Recrystallization & Acid–Base Extraction – Comprehensive Study Notes

Lab Exercise 4 – Purification of Organic Solids by Recrystallization

Experimental Goals

• Purify an impure organic solid that contains:

  • Target compound (unknown)

  • $\approx$ 1–2 % acetanilide impurity

  • Colored impurity

• Perform a complete recrystallization procedure

• Measure:

  • Melting-point (MP) range with a Digimelt apparatus

  • Percent recovery of purified crystals

Core Techniques & Apparatus

• Hot-gravity filtration (T-3.1 & 3.2)

  • Components: stem-less or short-stem funnel, fluted paper, ring support, hot receiver flask

  • Purpose: remove insoluble impurities while solution is still hot

• Vacuum filtration (T-3.1)

  • Büchner funnel + filter flask + adaptor + trap + vacuum source

  • Purpose: collect cold crystals, speed drying, remove soluble impurities

• Digimelt (T-1.1 & 1.2)

  • Digital melting-point apparatus; yields sharp, reproducible ranges

Recrystallization Process (Conceptual Flow-Chart)

• Impure solid → Hot solution → Hot-gravity filtration (remove insoluble matter)

• Optional: add activated charcoal (Norit)

  • Adsorbs colored micro-impurities via surface interaction

  • Use small aliquots; repeat until solution is clear

• Slow cooling (room-T then ice bath)

  • Favors nucleation of pure lattice, rejects soluble contaminants

• Vacuum filtration of crystals

• Dry → Weigh → MP → % recovery

Choosing a Recrystallization Solvent

• Requirements

  • Minimal solubility of solute at low $T$ (to maximize recovery)

  • High solubility at solvent b.p. (to ensure complete dissolution)

  • Either dissolves impurity at all $T$ or never dissolves it

• Graphical criterion: steep solubility–temperature curve for compound, flat/contrasting curve for impurity

"Right" Amount of Solvent

• Too little → high recovery, low purity (impurities occluded)

• Too much → high purity, low recovery (excess solute stays in mother liquor)

• Strategy: use the minimum volume that fully dissolves solute at b.p.

  • Heat incrementally, swirling; add solvent dropwise until last trace disappears

Worked Example (5.00 g Benzoic Acid)

1. Solubility data: $5.60\;\text{g}/100\,\text{mL @ }100^{\circ}\text{C}$

2. Minimum volume $V<em>{100}$: $V</em>{100}=\frac{5.00\,\text{g}}{5.60\,\text{g}}\times100\,\text{mL}=89.3\,\text{mL}$

3. Expected loss on cooling to $25^{\circ}\text{C}$ (solubility $0.38\,\text{g}/100\,\text{mL}$):
   $m_{lost}=0.38\times\frac{89.3}{100}=0.34\,\text{g}$

4. Ideal recovery:
   $5.00-0.34=4.66\,\text{g}\;(\text{or }93.2\,\%)$

Soluble Impurity Check (Acetanilide)

• Impurity mass = $0.02\times5.00=0.10\,\text{g}$

• Solubility @ $25^{\circ}\text{C}:\;0.53\,\text{g}/100\,\text{mL}$

• Capacity of 89.3 mL water ≈ $0.47\,\text{g}$

• ⇒ Entire 0.10 g impurity remains dissolved; removed with mother liquor

Practice Problem (Salicylic Acid)

• Data supplied in lecture; follow same 4-step logic

  • Minimum $V_{100}$ for 6.40 g given $7.50\,\text{g}/100\,\text{mL}$

  • Loss at $20^{\circ}\text{C}$ using $0.20\,\text{g}/100\,\text{mL}$

  • Compute % recovery and amount of acetanilide (0.25 g) dissolving at 20 °C

Melting-Point Theory & Interpretation

• A range (first-drop → clear-liquid) is reported; pure organics: $1–2^{\circ}\text{C}$ width

• Impurities depress $T_m$ and broaden range

• Binary phase diagram concepts

  • Eutectic point = lowest possible $T_m$ for any mixture A/B

  • MP depression magnitude depends on mole-fraction of impurity

• Application

  • Sharp MP of recrystallized sample (e.g., $122.4^{\circ}\text{C}$) vs literature tables ⇒ confirm identity

  • If measured MP widens or drops (e.g., $101.2–105.4^{\circ}\text{C}$) ⇒ still impure or mis-identified

Take-Home Challenge (Stoichiometry → MP Range)

• Given masses 3.97 g A ($M<em>W=113.45$) & 0.903 g B ($M</em>W=60.18$)

• Convert to mole-fraction; locate point on phase diagram; predict depressed range using provided curves (students complete)

Lab Exercise 5 – Separation of Components by Acid-Base Extraction

Objectives

• Master concept of extraction and partitioning

• Safely operate a separatory funnel

• Perform successive acid/base washes to resolve a trinary mixture (acid + base + neutral)

• Dry, isolate, and identify each fraction by MP

Extraction Fundamentals

• Liquid–liquid extraction = distribution of solute between two immiscible phases (usually water + organic)

• Partition coefficient Kd: Kd=grams A in S2VS2grams A in S1VS1Kd=VS1grams A in S1​VS2grams A in S2​​
  where S1 represents the volume of the first phase (usually the organic phase) and S2 represents the volume of the second phase (usually the aqueous phase). This coefficient is crucial for understanding how much of the solute will be extracted into each phase during the extraction process.

