Organic Chemistry Fundamentals

Lewis Structures and Free Radicals

Lewis structures are diagrams that represent the bonding between atoms and the distribution of electrons in a molecule. Free radicals are species with an unpaired electron, often highly reactive.

• Lewis Structure: Shows all valence electrons, including lone pairs and bonding pairs.

• Free Radical Example: CH3 and CH(CH3)2 have an unpaired electron on a carbon atom.

• Application: Free radicals are important in organic reactions such as halogenation and polymerization.

Formal Charges in Lewis Structures

Formal charge is a bookkeeping tool used to determine the charge distribution within a molecule or ion. It is calculated as:

• Formula:

• Example: For NaOC(CH3)3, the oxygen atom may carry a negative formal charge.

• Application: Correct assignment of formal charges helps predict reactivity and stability of molecules.

Multiple Bonds and Non-Bonding Electrons

Organic molecules can contain single, double, or triple bonds. Non-bonding electrons (lone pairs) are important for molecular geometry and reactivity.

• Double Bond: Two pairs of electrons shared between two atoms (e.g., C2H4).

• Triple Bond: Three pairs of electrons shared (e.g., C4H6).

• Marking Non-Bonding Electrons: Lone pairs are shown as dots on atoms in Lewis structures.

Skeletal and Condensed Structures

Skeletal structures are simplified representations of organic molecules, showing bonds between carbon atoms and omitting hydrogen atoms bonded to carbon.

• Condensed Structure: Shows all atoms but groups hydrogens with their attached carbons (e.g., CH3CH2COOH).

• Skeletal Structure: Lines represent carbon-carbon bonds; vertices and ends represent carbon atoms.

• Application: Used for quick visualization of complex organic molecules.

Condensed Structures from Molecular Models

Molecular models use color-coded spheres to represent atoms. Condensed structures translate these models into chemical formulas.

• Color Coding: Black = Carbon, White = Hydrogen, Red = Oxygen, Green = Chlorine.

• Example: (CH3)2CHCH2CH2Cl represents a molecule with a chlorine atom attached to a carbon chain.

Index of Hydrogen Deficiency (IHD)

The Index of Hydrogen Deficiency (IHD) indicates the number of rings and multiple bonds in a molecule. It is calculated as:

• Formula: where C = number of carbons, N = number of nitrogens, H = number of hydrogens, X = number of halogens.

• Application: Used to deduce structural features from molecular formulas.

• Example: For C12H8O2BrF, calculate IHD to determine rings/double bonds.

Elements of Unsaturation

Elements of unsaturation refer to rings and multiple bonds in a molecule. Each double bond or ring increases the element of unsaturation by one; each triple bond by two.

• Calculation: Use IHD formula above.

• Example Table:

Constitutional Isomers

Constitutional isomers are compounds with the same molecular formula but different connectivity of atoms.

• Example: C8H18 has several constitutional isomers, each with a unique carbon skeleton.

• Application: Isomerism is important in drug design and material science.

Structural Isomers and Double Bonds

Structural isomers differ in the arrangement of atoms. For alkenes (e.g., C4H8), isomers may or may not show cis/trans (geometric) isomerism.

• Double Bond Location: Isomers can have the double bond in different positions or within rings.

• Application: Used to predict physical and chemical properties.

Acyclic Isomers with Halogens

Acyclic isomers are open-chain compounds. For a formula like C3H5Cl, isomers differ by the position of the chlorine atom and the arrangement of the carbon chain.

• Stereoisomers: If applicable, include isomers with different spatial arrangements.

Resonance Structures

Resonance structures are different Lewis structures for the same molecule, showing delocalization of electrons. The true structure is a hybrid of all resonance contributors.

• Pushable Electron Pairs: Lone pairs and pi electrons can be moved to form resonance structures.

• Example: Acetate ion has resonance between two oxygen atoms.

• Resonance Hybrid: Represents the delocalization of electrons across the molecule.

Relative Stability of Resonance Contributors

Resonance contributors are not always equally stable. Stability is influenced by charge distribution, electronegativity, and octet fulfillment.

• More Stable Contributor: Has charges on more electronegative atoms and complete octets.

• Example Table:

Additional info: These topics are foundational in organic chemistry and are not directly related to Personal Health, but are essential for students studying chemistry, biochemistry, or related health sciences.