Notes on CHEM 153: Basic Organic Chemistry

Instructor: Dr. Akwasi Acheampong
Qualifications: BSc. (Hons) Chemistry, MSc. Pharm, PhD (Paris-Sud, Orsay), PhD (KNUST, Kumasi)

Molecular Composition and Structure
a) Definition and calculation of empirical and molecular formulae
b) Pictorial treatment of hybridization
Organic Reactions
a) Bond formation and fission
b) Classification of reagents and reactions
c) Reaction intermediates: carbocations, free radicals, and carbanions
Hydrocarbons
a) Structure and nomenclature
b) Homologous series and gradation of properties
c) Preparation
d) Reactions

Organic Chemistry by Robert Thornton Morrison and Robert Neilson Boyd. Published by Prentice Hall Int. Inc., New York. 6th Edition or a later edition.
Organic Chemistry by T.W. Graham Solomons. Published by John Wiley and Sons, New York. 4th Edition or later edition.
Organic Chemistry by Francis Carey

The structural theory is based on two central ideas:
- Bond Formation: The atoms of the elements in organic compounds can form a fixed number of bonds, known as valence. Carbon is tetravalent, meaning it can form four bonds.
- Bonding Between Carbons: A carbon atom can utilize one or more of its valences to bond with other carbon atoms. This theory assists in distinguishing between compounds that share the same molecular formula but exhibit significantly different properties.

Definition: The simplest chemical formula that shows the relative numbers of different kinds of atoms in a molecule.
Characteristics: It gives the smallest whole number ratios of the atoms present.
Examples:
- $CH_2$
- $CHO$
- $C2H3$

Definition: A chemical formula that reveals the actual number of each kind of atom present in a molecule.
Characteristics: Combines chemical symbols with subscripts indicating the count of atoms for each element in a molecule.

Conduct qualitative elemental analysis to identify the types of atoms within the molecule.
Perform quantitative elemental analysis to ascertain the relative numbers of different atoms, yielding the empirical formula.
Determine the molecular weight of the compound.

Formula Relation:
- $Molecular \, formula = Empirical \, formula imes n$, where $n \geq 1$
Molecular Weight Relation:
- $Molecular \, Weight = Empirical \, formula \times n$, where $n$ is the number of carbon atoms.

Example 1: What is the empirical formula of the compound that has the following percent compositions?
- Given:
  - H = 7.75%, C = 37.21%, Cl = 55.04%
- Molar Masses: H = 1.0, C = 12.0, Cl = 35.5
Calculation Steps:
- Moles (n):
  - $C: \frac{37.21}{12.0} = 3.1$
  - $H: \frac{7.75}{1.0} = 7.75$
  - $Cl: \frac{55.04}{35.5} = 1.55$
- Ratio Calculation:
  - Divide by the smallest mole:
  - $\frac{3.1}{1.55} = 2.0$
  - $\frac{7.75}{1.55} = 5.0$
  - $\frac{1.55}{1.55} = 1.0$
- Empirical Formula Found: $C2H5Cl$
Example 2: An organic compound A contains 48.6% C, 8.1% H, remainder being oxygen. The vapor density of A is 37.
- Steps to calculate:
1. Calculate the empirical formula of A.
2. Determine the molecular formula of A.
3. Deduce structures of A and B.

Quantum Mechanics Summary:
1. Atomic Orbital: A region about the nucleus where there is a high probability of finding an electron.
2. Molecular Orbitals: Formed when atomic orbitals overlap.
3. Bonding Molecular Orbital: Formed when orbitals with the same phase sign overlap.
4. Antibonding Molecular Orbital: Forms when opposite phase sign orbitals overlap.
5. Energy Comparison: Electrons in bonding orbitals have less energy than their atomic orbital counterparts.
6. Molecular Orbital Count: Matches the number of atomic orbitals used.
7. Hybrid Atomic Orbitals: Created by mixing the wave functions of different types of orbitals from the same atom.
8. Hybridization Types:
  - sp³: Tetrahedral hybridization from three p and one s orbital.
  - sp²: Trigonal planar from two p and one s orbital.
  - sp: Linear from one p and one s orbital.

Sigma (σ) Bonds:
- Definition: Formed when the electron density is circular along the bond axis.
- Example: All single bonds are σ bonds.
Pi (π) Bonds:
- Definition: Formed when the electron densities of adjacent parallel p orbitals overlap to form a bonding π molecular orbital.
- Example: Present in double and triple bonds.

Energy Barrier: There is a significant energy barrier to rotation for groups joined by a double bond — approximately 264 kJ/mol due to the strength of the π bond.
Comparison: Rotation around C-C single bonds typically requires around 13-26 kJ/mol.

Methane (CH₄):
- Structure: Tetrahedral with bond angles of $109.5°$.
- Hybridization: sp³ hybridization.
Ethane (C₂H₆):
- Structure: Formed from two sp³ hybridized carbons.
Ethene (C₂H₄):
- Structure: Planar structure with bond angles of ~$120°$ and contains one σ bond and one π bond.
- Hybridization: sp² hybridization.
Ethyne (Acetylene, C₂H₂):
- Structure: Linear structure with bond angles of $180°$.
- Hybridization: sp hybridization.

VSEPR Theory:
- Definition: Predicts molecular shape based on electron pair repulsion.
- Electron Consideration: Both bonding and nonbonding electron pairs are considered.
- Geometry: Determined primarily by the position of nuclei, not electron pairs.

Homolysis:
- Definition: Bond cleavage that results in free radicals (e.g., A B → radicals).
Heterolysis:
- Definition: Bond cleavage resulting in ions (e.g., A B → A⁺ + B⁻).
- Commonly requires polarized bonds and external assistance.

Carbocations:
- Definition: Electron-deficient species with only six electrons in their valence shell.
- Classification: Considered Lewis acids due to their electron-seeking nature.
Carbanions:
- Definition: Electron-rich anions that donate electron pairs and behave as Lewis bases.

Electrophiles:
- Definition: Electron-deficient species that accept electron pairs (e.g., H₃O⁺, H₃C⁺, BF₃).
Nucleophiles:
- Definition: Electron-rich species that donate electron pairs (e.g., OH⁻, Cl⁻, RO⁻).

Categories:
1. Substitutions
  - Definition: One group replaces another in a molecule.
2. Additions
  - Definition: Two molecules combine to form one, characteristic of compounds with multiple bonds.
3. Eliminations
  - Definition: One molecule loses parts of another small molecule, aiding in forming double/triple bonds.
4. Rearrangements
  - Definition: A molecule reorganizes its constituent parts.