 1: A Review of General Chemistry

Atomic Structure and Bonding Patterns

This section reviews the fundamental atomic structure and bonding patterns of elements commonly found in organic molecules. Understanding these patterns is essential for predicting molecular structure and reactivity.

• Common Organic Atoms: Hydrogen (H), Oxygen (O), Nitrogen (N), Carbon (C), Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I).

• Bonding Patterns: Each element has a characteristic number of bonds it typically forms (e.g., C: 4, N: 3, O: 2, H: 1, halogens: 1).

• Lewis/Kekulé Structures: Visual representations of molecules showing all atoms, bonds, and lone pairs. Kekulé structures are a type of Lewis structure emphasizing bonding patterns.

• Formal Charge: The charge assigned to an atom in a molecule, calculated as:

• Deviations from Normal Bonding: Some structures may have atoms with unusual numbers of bonds or charges; these must be identified and rationalized.

Chemical Bonding Theories

Three main theories describe how atoms bond in organic molecules: VSEPR, valence bond (VB), and molecular orbital (MO) theory.

• VSEPR Theory: Valence Shell Electron Pair Repulsion theory predicts molecular geometry based on electron pair repulsions.

• Valence Bond (VB) Theory: Describes bonds as the overlap of atomic orbitals (e.g., sp3, sp2, sp).

• Molecular Orbital (MO) Theory: Considers electrons delocalized over the entire molecule, forming bonding and antibonding orbitals.

• Bond Types: Sigma (σ) bonds are single bonds formed by head-on overlap; pi (π) bonds are formed by side-on overlap in double/triple bonds.

• Hybridization: The mixing of atomic orbitals to form new hybrid orbitals (sp3, sp2, sp) that explain observed molecular geometries.

• Electron Geometry and Bond Angles: Determined by the number of electron groups around an atom (e.g., tetrahedral: 109.5°, trigonal planar: 120°, linear: 180°).

Representations of Organic Molecules

Organic molecules can be depicted in several ways, each with its own advantages for clarity and simplicity.

• Lewis Structures: Show all atoms, bonds, and lone pairs.

• Kekulé Structures: Emphasize bonding patterns, often used for aromatic compounds.

• Condensed Structures: Omit some or all bonds, grouping atoms together (e.g., CH3CH2OH).

• Skeletal Structures: Use lines for bonds and vertices for carbon atoms; hydrogens on carbons are usually omitted.

• Interpreting Skeletal Structures: Allows determination of molecular formula, bond types, hybridization, geometry, and bond angles.

Chapter 2: Acids, Bases, and Reaction Mechanisms

Electronegativity and Bond Polarity

Electronegativity differences between atoms determine the type and polarity of chemical bonds.

• Electronegativity (EN, χ): A measure of an atom's ability to attract electrons in a bond.

• Bond Types:

  • Ionic: Large EN difference; electrons transferred.

  • Polar Covalent: Moderate EN difference; electrons shared unequally.

  • Nonpolar Covalent: Little or no EN difference; electrons shared equally.

• Dipole Moment (μ): A measure of bond polarity, represented by an arrow pointing toward the more electronegative atom.

• Net Dipole Moment: The vector sum of individual bond dipoles in a molecule, shown with a crossed arrow.

Resonance and Electron Delocalization

Resonance structures depict the delocalization of electrons in molecules where a single Lewis structure is insufficient.

• Resonance Forms: Alternative Lewis structures differing only in the placement of electrons, not atoms.

• Curved Arrow Formalism: Used to show the movement of electrons between resonance forms.

• Resonance Hybrid: The true structure is a weighted average of all significant resonance forms.

• Stabilization by Resonance: Delocalization of electrons lowers the energy and increases stability.

• Evaluating Resonance Forms: The most significant contributor has the most atoms with complete octets, the least formal charge, and negative charges on more electronegative atoms.

• Inductive Effects: Electron withdrawal or donation through sigma bonds due to electronegativity differences, distinct from resonance effects.

Acids, Bases, and pKa

Acid-base chemistry is central to organic reactions. The strength of acids and bases is quantified by their dissociation constants and pKa values.

• Brønsted–Lowry Acid: Proton (H+) donor.

• Brønsted–Lowry Base: Proton acceptor.

• Lewis Acid: Electron pair acceptor.

• Lewis Base: Electron pair donor.

• Acidity Constant (Ka): Quantifies acid strength:

• pKa: The negative logarithm of Ka:

• Relationship: Lower pKa = stronger acid; higher pKa = weaker acid.

• Conjugate Acid/Base: The species formed after an acid donates a proton (conjugate base) or a base accepts a proton (conjugate acid).

• Acid-Base Equilibria: The equilibrium favors the side with the weaker acid/base (higher pKa).

Predicting Acid Strength: The CARIO Principle

Several factors influence acid strength, summarized by the acronym CARIO:

• Charge: More positive charge increases acidity.

• Atom: Acidity increases with electronegativity and size of the atom bearing the charge.

• Resonance: Delocalization of charge increases acidity.

• Induction: Electron-withdrawing groups increase acidity.

• Orbital: Greater s-character (sp > sp2 > sp3) increases acidity.

Curved Arrow Notation in Acid-Base Reactions

Curved arrows are used to depict the flow of electrons during acid-base and resonance processes.

• Electron Flow: Arrows start at electron sources (lone pairs or bonds) and point to electron sinks (atoms or bonds).

• Acid-Base Mechanism: Typically, a base uses a lone pair to form a bond with a proton, while the acid's bond to the proton breaks, forming the conjugate base.

Intermolecular Forces

Noncovalent interactions between molecules affect physical properties and reactivity.

• Dispersion Forces: Weak, temporary attractions due to momentary dipoles.

• Dipole-Dipole Interactions: Attractions between permanent dipoles in polar molecules.

• Hydrogen Bonds: Strong dipole-dipole interactions involving H bonded to N, O, or F.

• Electrostatic Potential Maps: Visual representations of charge distribution in molecules.

Key Terms Table

The following table summarizes key terms from Chapters 1 and 2:

Example: Acid-Base Reaction Using Curved Arrows

Consider the reaction of acetic acid with hydroxide ion:

• Acetic acid (CH3COOH) donates a proton to OH-, forming acetate (CH3COO-) and water (H2O).

• Curved arrows show the lone pair on OH- attacking the acidic proton, and the O-H bond in acetic acid breaking to form the acetate ion.

Additional info: The study guide emphasizes mastery of terminology, structure drawing, and mechanistic electron flow, which are foundational for all subsequent organic chemistry topics.