Organic Chemistry Exam 1 Study Guide: Structure, Bonding, Acids & Bases, and Stereochemistry

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Structure and Bonding

Lewis Structures and Formal Charges

Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist. Formal charges help determine the most stable structure and are calculated for each atom in a molecule.

Lewis Structure: Represents all valence electrons as either bonding or non-bonding (lone pairs).
Formal Charge Formula:

Example: For methimazole, formal charges are assigned to each atom based on the above formula.

Bond Types and Sigma Bonds

Sigma (σ) bonds are the first bonds formed between two atoms and are present in all single, double, and triple bonds. Counting sigma bonds helps understand molecular structure and stability.

Sigma Bonds: Each single bond is one sigma bond; double and triple bonds each contain one sigma bond (plus pi bonds).
Example: Methimazole contains 12 sigma bonds.

Bond Angles and Hybridization

The geometry around an atom is determined by its hybridization, which affects bond angles.

sp2 Hybridization: Trigonal planar geometry, bond angle ≈ 120°.
sp3 Hybridization: Tetrahedral geometry, bond angle ≈ 109.5°.
Example: In remdesivir, indicated carbons show sp2 and sp3 hybridization with corresponding bond angles.

Orbital Overlap in Bonding

Bonds are formed by the overlap of atomic orbitals. Sigma bonds are formed by the head-on overlap of hybrid orbitals.

sp–sp Overlap: Sigma bonds between two sp-hybridized atoms.

Functional Groups

Identification of Functional Groups

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

Common Functional Groups: Ester, arene (aromatic ring), amide, carboxylic acid, nitrile.
Example: In sacubitril, functional groups are identified and circled, including esters, arenes, amides, carboxylic acids, and nitriles.

Stereochemistry and Isomerism

Types of Isomers

Isomers are compounds with the same molecular formula but different structures or spatial arrangements.

Identical Structures: Same connectivity and arrangement.
Cis-Trans (Geometric) Isomers: Same connectivity, different spatial arrangement around a double bond or ring.
Constitutional Isomers: Different connectivity of atoms.
Example: Given pairs of structures are classified as identical, cis-trans isomers, or constitutional isomers.

Newman Projections and Conformational Analysis

Newman projections are used to visualize the conformation of a molecule by looking straight down a bond axis.

Eclipsed Conformation: Groups on adjacent carbons are aligned, leading to higher energy due to torsional strain.
Staggered Conformation: Groups are as far apart as possible, leading to lower energy.
Ranking: Staggered (anti) < staggered (gauche) < eclipsed (highest energy).
Example: Newman projections are drawn and ranked by energy based on steric interactions.

Acids and Bases

Acidity, Basicity, and Resonance

The acidity of a proton depends on several factors, including hybridization, resonance, and inductive effects.

Hybridization: The more s-character in the atom bearing the proton, the more acidic it is (sp > sp2 > sp3).
Resonance: Delocalization of negative charge stabilizes the conjugate base, increasing acidity.
Example: The most acidic proton is the one attached to an sp-hybridized atom with resonance stabilization.

Acid-Base Reactions and Equilibrium

Acid-base reactions favor the formation of the weaker acid and base. Resonance stabilization of the conjugate base shifts equilibrium toward products.

Equilibrium Direction: Favors side with weaker acid/base pair.
Example: Resonance-stabilized conjugate base makes equilibrium favor products.

Acid-Base Theories

There are different definitions of acids and bases:

Brønsted-Lowry Acid: Proton donor.
Brønsted-Lowry Base: Proton acceptor.
Lewis Acid: Electron pair acceptor.
Lewis Base: Electron pair donor.
Example: Reaction between a benzene ring and a boron compound is a Lewis acid-base reaction.

Nomenclature and Structure of Alkanes

IUPAC Naming of Alkanes

The International Union of Pure and Applied Chemistry (IUPAC) system provides rules for naming organic compounds.

Longest Chain: Identify the longest continuous carbon chain as the parent hydrocarbon.
Numbering: Number the chain to give substituents the lowest possible numbers.
Substituents: Name and number all substituents; list them alphabetically.
Example: 7-ethyl-5-isopropyl-4,4-dimethyldecane.

Classification of Carbon Atoms

Carbons are classified based on the number of other carbons to which they are attached.

Primary (1°): Attached to one other carbon.
Secondary (2°): Attached to two other carbons.
Tertiary (3°): Attached to three other carbons.
Quaternary (4°): Attached to four other carbons.
Example: Secondary carbons are circled in the given structure.

Resonance Structures

Drawing Resonance Structures

Resonance structures are different Lewis structures for the same molecule, showing delocalization of electrons.

Rules: Only electrons (not atoms) can move; resonance structures must have the same arrangement of atoms.
Formal Charges: All resonance structures must show correct formal charges.
Example: Three resonance structures are drawn for a given molecule, with all lone pairs and formal charges indicated.

Resonance Validity

Not all drawn resonance structures are valid. Atoms cannot exceed their octet, and only electrons can be moved.

Example: Oxygen can only be positively charged with a full octet; atoms cannot move, only electrons.

HTML Table: Classification of Isomers

Pair of Structures	Classification
First pair	Identical Structures
Second pair	Cis-Trans Isomers
Third pair	Constitutional Isomers

Additional info:

Some context and explanations have been expanded for clarity and completeness, such as the definitions of hybridization, resonance, and acid-base theories.
Examples are based on the exam questions and answers provided in the file.