Hydrocarbons and Nomenclature

Definitions and Classification of Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They are classified based on the types of bonds present and their structural features.

• Hydrocarbon: Compound composed of only carbon and hydrogen.

• Saturated Hydrocarbons (Alkanes): Compounds with only single bonds between carbon atoms.

• Unsaturated Hydrocarbons: Compounds with at least one double or triple bond between carbon atoms.

• Alkane: Hydrocarbon with only single bonds; general formula: .

• Cyclic Alkane: Hydrocarbon with carbon atoms arranged in a ring; general formula: .

Example: Butane () and cyclobutane ().

Heteroatoms and Heterocycles

Atoms in a molecule that are not carbon or hydrogen are called heteroatoms (e.g., nitrogen, oxygen). Cyclic structures containing heteroatoms are called heterocycles.

• Example: Pyrrole (contains nitrogen), Pyridine, Nicotinic acid.

Hybrid Orbitals and Bonding

Introduction to Hybridization

Hybridization is the process by which atomic orbitals mix to form new orbitals (hybrid orbitals) suitable for the pairing of electrons to form chemical bonds in molecules.

• sp3 Hybridization: Occurs in carbon atoms with four single bonds (tetrahedral geometry, bond angle ≈ 109.5°).

• sp2 Hybridization: Occurs in carbon atoms with one double bond (trigonal planar geometry, bond angle ≈ 120°).

• sp Hybridization: Occurs in carbon atoms with one triple bond (linear geometry, bond angle ≈ 180°).

Forming sp3 Hybridized Orbitals

Carbon atoms bonded to four atoms undergo sp3 hybridization, combining one s and three p orbitals to form four equivalent sp3 orbitals.

• Each sp3 orbital has 25% s character and 75% p character.

• These orbitals form four sigma () bonds.

• Example: Methane ().

Electron configuration: →

Forming sp2 Hybridized Orbitals

Carbon atoms bonded to three atoms (with one double bond) undergo sp2 hybridization, combining one s and two p orbitals to form three sp2 orbitals and one unhybridized p orbital.

• Each sp2 orbital has 33% s character and 66% p character.

• Three sp2 orbitals form sigma bonds; the unhybridized p orbital forms a pi () bond.

• Example: Ethylene ().

Electron configuration:

Forming sp Hybridized Orbitals

Carbon atoms bonded to two atoms (with one triple bond) undergo sp hybridization, combining one s and one p orbital to form two sp orbitals and two unhybridized p orbitals.

• Each sp orbital has 50% s character and 50% p character.

• Two sp orbitals form sigma bonds; two unhybridized p orbitals form two pi bonds.

• Example: Acetylene ().

Electron configuration:

Bond Types and Properties

• Sigma () bonds: Formed by head-to-head overlap of orbitals; all single bonds are sigma bonds.

• Pi () bonds: Formed by side-to-side overlap of unhybridized p orbitals; present in double and triple bonds.

• Double bond: One sigma and one pi bond.

• Triple bond: One sigma and two pi bonds.

Bond Length and Strength:

• Bond length:

• Bond strength:

Summary Table: Hybridization and Bonding

Key Points and Summary

• Bonding in organic chemistry is explained using hybrid orbitals.

• Four-bonded carbons undergo sp3 hybridization.

• Carbons with one double bond undergo sp2 hybridization.

• Carbons with one triple bond undergo sp hybridization.

• All single bonds are sigma bonds.

• A double bond consists of one sigma and one pi bond.

• A triple bond consists of one sigma and two pi bonds.

• Bond length: triple < double < single; bond strength: triple > double > single.

Additional info: These notes are based on lecture slides and handwritten annotations for Organic Chemistry I, Chapter 2, Fall 2025. The content covers foundational concepts in hydrocarbon classification, nomenclature, and the theory of orbital hybridization as applied to organic molecules.