Organic Chemistry I: Key Concepts and Practice for Exam 1

Organic Molecules: Bonds and Structure

Types of Bonds in Organic Molecules

Organic molecules are composed of atoms connected by covalent bonds, which can be classified as sigma (σ) or pi (π) bonds. Understanding these bonds is fundamental to predicting molecular properties and reactivity.

• Sigma (σ) bonds: Formed by the head-on overlap of atomic orbitals; present in all single bonds.

• Pi (π) bonds: Formed by the side-to-side overlap of p orbitals; present in double and triple bonds in addition to a sigma bond.

• Example: In Tylenol (acetaminophen), count the number of σ and π bonds by analyzing the structure. Each single bond is a σ bond; each double bond contains one σ and one π bond.

Hybridization of Atoms

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding. The type of hybridization affects molecular geometry and reactivity.

• sp3 hybridization: Tetrahedral geometry, found in saturated carbons (alkanes).

• sp2 hybridization: Trigonal planar geometry, found in alkenes and carbonyl groups.

• sp hybridization: Linear geometry, found in alkynes.

• Example: In Aspartame, identify the hybridization of atoms at specific positions and name three functional groups (e.g., ester, amine, carboxylic acid).

Functional Groups in Organic Chemistry

Common Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for characteristic chemical reactions.

• Alcohols: -OH group attached to a carbon atom.

• Amines: -NH2, -NHR, or -NR2 groups.

• Carboxylic acids: -COOH group.

• Esters: -COOR group.

• Alkenes: C=C double bond.

• Alkynes: C≡C triple bond.

• Example: In aspartame, the molecule contains ester, amine, and carboxylic acid functional groups.

Acids, Bases, and Reaction Mechanisms

Acid-Base Reactions

Organic acid-base reactions involve the transfer of a proton (H+) between molecules. The direction of the reaction depends on the relative strengths (pKa values) of the acids and bases involved.

• pKa: A measure of acid strength; lower pKa means stronger acid.

• Mechanism: Show electron flow using curved arrows to indicate bond formation and breaking.

• Reactant vs. Product Favored: Compare pKa values to determine which side is favored.

• Example: For a reaction between a carboxylic acid and a base, draw the mechanism and predict the favored side.

Formal Charge Calculation

Formal charge helps determine the distribution of electrons in a molecule and predict reactivity.

• Formula: $\text{Formal charge} = \text{Valence electrons} - \text{Nonbonding electrons} - \frac{1}{2} \times \text{Bonding electrons}$

• Application: Assign formal charges to atoms in a molecule to identify reactive sites.

Orbital Interactions: Sigma and Pi Bonds

Orbital Overlap and Bond Types

The type of orbital overlap determines whether a bond is a sigma or pi bond.

• 1s-1s overlap: Forms a σ bond.

• 2p-2p head-on overlap: Forms a σ bond.

• 2p-2p side-on overlap: Forms a π bond.

• Example: Identify the bond type based on orbital diagrams.

Acid Strength and Trends

Comparing Acid Strengths

Acid strength in organic molecules is influenced by factors such as electronegativity, resonance, and atom size.

• Electronegativity: More electronegative atoms stabilize negative charge, increasing acid strength.

• Resonance: Delocalization of charge increases acid strength.

• Atom Size: Larger atoms stabilize charge better (e.g., HI is a stronger acid than HCl).

• Example: Compare phenol, cyclohexanol, and cyclohexanethiol for acid strength.

Resonance Structures and Stability

Drawing Resonance Structures

Resonance structures represent different ways electrons can be distributed in a molecule. The most stable resonance structure minimizes formal charges and places negative charge on the most electronegative atom.

• Rules: Only move electrons, not atoms; total number of electrons remains constant.

• Stability: Structures with full octets and minimal formal charge are most stable.

• Example: Draw resonance structures for carboxylate and amide ions, and identify the most stable form.

Nomenclature and Drawing Structures

Systematic Naming and Drawing

Organic molecules are named using IUPAC rules, which allow for systematic identification and drawing of structures.

• Alkanes: Name longest chain, number to give substituents lowest possible numbers.

• Cycloalkanes: Name ring, number substituents for lowest set of locants.

• Example: Draw 3-ethyl-2,6-dimethylheptane and 1,4-diethyl-2,6-dimethylcyclohexane.

Physical Properties: Boiling Point

Factors Affecting Boiling Point

Boiling point depends on molecular weight, intermolecular forces, and structure.

• Hydrogen bonding: Molecules with -OH or -NH groups have higher boiling points.

• Chain length: Longer chains generally have higher boiling points.

• Branching: More branching lowers boiling point.

• Example: Compare alcohols, ethers, and hydrocarbons for boiling point.

Molecular Formula Determination

Counting Atoms in a Structure

To determine the molecular formula, count the number of each type of atom in the molecule.

• Example: For a molecule with a benzene ring, nitrogen, and side chains, count C, H, N atoms to write the formula.

Reference Data: pKa Values and Periodic Table

Important pKa Values

pKa values are essential for predicting acid-base reaction outcomes.

Group	pKa
Strong acids, H3O+	<0
Carboxylic acids	~5
Protonated amines	~10
Phenol	~10
Alcohols, H2O	~16
Alkynes	~25
Amines	~35
Alkenes	~40
Alkanes	~50

The periodic table is used to determine atomic properties, electronegativity, and trends relevant to organic chemistry.

Additional info:

• Topics covered align with Ch.1 (Introduction), Ch.2 (Molecules and Forces), Ch.3 (Acids/Bases/Mechanisms), Ch.4 (Alkanes/Cycloalkanes), Ch.5 (Stereochemistry, via resonance and structure), and Ch.6 (Thermodynamics of Reactions).

• Questions require application of concepts, drawing structures, and analysis of molecular properties.