What is Organic Chemistry?

Introduction to Organic Chemistry

Organic chemistry is the study of the structure, properties, and reactions of compounds containing carbon. It forms the basis for understanding biological molecules, pharmaceuticals, and many materials used in daily life.

The Atom and the Periodic Table

Atomic Structure

Atoms are composed of three fundamental particles:

• Protons: Positively charged, mass = 1.6726 × 10-27 kg

• Neutrons: Neutral, mass = 1.6750 × 10-27 kg

• Electrons: Negatively charged, mass = 9.1096 × 10-31 kg

The atomic number (Z) is the number of protons in the nucleus and defines the element. The mass number (A) is the sum of protons and neutrons.

Electron Configuration and Orbitals

Electrons occupy shells and subshells according to the Aufbau Principle, filling lower energy levels first. The Pauli Exclusion Principle states that no two electrons in the same atom can have the same set of four quantum numbers.

• s Orbitals: Spherical shape

• p Orbitals: Dumbbell shape

• d Orbitals: More complex shapes

Electronic Configurations in the Periodic Table

Electronic configurations describe the arrangement of electrons in an atom. For example, carbon is 1s22s22p2.

Valence Electrons

Valence electrons are the outermost electrons and are crucial for chemical bonding. For main group elements, the number of valence electrons equals the group number.

Bonding and Molecular Structure

The Octet Rule

Atoms gain, lose, or share electrons to achieve a stable configuration with eight valence electrons (the octet rule).

Types of Bonds

• Ionic Bonding: Electrostatic attraction between oppositely charged ions.

• Covalent Bonding: Sharing of electron pairs between atoms.

Electronegativity and Bond Polarity

Electronegativity is a measure of an atom's ability to attract electrons. The difference in electronegativity (Δχ) determines bond type:

• Δχ < 0.5: Non-polar covalent

• 0.5 < Δχ < 1.7: Polar covalent

• Δχ > 1.7: Ionic

Bond Dipoles and Molecular Dipole Moments

Bond dipoles arise from differences in electronegativity and bond distance. The dipole moment is a vector quantity indicating charge separation.

Molecular Representations

Types of Molecular Representations

• Molecular Formula: Shows the number of each atom type, no structural info.

• Lewis Structures: Explicitly show atoms and bonds, practical for small molecules.

• Condensed Structures: Groups atoms together, omits some bonds.

• Bond-Line Formulas: Carbon atoms are implied at intersections/ends; hydrogens on carbon are not shown.

Three-Dimensional Bond-Line Structures

Wedges and dashes indicate three-dimensionality: wedges come out of the page, dashes go behind.

Identifying Functional Groups

Functional groups are structural units responsible for characteristic chemical behavior.

Lewis Structures and Resonance

Lewis Structures

Lewis structures represent molecules pictorially, showing valence electrons as dots. Steps to determine Lewis structures:

1. Predict connectivity

2. Count valence electrons

3. Add electrons for bonds

4. Calculate unused electrons

5. Add electrons to fill valences

6. Add multiple bonds if needed

7. Check formal charges

Resonance Structures

Resonance occurs when more than one valid Lewis structure exists for a molecule. Rules for resonance:

• Connectivity must remain the same

• Same number of electrons and net charge

• Same number of unpaired electrons

• Second-row elements cannot exceed the octet rule

• Major contributors maximize bonds, minimize charge, and place negative charge on the most electronegative atom

Resonance stabilizes molecules by electron delocalization.

VSEPR Theory and Molecular Geometry

Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory predicts molecular shape by minimizing repulsions between electron pairs. Non-bonding pairs require more space than bonding pairs.

Molecular Dipole Moments

Predicting Dipole Moments

The shape and bond polarity determine the presence and direction of a molecular dipole moment. Examples: HF (polar), CO2 (nonpolar), H2O (polar).

Isomers

Types of Isomers

Isomers are compounds with the same molecular formula but different structures:

• Constitutional Isomers: Different connectivity

• Stereoisomers: Different spatial arrangement

Acids and Bases

Bronsted-Lowry and Lewis Definitions

• Bronsted-Lowry Acid: Proton donor

• Bronsted-Lowry Base: Proton acceptor

• Lewis Acid: Electron pair acceptor

• Lewis Base: Electron pair donor

Acid Strength and pKa

The strength of an acid is measured by its acid dissociation constant (Ka) and pKa value:

• Stronger acid: larger Ka, smaller pKa

• Weaker acid: smaller Ka, larger pKa

Factors Affecting Acid Strength

• Atom: Charge location, electronegativity, size

• Resonance: Stabilization of conjugate base

• Induction: Presence of electronegative atoms or alkyl groups

• Hybridization: s-character increases acidity

Acid-Base Equilibria

The equilibrium in an acid-base reaction lies to the side of the weaker acid. The reaction is favorable when the stronger acid is on the left and the weaker acid is on the right.

Examples of Lewis Acids and Bases

Examples of Lewis Acid-Base Reactions

Additional info: These notes cover foundational topics in organic chemistry, including atomic structure, bonding, molecular representations, and acid-base theory, which are essential for further study in organic reactions and mechanisms.