Recrystallization Techniques and Principles

Comprehensive vocabulary flashcards covering the theory, solvent selection criteria, filtration methods, and procedural troubleshooting for recrystallization based on lecture notes.

Recrystallization

A purification process used to purify an organic compound that is solid at room temperature by dissolving it in a hot solvent and allowing it to recrystallize as the solvent cools to room temperature.

Precipitation

A rapid, non-selective crystal lattice formation process that does not aid in purification because impurities become trapped within the crystal lattice, yielding an impure solid.

Crystallization

A very slow process where a crystal lattice grows with a high degree of selectivity, excluding impurities over time to produce pure crystals.

Three Laboratory Steps of Recrystallization

1. Dissolving the solid in hot solvent; 2. Filtering insoluble impurities; 3. Allowing desired crystals to form in the filtrate while impurities remain dissolved in the solvent.

Recrystallization Solvent Requirements

A suitable solvent must not react with the compound, must form desirable well-formed crystals, must be volatile (easily removed), must dissolve the substance poorly at room temperature and well at high temperatures, and must have high solvency for impurities.

"Like Dissolves Like" Principle

The solubility rule stating that polar compounds (containing functional groups such as $-\text{OH}$, $-\text{COOH}$, $-\text{CONH}_2$, $-\text{NH}_2$, or $-\text{SH}$) dissolve in polar solvents, while non-polar compounds dissolve in non-polar organic solvents (such as benzene, hexane, petroleum ethers, or chlorohydrocarbons).

Ideal Solvent Criterion for Acetanilide

The criterion stating that the best recrystallization solvent is insoluble at room temperature and fully soluble at elevated/boiling temperature, making water the best solvent for acetanilide among tested solvents.

Gravity Filtration

A crude filtration technique that uses gravity to separate large pieces of insoluble impurities from a solution.

Vacuum Filtration (Büchner Funnel)

A rapid filtration method using reduced pressure to collect fine crystals; it may lose more product and can minimize solute crystallization through vaporization under reduced pressure.

Oiling Out

A condition in which a solute fails to recrystallize from the mother liquor and instead separates as an insoluble oil.

Methods for Inducing Crystallization

Techniques used when crystallization fails or oiling out occurs: scratching the flask wall with a glass stir rod, seeding the solution with pure crystals or starting material, cooling in a freezing mixture, adding dry ice crumbs, or allowing the mixture to stand for an extended period.

Decolorization

The process of removing colored impurity pigments by adding finely powdered activated carbon, which adsorbs the colored contaminants into its crystal lattice prior to hot gravity filtration.