Organic Chemistry Fundamentals: Structure, Nomenclature, and Properties of Alkanes, Alkenes, Alkynes, and Aromatic Compounds

Chapter 11: Intro to Organic Compounds

Functions of Organic Compounds

Organic compounds are primarily composed of carbon atoms bonded to hydrogen, and may also include other elements such as oxygen, nitrogen, sulfur, phosphorus, and halogens. The structure and bonding in organic molecules determine their chemical properties and reactivity.

• Covalent bonds: Organic compounds mainly feature covalent bonds, which involve the sharing of electron pairs between atoms.

• Bonding patterns: Carbon forms four covalent bonds, often with hydrogen, oxygen, nitrogen, or halogens.

• Types of covalent bonds:

  • Single bond: $C-C$

  • Double bond: $C=C$

  • Triple bond: $C\equiv C$

• Functional groups: Specific groups of atoms within molecules that determine characteristic chemical reactions (e.g., alcohols, amines).

Example: Methane ($CH_4$) is the simplest organic molecule, with carbon bonded to four hydrogens.

Shapes of Atoms within Organic Molecules (VSEPR Theory)

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the three-dimensional shapes of molecules based on electron pair repulsion around the central atom.

• Tetrahedral: Four single bonds (e.g., $CH_4$)

• Trigonal planar: One double bond (e.g., $C_2H_4$)

• Linear: One triple bond (e.g., $C_2H_2$)

Example: Ethylene ($C_2H_4$) has a planar structure due to the double bond between carbons.

Bond Polarity and Intermolecular Forces

Bonds can be polar or nonpolar depending on the difference in electronegativity between atoms. Intermolecular forces influence boiling and melting points.

• Types of intermolecular forces:

  • London dispersion forces (weakest)

  • Dipole-dipole interactions

  • Hydrogen bonding (strongest)

• Boiling and melting points: Increase with stronger intermolecular forces.

Example: Water ($H_2O$) has a high boiling point due to hydrogen bonding.

Chapter 12: Alkanes

Constitutional Isomers

Constitutional isomers have the same molecular formula but different connectivity of atoms.

• Example: Butane ($C_4H_{10}$) and isobutane are constitutional isomers.

Nomenclature of Alkanes and Cycloalkanes (IUPAC)

The IUPAC system provides rules for naming alkanes and cycloalkanes based on the number of carbon atoms and branching.

• Steps:

  1. Find the longest continuous carbon chain.

  2. Number the chain from the end nearest a substituent.

  3. Name and number substituents as prefixes.

• Example: 2-methylpropane

Classification of Carbons

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

Type	Description
Primary	Bonded to one other carbon
Secondary	Bonded to two other carbons
Tertiary	Bonded to three other carbons
Quaternary	Bonded to four other carbons

Physical Properties of Alkanes

Alkanes are nonpolar molecules with relatively low boiling and melting points. Their physical properties depend on molecular size and branching.

• Boiling point: Increases with molecular size; decreases with branching.

• Solubility: Insoluble in water; soluble in nonpolar solvents.

Example: Hexane ($C_6H_{14}$) is a liquid at room temperature.

Combustion of Alkanes

Alkanes undergo combustion reactions with oxygen to produce carbon dioxide and water.

• General equation: $C_nH_{2n+2} + O_2 \rightarrow CO_2 + H_2O$

• Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Chapter 13: Alkenes, Alkynes, and Aromatic Compounds

Classification of Unsaturated Compounds

Unsaturated compounds contain double or triple bonds. Alkenes have C=C double bonds, alkynes have C≡C triple bonds, and aromatic compounds contain conjugated ring systems.

• Alkene: Contains at least one C=C bond

• Alkyne: Contains at least one C≡C bond

• Aromatic: Contains a benzene ring or similar structure

Nomenclature of Alkenes and Alkynes (IUPAC)

Alkenes and alkynes are named by identifying the longest chain containing the double or triple bond and numbering the chain to give the bond the lowest possible number.

• Example: 1-butene, 2-butyne

Structural Formulas and Isomers

Alkenes can exhibit cis-trans (geometric) isomerism due to restricted rotation around the double bond.

• Cis isomer: Substituents on the same side of the double bond

• Trans isomer: Substituents on opposite sides

Reactions of Alkenes and Alkynes

Alkenes and alkynes undergo addition reactions, where atoms are added across the multiple bond.

• Hydrogenation: Addition of $H_2$

• Halogenation: Addition of $X_2$ (e.g., $Br_2$, $Cl_2$)

• Hydration: Addition of $H_2O$

Example: Ethene reacts with bromine: $CH_2=CH_2 + Br_2 \rightarrow CH_2Br-CH_2Br$

Markovnikov's Rule

When adding HX or H2O to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with more hydrogens (the "rich get richer" rule).

• Example: Propene reacts with HBr: $CH_3-CH=CH_2 + HBr \rightarrow CH_3-CHBr-CH_3$

Additional info: These notes cover foundational concepts in organic chemistry relevant to a general chemistry curriculum, including structure, nomenclature, physical properties, and basic reactions of hydrocarbons.