Extraction and Distillation (5C) - MCAT Chemical and Physical Foundations of Biological Systems

A fractionating column enables repeated vaporization–condensation (more theoretical plates). The column provides multiple vapor-liquid equilibria stages, enriching the vapor in the more volatile component for better separation.

It is best when boiling points differ greatly (roughly $\ge 25,^{\circ}\text{C}$). Significant boiling point differences allow the more volatile component to vaporize and condense separately without extensive overlap.

Drying agent remains free-flowing and no longer clumps. Free-flowing drying agent indicates excess capacity remains after all trace water has been absorbed, preventing clumping from hydration.

To remove trace water from the organic layer. Anhydrous salts like MgSO4 absorb residual water through hydration, ensuring the organic layer is dry for further processing or analysis.

Mostly protonated when $\text{pH} < \text{p}K_a$ (of the conjugate acid). Below the conjugate acid's pKa, the base is more likely to accept a proton, favoring the charged, water-soluble form.

Mostly deprotonated when $\text{pH} > \text{p}K_a$. When pH exceeds pKa, the equilibrium shifts toward the deprotonated conjugate base form according to Henderson-Hasselbalch.

$\text{pH} = \text{p}K_a + \log\left(\frac{[A^-]}{[HA]}\right)$. This equation relates pH to the ratio of conjugate base to acid, determining the extent of ionization at equilibrium.

$\text{RNH}_3^+$ partitions into the aqueous layer. Protonation creates a charged ammonium ion that is highly soluble in water due to its polarity and ability to hydrogen bond.

$\text{RCOO}^-$ partitions into the aqueous layer. The charged carboxylate ion forms strong ion-dipole interactions with water, enhancing its solubility in the aqueous phase.

Convert neutral compounds to ionic salts to change solubility between phases. pH adjustment protonates or deprotonates the compound, altering its polarity and solubility to favor one phase over the other.

It decreases solubility of organics in water, pushing them into the organic phase. High salt concentration hydrates water molecules, reducing their availability to solvate organics and forcing them into the nonpolar phase.

Add brine (salt) to increase ionic strength and promote phase separation. Brine increases the aqueous phase's ionic strength, reducing solubility of organic droplets and promoting their coalescence into a separate layer.

A stable dispersion of tiny droplets that prevents clean phase separation. Vigorous mixing can create fine, stable droplets of one phase in the other, impeding coalescence and clear layer formation.

To release built-up pressure from volatile solvents or gas-forming reactions. Venting prevents excessive pressure buildup that could dislodge the stopper or rupture the funnel during shaking.

The organic layer is on the bottom. Higher density of the organic solvent positions it below the less dense aqueous phase due to gravitational settling.

$q^n = \left(\frac{V_{\text{aq}}}{V_{\text{aq}} + K_D V_{\text{org}}}\right)^n$. For independent successive extractions, the fraction remaining multiplies with each step, resulting in an exponential decrease in solute left behind.

$q = \frac{V_{\text{aq}}}{V_{\text{aq}} + K_D V_{\text{org}}}$. Derived from the distribution coefficient, this formula calculates the proportion of solute that remains unextracted in the aqueous phase after equilibration.

It must preferentially dissolve the target solute and be immiscible with the other phase. This ensures selective partitioning of the solute into the extracting phase while maintaining two distinct layers for easy separation.

$K_D = \frac{[S]{\text{org}}}{[S]{\text{aq}}}$ at equilibrium. This ratio quantifies the equilibrium concentration of the solute in the organic phase relative to the aqueous phase, indicating partitioning efficiency.

Several small extractions remove more solute (greater overall recovery). Multiple extractions successively remove portions of the remaining solute, leading to higher total recovery than a single extraction with the same total volume.

High-boiling, water-immiscible, heat-sensitive compounds (often aromatic oils). Steam lowers the effective boiling point of immiscible mixtures, allowing isolation of sensitive compounds without thermal degradation.

$P_{\text{total}} = P_1^* + P_2^*$ (sum of pure-component vapor pressures). For immiscible liquids, Dalton's law states the total vapor pressure is the sum of each component's vapor pressure, enabling lower boiling temperatures.

Lower pressure lowers the boiling point. Boiling occurs when vapor pressure equals external pressure, so reduced pressure allows boiling at lower temperatures to avoid decomposition.

Separation by differences in volatility (vapor pressure) and boiling points. Components with higher vapor pressure or lower boiling points evaporate preferentially, enabling separation upon controlled condensation.