Chemistry: Boiling Point Concepts

Boiling Point

• Boiling point is the temperature at which a liquid vaporizes at a specified pressure.
• The heat absorbed when a liquid vaporizes at its normal boiling point is called the latent heat of vaporization.
• Boiling point depends on intermolecular forces of attraction between molecules.
• Increased molecular weight generally increases boiling point.
• Branching of a molecule's chain decreases boiling point.
• Alcohols have higher boiling points than hydrocarbons with the same molecular weight due to hydrogen bonding.
• Carboxylic acids exhibit abnormally high boiling points due to dimer formation through hydrogen bonding.
• Nonpolar molecules have lower boiling points than polar molecules.

Boiling Point of Specific Compounds

• The boiling point of alkanes, chloroalkanes, alkenes, alcohols, aldehydes, ketones, carboxylic acids, and ethers increases with increasing molar mass.

Pentane Isomer Boiling Points

• n-pentane (pentane): 36.0 °C
• Isopentane (2-methylbutane): 27.7 °C
• Neopentane (2,2-dimethylpropane): 9.5 °C

Distribution and Partitioning

• Immiscible liquids form two separate phases.
• Examples: water/ether, water/amyl alcohol, water/octanol, water/peanut oil.
• Excess solute distributes itself between the phases to reach saturation.

Qualitative Description of Distribution and Partitioning

• When two immiscible liquids are combined, they separate into distinctly different phases.
• Systems of water and various organic solvents (ether, amyl alcohol, octanol, or peanut oil) are common liquid systems used in these processes.
• Adding excess solute or liquid will distribute across the phases to achieve saturation in each phase.

Quantitative Description of Distribution and Partitioning

• When two solvents are in equilibrium with a substance the ratio of the equilibrium concentrations of the substances in the solvents is constant.
• This constant is the distribution coefficient (or partition coefficient, K).
• For calculating the partition coefficient the numerator should be the concentration of the solute in the organic phase and the denominator the one in the aqueous phase.
• The partition coefficient is a measure of the relative affinity of a substance for each of the two immiscible solvents.
• High values of the partition coefficient indicate that the substance has a higher affinity for the organic component of the system vs the aqueous component.

Shake-Flask Method

• This method is widely used to determine the partition coefficient of a drug.
• The drug is placed in a separating funnel with two immiscible solvents.
• The pH of the aqueous layer is carefully adjusted to minimize ionization of the drug (generally pH = pKa-2 for weak acids or pH = pKa+2 for weak bases).

Pharmaceutical Relevance of Distribution and Partitioning

• Partitioning is important in various aspects of drug action, including drug preservation in oil/water systems, absorption, and distribution of drugs throughout the body.

Effect of Ionic Dissociation on Partitioning

• Molecules can exist in different forms in various solvents; they might dissociate into ions in aqueous solutions or associate in separate phases.
• The solubility of certain compounds in aqueous environments is dependent on the pH of the solution.
• Ionization can significantly affect partitioning behavior.
• A non-ionized solute shows little difference in solubility between oil and water. In contrast, ionized forms are much more soluble in water.

Preservative Action in Oil-Water Systems

• Preservatives are frequently used to prevent microbial attack on solutions, suspensions, and emulsions.
• Benzoic acid, often in the form of sodium benzoate, is a common preservative.
• The effectiveness of benzoic acid depends mainly on its undissociated form, which readily penetrates biological membranes.

Extraction

• Extraction is a process for separating a compound from a solvent system.
• Extraction relies on the differences in the distribution between phases of the target substance.
• The distribution (or partition) coefficient is a measure of the relative tendency of a particular solute to be in two liquid phases.
• The K value should be high to ensure a good separation.

Solubility and Distribution Phenomena

• Solubility is a quantitative parameter; it's the concentration of the solute that's saturated in a solution at a particular temperature and pressure.
• Solubility depends on the physical and chemical properties of the solute and solvent (including temperature, pressure, and pH).
• Miscibility is the mutual solubility of liquids.
• The solubility (in g/mL) of solids is also greatly affected by changes in temperature and changes in pH.

Solubility Definitions

• The USP expresses solubility as parts of solvent required for 1 part of solute.
• Solubility is also defined quantitatively in terms of molality, molarity, and percentage.
• Many official compendia (e.g., USP, Merck Index) provide exact solubility values for various substances.

Solvent-Solute Interactions

• "Like dissolves like"
• Polar solvents dissolve polar solutes due to similar intermolecular forces.
• Nonpolar solvents dissolve nonpolar solutes.
• Semipolar solvents can act as intermediates between polar and nonpolar systems and, as such, can increase the solubility of polar compounds in nonpolar systems.
• Aliphatic alcohols are examples that fit this trend. Solubility in water decreases as the length of the nonpolar hydrocarbon chain increases.

Solubility of Compounds in Liquids

• The influence of temperature, pressure and pH also play a role in the solubility of compounds. Endothermic processes increase solubility, whereas exothermic processes reduce it.

Crystallisation

• Crystallisation describes methods to obtain crystals from a solution.
• Supersaturation is a key step in these processes. It means the solution contains more solute than would normally dissolve under equilibrium conditions at a given temperature and pressure.
• Methods to induce supersaturation include cooling, evaporation, increasing solute concentration, chemical reaction, addition of seed crystals, and deliberate seeding.

Crystal Growth

• Molecules move or are transported to the surface of a seed crystal to form a new solid structure.
• Increased agitation in a system affects the rate of diffusion of molecules to the surface of growing crystals.

Precipitation

• Precipitation is an induced process that may occur due to altering the pH of the solution, or by chemical reactions to form a precipitate from a homogeneous solution.

Polymorphism

• Polymorphism refers to the ability of a compound to exist in multiple crystal forms.
• The physical and chemical properties may vary with the crystal structure.
• The molecules pack together in slightly different ways in each crystal structure.
• These differences can influence the rate of dissolution, solubility, stability, and pharmacological action.

Amorphous Solids

• Amorphous solids do not have a regular, repeating crystal structure.
• Their properties are often more isotropic, meaning they exhibit similar properties in all directions.
• Amorphous solids have no specific melting point; rather, they exhibit a gradual transition in temperature from solid to liquid state.

Explore the essential concepts of boiling points in this chemistry quiz. Learn about how molecular weight, intermolecular forces, and molecular structure affect the boiling points of various compounds. Test your understanding of alcohols, carboxylic acids, and other organic compounds.