  
  

  • Kd>1 → A prefers S2S2; Kd<1Kd<1 → A prefers S1S1

• Multiple smaller extractions give higher overall transfer than one large extraction (not explicitly shown but implied by equilibrium graphic)

Ideal Extracting Solvent Characteristics

• Immiscible with water (creates two distinct layers)

• High capacity for target solute (high $K_d$)

• Chemically inert toward solute & other layer

• Low b.p. (facilitates removal by evaporation)

Separatory Funnel Technique (T-5)

• Components: stopper, ground-glass joint, funnel body, stopcock, drain stem

• Critical operations

  • Vent to release pressure (gas evolution, vapor pressure); modes: upright (stopcock closed) or inverted (stopcock open)

  • Identify upper vs lower layer by density (e.g., $\rho<em>{\text{DCM}}=1.33>\rho</em>{\text{H}_2\text{O}}$)

  • Drain lower layer via stem; decant upper layer from mouth

Acid-Base Extraction Logic

1. Dissolve mixture in organic solvent (S1; e.g., diethyl ether or DCM)

2. Treat with aqueous reagent (S2) to ionize one component → converts it to water-soluble salt

   • Acids (e.g., benzoic acid) become carboxylate $\text{RCOO}^-\text{Na}^+$ in base wash (NaOH or NaHCO₃)

   • Bases (e.g., aniline, benzimidazole) become ammonium salt $\text{RNH}_3^+\text{Cl}^-$ in acid wash (HCl)

3. Separate layers; back-neutralize salt to regenerate neutral acid/base ➞ precipitation, filtration, drying

4. Neutral component stays in organic layer throughout; recovered after solvent evaporation

Solubility Table (20 °C)

• Table reinforces conversion → partition behavior

Worked Example (Benzoic Acid + Aniline)

• Step 1: Dissolve mixture in ether (organic)

• Step 2: Extract with 3 M HCl

  • Aniline → anilinium chloride (enters aqueous)

  • Benzoic acid remains in ether

• Step 3: Separate layers

• Step 4: Neutralize aqueous HCl layer with 20 % NaOH to pH > 10 → free aniline precipitates, vacuum filter

• Step 5: Extract ether layer with 10 % NaHCO₃

  • Benzoic acid → sodium benzoate (aqueous)

• Step 6: Re-acidify with 6 M HCl (pH < 2) → benzoic acid precipitates; collect by filtration

• Step 7: Dry ether layer, evaporate solvent to recover neutral residue (if any)

Trinary Mixture Flow-Chart (Provided in Slides)

• Starting solution: 15 mL DCM + unknown A/B/C

1. Wash 1: 3 M HCl (10 mL)

   • Transfers basic component B to aqueous; later basify (20 % NaOH) to precipitate B

2. Wash 2: 10 % NaHCO₃ (10 mL)

   • Transfers acidic component A to aqueous; later acidify (6 M HCl) to precipitate A

3. Residual DCM layer → neutral component C; evaporate DCM to isolate C

Partition Coefficient Practice (slide 30)

• Students calculate $K<em>d$ using 12 mL (S1) & 8 mL (S2) volumes, masses from diagram (5 → 2 g in S1; 3 g in S2): $K</em>d=\frac{\frac{3\,\text{g}}{8\,\text{mL}}}{\frac{2\,\text{g}}{12\,\text{mL}}}=\frac{0.375}{0.167}=2.25$

• Interpretation: solute prefers S2; second extraction would raise overall transfer

Multi-Component Practice Problem

• 1 g equimass mixture: o-chlorobenzoic acid, ethyl-4-aminobenzoate, anthracene in 15 mL ether

• Reagents available: 6 M HCl, 20 % NaOH, 20 % NaHCO₃

• Recommended sequence:

  1. Extract ether with 6 M HCl (remove amine as salt)

  2. Basify aqueous to pH > 10, cool → precipitated ethyl-4-aminobenzoate

  3. Extract original ether layer with 20 % NaHCO₃ (remove o-chlorobenzoic acid)

  4. Acidify bicarbonate extract to pH < 2 → precipitated o-chlorobenzoic acid

  5. Dry remaining ether, evaporate → anthracene

• Verify each MP: acid 122–123 °C, base 88–90 °C, neutral 80–81 °C (example table)

