Chapter 2: Organic Chemistry

Organic Chemistry Overview

• Definition of Organic Chemistry: The study of carbon-based compounds.

• Origin of Term "Organic":

  • Derived from historical classifications of chemical compounds into two types:

  • Inorganic Compounds: Originated from minerals.

  • Organic Compounds: Originated from living organisms (plant or animal sources).

Theoretical Background

• Vital Force Theory:

  • Until the 18th century, many chemists believed that organic compounds could only arise from living organisms.

• Friedrich Wöhler's Contribution (1828):

  • Synthesis of Urea from inorganic compounds disproving the Vital Force Theory.

  • This breakthrough opened the door for laboratory synthesis of various organic compounds.

Composition and Classification of Organic Compounds

• Basic Composition: Most organic compounds are primarily made up of:

  • Carbon (C)

  • Other Elements: Hydrogen (H), Oxygen (O), Nitrogen (N)

• Diversity: More than 10 million known compounds contain carbon and its primary elements.

• Retention of Naming: Despite laboratory synthesis, the term "organic" remains in use for these compounds.

Types of Organic Compounds

• Sugars

• Nitrogenous Bases:

  • Include Adenine, Thymine, Guanine, Cytosine.

• Pharmaceuticals: Examples like Remdesivir.

• Plastics, Pesticides, Vitamins, Fibers

Alkanes and Cycloalkanes

Hydrocarbons

• Definition of Hydrocarbon:

  • Compounds consisting solely of carbon and hydrogen (e.g., $ext{CH}<em>4$, $ext{C}</em>6 ext{H}_6$).

• Alkanes:

  • Saturated hydrocarbons containing only carbon-carbon single bonds.

  • Saturated means each carbon atom is bonded to the maximum number of hydrogen atoms.

  • Also referred to as aliphatic hydrocarbons.

  • Similar properties to molecules in animal fats and plant oils (Greek: aleiphar means fat or oil).

General Molecular Formulas

• Alkanes: $\text{C}<em>n\text{H}</em>{2n+2}$

• Cycloalkanes: $\text{C}<em>n\text{H}</em>{2n}$

  • Indicates one degree of unsaturation.

Where:

• $\text{C}$ represents the element Carbon.

• $\text{H}$ represents the element Hydrogen.

• $n$ represents the number of carbon atoms in the molecule.

  • For alkanes, $n$ is a positive integer ($n \geq 1$).

  • For cycloalkanes, $n$ is an integer ($n \geq 3$) because a ring requires at least three carbon atoms.

Specific Examples

• Methane (CH4): Simplest and most abundant alkane. Found in the atmosphere, ground, oceans, and even other planets (e.g., Mars, Jupiter).

• Natural Gas Composition: Approximately 75% Methane, 10% Ethane, 5% Propane.

Boiling Points of Alkanes

• Trend: Boiling point increases with the number of carbon atoms.

  • Alkanes from C1 to C4 are gases at room temperature.

  • C5 to C15 are liquids.

  • Greater than C15 are solids.

Molecular Structure

• Methane Structure:

  • Shape: Tetrahedral, with bond angles of 109.5°.

• Ethane Structure:

  • Each carbon is tetrahedral, bond angles approximately 109.5°.

Propane Structure:

• three carbon atoms linked in a chain with single covalent bonds, each surrounded by enough hydrogen atoms to satisfy the tetravalence of carbon, resulting in a molecular formula of C3H8.

• The bond angles between carbons and hydrogens are approximately 109.5°.

• The central carbon atom is bonded to two other carbon atoms and two hydrogen atoms.

• The two terminal carbon atoms are each bonded to one other carbon atom and three hydrogen atoms.

• Molecular formula: C3H8C3H8

• Condensed structural formula: CH3CH2CH3CH3CH2CH3

Naming Alkanes: IUPAC Rules

• Principles of Naming Organic Compounds:

  • Governed by the International Union of Pure and Applied Chemistry (IUPAC) rules.

  • Common names persist despite systematic nomenclature.

• Basic IUPAC Naming:

  • Name based on the number of carbon atoms (#Cs), type of C-C bonds, and functional groups present.

