Section 2: Intramolecular & Intermolecular Binding Forces

Binding Forces Roles

Bulk forces that hold atoms, ions, or molecules together, directly governing solubility, melting point, boiling point, stability, and drug-excipient interactions.

Intramolecular Forces Location

Chemical bonding forces acting "within" molecules or compounds to securely hold atoms together.

Intramolecular Forces Strength

Generally much stronger than intermolecular forces.

Intermolecular Forces Location

Chemical bonding forces operating "between" separate molecules.

Ionic Bonds Mechanism

Formed via the absolute transfer of electrons between a metal and a nonmetal atom.

Covalent Bonds Mechanism

Formed via the sharing of electron pairs between nonmetal atoms.

Electronegativity Definition

The inherent chemical ability of an atom within a covalent bond to attract shared electrons to itself.

Pure Covalent Bond Parameter

A bond possessing an electronegativity difference strictly less than 0.4.

Polar Covalent Bond Parameter

A bond possessing an electronegativity difference ranging from 0.4 to 1.7 (or 2.0).

Ionic Bond EN Difference Parameter

A bond possessing an electronegativity difference strictly greater than 1.7 (or 2.0).

Four Types of Intramolecular Forces

Categorized as ionic, covalent, metallic, and intramolecular hydrogen bonds.

Van der Waals Forces Definition

The weakest class of intermolecular forces, subdivided into Keesom, Debye, and London forces.

Keesom Forces Definition

Dipole-dipole interactions occurring between two polar molecules due to the Orientation or Alignment effect.

Keesom Force Energy Boundary

Requires 1 to 7 kcal/mole of energy.

Keesom Force Solute Examples

Readily observed in water, alcohols, and acetone (WAA).

Debye Forces Definition

Dipole-induced dipole interactions occurring between a polar molecule and a nonpolar molecule due to the Induction effect.

Debye Force Energy Boundary

Requires 1 to 3 kcal/mole of energy.

Debye Force Solute Examples

Readily observed in ether and ethyl acetate (EE).

London Forces Definition

Extremely weak dispersion forces originating from transient molecular vibrations between nonpolar molecules.

London Forces Energy Boundary

The weakest Van der Waals subclass, requiring 0.5 to 1 kcal/mole of energy.

London Force Solute Examples

Readily observed in hexane and carbon tetrachloride (CCl4).

Attractive Forces Definition

Forces that bring molecules "together", subdivided into cohesive and adhesive interactions.

Cohesive Forces Definition

Attractive intermolecular forces operating between identical, like molecules.

Adhesive Forces Definition

Attractive intermolecular forces operating between distinct, unlike molecules.

Repulsive Forces Definition

Intermolecular forces that push molecules "apart".

Equilibrium Distance of Forces

The absolute distance parameter (3 to 4 x 10^-8 cm) where molecular attractive and repulsive forces become completely equal.

Ion-Dipole Forces Definition

Interactions occurring between a charged ion (+/-) and a polar molecule.

Ion-Dipole Forces Role

Strongly governs and enhances the aqueous solubility of salts in water.

Ion-Dipole Force Examples

Dissolution of quaternary ammonium compounds or tertiary amines in water; Na+ attracts the negative side of water while Cl- attracts the positive side.

Ion-Induced Dipole Interactions

Interactions occurring between a charged ion (+/-) and a nonpolar molecule.

Ion-Induced Dipole Example

The chemical interaction between molecular iodine (I2) and potassium iodide (KI) to form a soluble tri-iodide complex.

Hydrogen Bonds Definition

A highly electronegative attraction between a hydrogen atom and a highly electronegative atom (F, O, N, S).

Hydrogen Bond Energy Range

Requires 2 to 8 kcal/mole of energy.

Electronegative FONS Atoms

Represents Fluorine, Oxygen, Nitrogen, and Sulfur.

Hydrogen Bond Subclass

Considered a special, highly powerful type of dipole-dipole interaction.

Hydrogen Bonding Solute Examples

Readily forms in water, alcohols, carboxylic acids (R-COOH), phenol (C6H5OH), esters, and aldehydes.

Molecules Lacking Intermolecular H-Bonding

Ethers and ketones do not possess the structural capability to form hydrogen bonds with themselves.

Water Anomalies from H-Bonding

Responsible for water's uniquely high dielectric constant (universal solvent), abnormally low vapor pressure, and high boiling point.

Protein Structural Hydrogen Bonding

Dictates and stabilizes alpha helices and beta sheets within the secondary structure of proteins.

Nucleic Acid Hydrogen Bonding

Stabilizes DNA base-pairing, forming 2 hydrogen bonds between Adenine-Thymine (A-T) and 3 hydrogen bonds between Guanine-Cytosine (G-C).

Boiling Point Comparison of Small Organics

Ordered sequentially as Methane (-161°C) < Chloromethane (-24°C) < Methanol (80°C).

IMF Class Comparison

Ordered by increasing strength as London Dispersion < Dipole-Dipole < Hydrogen Bonding.