Safety & Practical Tips

• Always label layers immediately ("organic top", "aqueous bottom")

• Vent every 10 sec of shaking, especially when using bicarbonate (CO₂ evolution)

• Never discard a layer until desired product confirmed in another layer

• Dry organic solutions with anhydrous $\text{Na}<em>2\text{SO}</em>4$ or MgSO₄ before evaporation

• Dispose of acidic/basic aqueous wastes in designated containers; neutralize if required

Cross-Lecture Connections & Real-World Relevance

• Pharmaceutical purification: recrystallization ensures API purity (removal of colored by-products)

• Environmental chemistry: liquid-liquid extraction used for pollutant monitoring (e.g., PCBs from water)

• Forensic labs: MP comparison remains a quick screening tool for seized substances

• Industrial scale-up considers solvent economy and waste minimization; principles identical to bench-scale protocols

Ethical & Practical Considerations

• Choice of solvent impacts environmental footprint; greener alternatives (ethyl acetate, ethanol/water) encouraged

• Accurate MP data relies on properly calibrated apparatus; misidentification can propagate errors in literature and product labeling

• Safe handling of strong acids/bases and volatile solvents protects both personnel and laboratory infrastructure

Lab Exercise 4 – Purification of Organic Solids by Recrystallization

Experimental Goals

• Purify an impure organic solid that contains:

  • Target compound (unknown)

  • $\approx$ 1–2 % acetanilide impurity

  • Colored impurity

• Perform a complete recrystallization procedure

• Measure:

  • Melting-point (MP) range with a Digimelt apparatus

  • Percent recovery of purified crystals

Core Techniques & Apparatus

• Hot-gravity filtration (T-3.1 & 3.2)

  • Components: stem-less or short-stem funnel, fluted paper, ring support, hot receiver flask

  • Purpose: remove insoluble impurities while solution is still hot; a stem-less or short-stem funnel prevents premature crystallization, and fluted paper increases the surface area for faster filtration.

• Vacuum filtration (T-3.1)

  • Büchner funnel + filter flask + adaptor + trap + vacuum source

  • Purpose: collect cold crystals, speed drying (by drawing air through the crystals to evaporate residual solvent), and remove soluble impurities (as the solvent containing them is drawn away).

• Digimelt (T-1.1 & 1.2)

  • Digital melting-point apparatus; yields sharp, reproducible ranges due to precise temperature control.

Recrystallization Process (Conceptual Flow-Chart)

• Impure solid → Hot solution → Hot-gravity filtration (remove insoluble matter)

• Optional: add activated charcoal (Norit)

  • Adsorbs colored micro-impurities via surface interaction. The charcoal's porous structure traps the impurities.

  • Use small aliquots; repeat until solution is clear. Using too much activated charcoal can lead to the adsorption of the target compound, reducing yield.

• Slow cooling (room-T then ice bath)

  • Favors nucleation of a pure crystal lattice and rejects soluble contaminants. Slow cooling allows impurities to remain dissolved in the mother liquor and prevents

Based on the "Melting-Point Theory & Interpretation" section, if benzoic acid exhibits a lower melting point than expected, coupled with a potentially broadened range, the primary conclusion is that the sample is still impure. The note explicitly states, "Impurities depress $T_m$ and broaden range."

Additional conclusions for a lower than expected melting point include:

• Incomplete Purification: The recrystallization process may not have been fully effective in removing all soluble impurities. This could be due to too rapid cooling (which locks impurities into the crystal lattice), using too little solvent (leading to occlusion of impurities), or an insufficient number of recrystallization cycles.

• Residual Solvent: The purified crystals might not have been thoroughly dried, meaning residual solvent is still present and acting as an impurity, thus lowering the melting point.

• Contamination: The sample could have been contaminated during handling, drying, or transfer after the recrystallization-purification step.

• Possible Mis-identification: Although less likely if starting with a known compound, a significantly lower and broader MP, especially if vastly different from the literature value, could also imply mis-identification of the substance, as mentioned in the note: "If measured MP widens or drops… $\Rightarrow$ still impure or mis-identified."

To address this, several options are available:

• Repeat Recrystallization: Perform another recrystallization on the already purified sample to further enhance its purity. This often involves using a fresh portion of the ideal solvent.

• Optimize Drying: Ensure the crystals are completely dry using appropriate methods like extended vacuum filtration, placement in a desiccator, or a drying oven (if suitable for the compound).

• Re-evaluate Solvent Choice: If possible, consider if the chosen recrystallization solvent was truly optimal. Refer back to the "Choosing a Recrystallization Solvent" requirements.

