Recrystallization, Melting Point, and Extraction Techniques – Comprehensive Exam Notes

Recrystallization

• Goal: Purify a solid by exploiting differences in solubility as a function of temperature.

• Starting material: An impure solid (may contain

  • Decomposition products,

  • Side-products from synthesis,

  • Adventitious contaminants such as sand, glass shards, or starch).

• General workflow

  1. Choose an appropriate solvent.

  • High solubility for the target compound at the solvent’s boiling point ("hot" condition).

  • Minimal to zero solubility at room / ice-bath temperature ("cold" condition).

  1. Dissolution step (hot):

  • Heat the heterogeneous mixture + solvent to boiling.

  • Target compound dissolves completely.

  • Insoluble impurities remain as suspended solids.

  1. Hot gravity filtration:

  • Removes insoluble impurities (sand, starch, charcoal, glass, etc.).

  • Filtrate = clear, hot solution of compound + any soluble impurities.

  1. Cooling / crystallization:

  • Slowly cool to room temperature, then (optionally) ice-bath.

  • Target compound precipitates (re-crystallises).

  • Soluble impurities stay dissolved because solvent still dissolves them even when cold.

  1. Cold (vacuum or gravity) filtration:

  • Mother liquor (filtrate) = solvent + soluble impurities.

  • Collected solid = purified product.

  1. Drying & analysis (melting point, mass, spectroscopy).

Types of impurities & when they are removed

Activated charcoal (decolourising carbon)

• Added after dissolution while solution is still hot.

• Mechanism: adsorption of coloured molecules onto high-surface-area carbon.

• Non-selective → can adsorb desired product too.

  • Use the minimum effective amount.

  • Over-use ↓ recovery ("charcoal eats up your compound").

Solvent selection

• Temperature–solubility curve shapes

  • Ideal: steep curve (large Δsolubility between high & low T).
    Curve a on lecturer’s sketch.

  • Poor choices:
    Curve b: Already very soluble at low T → low recovery.
    Curve c: Barely soluble even at high T → compound never fully dissolves.

• Interaction with impurities

  • Prefer solvent that always dissolves soluble impurities (so they never crystallise).

  • Prefer solvent that never dissolves insoluble impurities (so they can be filtered hot).

• Balancing purity vs recovery

  • More solvent (beyond minimum) → higher purity, lower recovery.

  • Less solvent → higher recovery, lower purity.

  • Strategy: calculate minimum solvent volume that will dissolve the entire solid at its boiling point.

Example calculation (numbers from lecture)

• Solubility data (illustrative):
  \text{100 °C: } 5\,\text{g compound} \; \xrightarrow[]{89.3\,\text{mL solvent}} \text{fully dissolves}

• Cool to room T, solubility = 0.34\,\text{g per }89.3\,\text{mL}.

• Material remaining in solution after cooling: 0.34\,\text{g}.

• Theoretical recovery:
  \text{Recovered mass}=5.00\,\text{g}-0.34\,\text{g}=4.66\,\text{g}

• % Recovery (ideal):
  \%\text{Recovery}=\frac{4.66}{5.00}\times100\approx93.2\%

• If solvent volume chosen < 89 mL → incomplete dissolution → insoluble impurity step fails → purity suffers.

• If volume ≫ 89 mL → more product stays in mother liquor → recovery suffers.

Melting-point analysis

• Reported as a range: T{\text{first melt}} - T{\text{last crystal}}.

• Sharp MP (narrow range) usually indicates purity, but not always.

• Binary mixture phase diagram (A/B system):

  • Pure A: T_m \approx 77\,^\circ\text{C} (sharp).

  • Pure B: T_m \approx 120\,^\circ\text{C} (sharp).

  • Any mixture → MP depresses & broadens except at the eutectic composition.

  • Eutectic point: lowest MP in the system; mixture there can melt sharply even though it’s impure.

• Practical rule: • Sharp MP alone ≠ proof of purity. • Confirm by mixed-melting-point test: mix unknown with authentic pure sample.

  • If range stays sharp & unchanged → same compound.

  • If range depresses / broadens → mixture ≠ same compound.

Liquid–Liquid Extractions

Definitions

• Extraction: transfer of a solute from one immiscible solvent (S₁) to another (S₂).

• Partition (distribution) coefficient KD: KD = \frac{\frac{\text{mass of solute in }S2}{\text{volume }S2}}{\frac{\text{mass of solute in }S1}{\text{volume }S1}}

  • K_D>1 → solute prefers S₂.

  • K_D<1 → solute prefers S₁.

• Equilibration: achieved when concentrations in each layer become time-independent (shaking + venting + settling).

Acid–Base extractions

• Convert an acidic or basic organic compound into its ionic salt, increasing water solubility.

• Typical solvent pair in teaching labs:
  • Organic layer = diethyl ether or methylene chloride (DCM).
  • Aqueous layer = water ± acid/base.

Generic reactions

1. Acidic compound (R–COOH)
   \text{R–COOH} + \text{OH}^- \;\longrightarrow\; \text{R–COO}^- + \text{H}_2\text{O}
   Salt (carboxylate) moves to aqueous layer.

2. Basic compound (R–NH₂)
   \text{R–NH}2 + \text{H}^+ \;\longrightarrow\; \text{R–NH}3^+
   Ammonium salt moves to aqueous layer.

3. Recovery ("back-extraction")

   • Acidify carboxylate salt with strong acid (e.g., \text{HCl}) to regenerate R–COOH → precipitates.

   • Basify ammonium salt with strong base (e.g., \text{NaOH}) to regenerate R–NH₂ → separates into organic solvent.

Worked multi-component example

Mixture: benzoic acid (acid) + aniline (base) + naphthalene (neutral) dissolved in methylene chloride.

Step-wise separation in separatory funnel:

Result: three purified components.

Practical & Safety Notes

• Vent separatory funnel after every shake (pressure build-up by volatile solvents or CO₂ formation).

• Keep charcoal suspensions hot during filtration to avoid premature crystallisation on filter paper.

• Record exact solvent volumes used; 1 mL error can shift purity vs recovery balance noticeably.

• Always label filtrates & precipitates immediately to avoid mix-ups.

• Melting-point apparatus must be calibrated; mis-calibration broadens/lowers ranges artificially.

Summary Checklist for the Lab & Exam

• [ ] Select solvent with steep \text{solubility}(T) curve.

• [ ] Use minimal solvent; verify complete dissolution at boil.

• [ ] Add decolourising carbon sparingly while hot; hot-filter quickly.

• [ ] Cool slowly, then ice; collect crystals by cold filtration.

• [ ] Dry thoroughly before weighing or measuring MP.

• [ ] Interpret MP: sharp? broad? eutectic?

• [ ] For liquid–liquid extractions: know layer densities (e.g., \rho{\text{DCM}} > \rho{\text{H}_2O} → DCM bottom).

• [ ] Write balanced acid/base equations & predict which component moves to which layer.

• [ ] Calculate K_D and theoretical recovery when asked.

These notes consolidate the lecture’s full discussion of recrystallisation, melting-point analysis, solvent choice, impurity handling, charcoal use, partition coefficients, and acid-base extraction strategies—providing a standalone study guide for your upcoming exam.