Distillation and Boiling Points

• Distillation separates components from a liquid mixture through selective evaporation and condensation.
• It's a basic method for purifying liquids and separating liquid mixtures.
• The process involves heating a liquid to boiling, then collecting and condensing the resulting vapors.

Uses for Distillation

• Separating liquids with slightly different boiling points.
• Separating liquids from non-volatile components.
• Purifying liquids.

Boiling Point

• The temperature at which a liquid's saturated vapor pressure equals the surrounding atmospheric pressure.
• When a liquid is heated at constant pressure, its vapor pressure increases.
• A liquid boils when its vapor pressure equals the outside atmospheric pressure.
• The boiling point is the temperature at which the vapor pressure equals the atmospheric pressure.
• Adding more heat at the boiling point doesn't increase temperature, it fuels vaporization.
• Boiling point is the temperature a substance changes from liquid to gas throughout its volume.
• At the boiling point, molecules anywhere in the liquid can vaporize.
• Liquids partially vaporize until the vapor pressure reaches a characteristic value at that temperature.
• As temperature increases, vapor pressure increases, and bubbles form within the liquid at boiling point.
• A liquid's boiling point varies with applied pressure.
• Normal boiling point occurs when vapor pressure equals standard sea-level atmospheric pressure (760 mm of mercury).

Factors Affecting Boiling Point

• Boiling point relies on molecular mass and attractive forces between molecules.
• Intermolecular force strength order: Ionic > Hydrogen bonding > dipole dipole > Van der Waals dispersion forces.
• Stronger attractive forces generally lead to a higher boiling point.
• Boiling points increase with more carbons but decrease with branching. Boiling point of associative polar liquids is higher than non associative polar liquids.
• Example: ethanol has a higher boiling point than diethyl ether. Both have higher boiling points than nonpolar propane.

Boiling Point at Different Pressures

• Boiling points are usually reported at normal atmospheric pressure (760 mmHg).
• Boiling points vary depending on pressure during distillation.
• New boiling points can be calculated using the formula: Log p = A/T + C, where p is vapor pressure, T is absolute temperature, and A and C are constants.

Types of Distillation

• Simple Distillation.
• Fractional Distillation.
• Steam Distillation.
• Vacuum Distillation.
• Molecular Distillation.
• Fractional Distillation Under Reduced Pressure.

Simple Distillation

• Suited for substances with simple structures, are volatile, and withstand their boiling heat
• Used to purify hydrocarbons, alcohols, esters, small molecule fatty acids, amines.
• Can separate two liquids with at least an 80°C boiling point difference.
• Vapors are richest in the lowest boiling component.
• Purified compounds boil and vaporize over a small temperature range (2-3°C).

Simple Distillation Procedure

• Check the thermometer's calibration.
• Fill the distillation flask no more than two-thirds full to prevent liquid from being propelled into the condenser.
• Add boiling chips to prevent superheating and bumping.
• Heat the flask slowly until boiling, allowing vapors to rise.
• Aim for a distillation rate of about 20 drops per minute, ensuring vapors condense in the condenser
• Monitor the thermometer; collect distillate in new receivers when the temperature stabilizes within a 2-3°C range or when it begins to change.
• Save all distillate fractions until desired compound separation is confirmed.
• Remove the heat source before all liquid vaporizes to prevent explosion hazards from peroxides in the residue.
• Avoid distilling to dryness to prevent potential peroxide explosions.
• Secure all joints tightly to prevent vapor leaks and ignition.
• Never heat a closed system to prevent explosions; ensure the thermometer isn't blocking vapor flow in tapered neck flasks.

Fractional Distillation

• Used for mixtures with closely boiling points (less than 80°C difference).
• A fractionating column sits between the flask and condenser.
• Procedures are similar to simple distillation, but equipment differs through the packed fractioning column.
• Fractionating columns increase surface area for rising vapors to condense and revaporize.
• The fractionating column establishes a temperature gradient for the distillation.

Fractional Distillation Process

• In an ideal scenario, temperature in the flask equals of liquid mixture boiling point.
• Temperature at the top of the fractionating column equals the boiling point of the lower boiling compound.
• All of the is lower boiling point compound is distilled before of the is higher boiling point.
• In reality, the distillation of the higher boiling point increases throughout the process.
• Collect several fractions over a small temperature range and re-distill them.

Raoult's Law

• The vapor pressure of a solvent above a solution equals the vapor pressure of the pure solvent at the same temperature, scaled by the mole fraction of the solvent present.
• P solution = X solvent * P° solvent, where P = vapor pressure and X = number of moles.
• Ideal solutions have uniform components and physical properties connected to their pure components.
• Interactions between solute and solvent molecules are the same as if they were by themselves.
• An example: is benzene and toluene.

Azeotropes

• Azeotropes don't conform to Raoult's law.
• Azeotrope vapors have the same composition as their liquid.
• Azeotropes behave like pure liquids and cannot be separated by distillation at specific concentrations.
• Examples include 95% ethanol-5% water (bp 78.1 °C), various benzene, ethanol, and acetone mixtures..
• Azeotropic compositions boil at temperatures sometimes lower or higher than their components.
• Azeotropic mixtures can be separated by: adding another liquid to alter the vapor pressure ratio or using chemical reactions to remove components.

Steam Distillation

• A separation process used to purify or isolate temperature sensitive materials, like natural aromatic compounds.
• Steam or water introduces to process or lowers boiling points.
• Steam distillation is useful for organic purification, or isolating a steam stripping.

Vacuum Distillation

• Distillation conducted under reduced pressure.
• Vacuum distillation is suitable for organic compounds that decompose at or below their boiling points.
• Lowering the pressure reduces the boiling point of liquids.
• Vacuum distillation is for purifying solids by recovering impurities from non-volatile materials.

Molecular Distillation

• Considered the safest method for separating and purifying thermally unstable molecules with low volatility and high boiling points.
• This method is distinguished by exposing substances to heat.
• This is done by keeping temperature exposure time to a minimum.
• The separation occurs in the zone of the molecular evaporator exposed to heat, under vacuum.
• It relies on the difference in molecular mean free path and requires collision-free passage of molecules.

Fractional Distillation Under Reduced Pressure

• Differs from vacuum through its use of liquid mixtures separated via a fraction header

Sublimation

• Sublimation is the direct change of a solid to vapour without passing through the liquid state.
• It separates volatile organic compounds from non-volatile impurities (e.g., naphthalene, benzoic acid, anthracene, camphor).
• Substances purified by sublimation must have impurities that do not sublime.
• After heating most organic compounds without liquid form will form vapors stage then condensation and cooling directly yields the solid.
• Sublimation can separate a mixture of two compounds.