• Verify Apparatus Calibration: Check the calibration of the Digimelt apparatus to rule out instrumental error as a cause for the inaccurate melting point.

• Perform Mixed Melting Point Test: If a pure sample of benzoic acid is available, a mixed melting point test could be performed. Mixing the impure sample with a pure

Given a sample in a storage vial labeled "benzoic acid" with a measured melting point (MP) range of $101.2^{\circ}\text{C} – 105.4^{\circ}\text{C}$:

Conclusions Regarding the Observed Melting Point

Based on the "Melting-Point Theory & Interpretation" section, the observed melting point leads to the following conclusions:

• Impure Sample: The primary conclusion is that the benzoic acid sample is impure. The note explicitly states: "Impurities depress $T_m$ and broaden range." The measured range ($101.2^{\circ}\text{C} – 105.4^{\circ}\text{C}$) is significantly lower than the literature melting point for pure benzoic acid (typically around $122^{\circ}\text{C}$) and broader than the expected $1–2^{\circ}\text{C}$ width for a pure organic compound.

• Potential Causes of Impurity:

  • Incomplete Purification: If this sample was a result of a previous purification attempt (e.g., recrystallization), the process might have been inefficient. This could be due to too rapid cooling, which can lead to impurities becoming occluded within the crystal lattice, or using too little solvent, which prevents complete dissolution of impurities at low temperatures.

  • Residual Solvent: The purified crystals might not have been thoroughly dried. Any residual solvent acts as an impurity, depressing the melting point and broadening the range.

  • Contamination during Storage or Handling: The sample could have become contaminated during storage in the vial (e.g., due to improper sealing, exposure to moisture/air, or cross-contamination) or during previous handling.

• Questionable Identity (Requires Further Confirmation): Although the vial is labeled "benzoic acid," a significantly lower and broader MP, especially if vastly different from the literature value, can imply mis-identification of the substance. As stated in the note: "If measured MP widens or drops… $\Rightarrow$ still impure or mis-identified."

How to Confirm the Identity of the Sample

To definitively confirm that the sample is benzoic acid and assess its purity, the following steps are recommended:

• Re-purification via Recrystallization: Given the impurity, the first step should be to re-purify a portion of the sample using the recrystallization procedure detailed in the Lab Exercise 4 notes. This involves:

  • Choosing an optimal solvent (e.g., water, based on the worked example for benzoic acid), ensuring minimal solubility at low $T$ and high solubility at solvent b.p.

  • Using the minimum volume of hot solvent to completely dissolve the solute.

  • Performing hot-gravity filtration to remove any insoluble impurities.

  • Implementing slow cooling (room temperature then ice bath) to favor the formation of a pure crystal lattice and reject soluble contaminants.

  • Collecting the purified crystals by vacuum filtration and ensuring thorough drying.

  • Measuring the MP of the re-purified crystals. If the MP is sharp (e.g., $1–2^{\circ}\text{C}$ range) and close to $122^{\circ}\text{C}$ (benzoic acid's literature MP), this strongly confirms the identity and implies the initial sample's impurity was the issue.

• Mixed Melting Point Test: This is a conclusive method, as suggested by the note, to confirm the identity of a substance:

  • Obtain a known, pure standard sample of benzoic acid.

  • Prepare two melting point capillaries: one containing a small amount of your re-purified benzoic acid mixed with an equal amount of the pure standard, and another containing only the pure standard.

  • Measure the melting point of both capillaries simultaneously using the Digimelt apparatus. If your purified sample is truly benzoic acid, the mixed melting point will be sharp and identical (or very close) to the melting point of the pure standard. If depression and broadening are still observed, it indicates that your sample is either still impure or not benzoic acid.

Class Discussion Activity: Addressing the Benzoic Acid Mystery

This scenario presents an excellent opportunity for a class discussion activity. Key discussion points could include:

• Troubleshooting: Brainstorming all possible reasons for the observed low and broad melting point, drawing from the "Melting-Point Theory & Interpretation" and practical aspects of the "Recrystallization Process."

• Recrystallization Optimization: Discussing how to ensure an effective recrystallization, particularly focusing on "Right" Amount of Solvent" and "Slow cooling" techniques to maximize purity.

• Importance of Drying: Emphasizing why complete drying is crucial before measuring MP and how residual solvent impacts results.

• Interpretation of Data: Analyzing the specific MP range ($101.2–105.4^{\circ}\text{C}$) and comparing it to ideal pure substance behavior ($1–2^{\circ}\text{C}$ width) and literature values ($\approx 122^{\circ}\text{C}$ for benzoic acid).

• Confirmation Techniques: Delving deeper into the utility and rationale behind the mixed melting point test as a definitive identification method.

• Real-World Relevance: Connecting this lab