Prefixes for Carbon Counts

• Types of Bonds:

  • Single Bond: suffix -an-

  • Double Bond: suffix -en-

  • Triple Bond: suffix -yn-

Naming Cycloalkanes

• Cycloalkanes Naming: Add prefix Cyclo- after applying the same rules as for linear alkanes.

Examples of Cycloalkanes

• Cyclopropane: $ext{C}<em>3 ext{H}</em>6$

• Cyclobutane: $ext{C}<em>4 ext{H}</em>8$

• Cyclopentane: $ext{C}<em>5 ext{H}</em>{10}$

• Cyclohexane: $ext{C}<em>6 ext{H}</em>{12}$

Homologous Structures

• Definition: A group of organic molecules that are alike in their basic structure and chemical behavior, where each member in the series differs from the next by just one $\text{CH}_2$ unit.

Constitutional Isomerism in Alkanes

• Definition of Constitutional Isomers: Compounds with the same molecular formula but different structural formulas due to different connectivity of their atoms.

  • Examples include various structural forms of pentane, isopentane, and neopentane.

Examples of Pentane Isomers

• Normal Pentane/n-Pentane: CH3CH2CH2CH2CH3

• Isopentane: CH3CHCH3CH2CH3

• Neopentane: (CH3)4C

  • Here's what it means in simpler terms:

    • The lonely CC at the end refers to a central Carbon atom.

    • The (CH3)4(CH3)4​ part means there are four CH3CH3 groups attached to that central carbon atom.

    • A CH3CH3 group is known as a methyl group. It consists of one carbon atom bonded to three hydrogen atoms.

    So, imagine a central carbon atom with four other carbon atoms (each with three hydrogens) directly connected to

Comparing Structural Isomers

• Boiling Points for C5H12 Isomers:

  • Normal pentane: 36.0°C

  • 2-methylbutane: 27.9°C

  • 2,2-dimethylpropane: 9.5°C

• Trends: More branching generally leads to lower boiling points due to weaker London dispersion forces.

Number of Isomers by Molecular Formula

Types of Carbon in Alkanes

• Primary Carbon: Bonded to only one other carbon atom (e.g., $ext{CH}_3-$)

• Secondary Carbon: Bonded to two other carbon atoms

• Tertiary Carbon: Bonded to three other carbon atoms

Alkyl Groups

• An alkyl group is essentially an alkane molecule that has lost one of its hydrogen atoms, allowing it to attach to other atoms or molecules in a larger chemical structure.

• Nomenclature: Change suffix “ane” to “yl” when identifying alkyl groups.

  • E.g., $ext{CH}<em>4 ightarrow ext{CH}</em>3$ = methyl; $ext{C}<em>2 ext{H}</em>6 <br>ightarrow ext{C}<em>2 ext{H}</em>5$ = ethyl.

Formation of Alkyl Groups

• Primary Alkyl Group: Formed by removing one hydrogen from propane.

• Secondary Alkyl Group: Like isopropyl, involves removing from a secondary position.

Nomenclature of Substituents

• Assign numbers indicating the parent chain's carbon to which the substituents are attached.

• Use alphabetization when multiple substituents are present and apply di-, tri-, and tetra- prefixes accordingly.

Examples of Naming Alkanes

• 2-Methyl-4-Methylhexane

• 5-sec-butyl-2,7-dimethylnonane

Stereochemistry of Alkanes

• Cis-Trans Isomerism:

  • Cis Isomer: Two identical groups are on the same side of the double bond.

  • Trans Isomer: Two identical groups are on opposite sides of the double bond.

Atom Movement in Molecules

• Atoms constantly move, impacting bond angles and interactions:

  • Bond Stretching

  • Bending

  • Rotational Movement

Stereoisomerism and Conformational Isomerism

• Differentiation between isomers present in different spatial arrangements despite identical molecular structure (Newman projections employed for visualization).

Types of Strain in Cycloalkanes

• Steric Strain: Occurs when bulky groups are forced close together causing repulsion.

• Torsional Strain: Arises from eclipsing interactions between bonds.

Cyclohexane Chair Conformation

• Chair Form: Most stable conformation, with no angle strain and minimal torsional strain.

• Ring-Flip: Transition between axial and equatorial orientations of substituents.

Conclusion

• Importance of Organic Chemistry: Critical for understanding biological systems, pharmaceutical development, and material science.