Chromatography & Distillation – Comprehensive Exam Study Notes

Thin-Layer Chromatography (TLC)

  • Fundamental separation principle

    • TLC relies on adsorption of analytes onto a polar solid stationary phase (usually \text{SiO}2 or \text{Al}2\text{O}_3) while a liquid mobile phase moves by capillary action.

    • The degree of adsorption (and hence migration) depends on the polarity of each analyte relative to the solvent.

    • More strongly adsorbed (more polar/ionic) compounds move more slowly and exhibit smaller R_f values.

  • Key definitions / relations

    • Retention factor: Rf = \dfrac{\text{distance travelled by solute (center of spot)}}{\text{distance travelled by solvent front}}, 0 \le Rf \le 1 under correct experimental conditions.

    • Interpretation of spots

    • R_f \approx 0
      \Rightarrow analyte hardly moved; very polar or stuck at origin.

    • R_f \approx 1
      \Rightarrow analyte travelled with solvent front; very non-polar under those conditions.

    • Over-run: R_f > 1 is analytically impossible; indicates mis-measurement or improper origin/solvent-front marking.

  • Illustrative multiple-choice reasoning

    • Q1: Main separation principle = Adsorption (stationary silica surface).

    • Q2: Student reported Rf = 1.80 with solvent distance 6.35\,\text{cm}. Because Rf cannot exceed 1, the value “does not seem valid.”

    • Q3: Three-component mixture (4-nitroaniline, benzoic acid, benzophenone). Dipole moments (polarity) listed 7.12>4.77>2.96\,\text{D}, but silica additionally H-bonds acids strongly
      \Rightarrow benzoic acid expected to show the smallest R_f.

    • Q4: Base extraction converts benzoic acid \rightarrow sodium benzoate (water-soluble). Its spot (smallest Rf) disappears; the higher Rf spots remain.

  • Practical pointers

    • Use pencil, not ink, to scribe origin line; ink may travel.

    • Chamber saturation and a tight lid minimize solvent evaporation streaking.

    • UV lamp or iodine/anisaldehyde staining reveals colourless spots.

Vapor Pressure, Boiling Point & Phase Equilibria

  • Vapor pressure (VP) correlates inversely with normal boiling point (NBP). At 1 atm (760 torr) a substance boils. Lower VP at a given T
    \Rightarrow stronger intermolecular forces
    \Rightarrow higher NBP.

    • Example at 100^{\circ} \text{C}:

    • Water P_{\text{eq}} = 760\,\text{torr} (boils at 100^{\circ} \text{C}).

    • Methanol P_{\text{eq}} = 2625\,\text{torr}, Ethanol =1694\,\text{torr}.

    • Highest NBP = Water (lowest VP).

  • Raoult’s & Dalton’s Laws (ideal behaviour)

    • PA = XA P^{\circ}A, PB = XB P^{\circ}B (Raoult).

    • P{\text{tot}} = PA + P_B (Dalton).

    • Vapor composition: YA = \dfrac{PA}{P{\text{tot}}}, YB = 1 - Y_A.

    • Algebraic manipulation: P{\text{tot}} - P^{\circ}B = XA (P^{\circ}A - P^{\circ}_B).

  • Worked example (Questions 12 & 13)

    • Data: P^{\circ}A = 0.52\,\text{atm},\; P^{\circ}B = 0.79\,\text{atm},\; P_{\text{tot}} = 0.60\,\text{atm}.

    • XA = \dfrac{P^{\circ}B - P{\text{tot}}}{P^{\circ}B - P^{\circ}_A} = \dfrac{0.79-0.60}{0.79-0.52}=0.70.

    • XB = 0.30;\; YB = \dfrac{XB P^{\circ}B}{P_{\text{tot}}}=\dfrac{0.30\times0.79}{0.60}=0.39.

  • Ideal vs Real solutions

    • Ideal: like–like and unlike–unlike interactions identical; obey Raoult’s law through full composition range.

    • Real: interaction energies differ \rightarrow positive or negative deviations.

Distillation Techniques

Simple vs Fractional Distillation

  • Simple distillation- Best when BP difference > 60^{\circ} \text{C} or to remove volatile solvent from non-volatile solute.

  • Fractional distillation- Employs a fractionating column packed (or Vigreux) to create multiple vaporization-condensation cycles (theoretical plates).

    • Continual change in distillation temperature observed as composition of still-pot shifts.

    • Efficiency metric

    • Height Equivalent to a Theoretical Plate (HETP): smaller HETP \Rightarrow higher efficiency (opposite of statement B in Q11).

  • True statements for fractional distillation (Q11)- Separates compounds with close BPs (\le 25–30^{\circ} \text{C}).

    • Process involves many vapor–condensation cycles.

    • Boiling temperature drifts as distillation proceeds.

    • It also lets you approximate individual component BPs from plateau temperatures.

Composition–Temperature Diagrams

  • Reading a T - x/y diagram- Horizontal tie line intersects liquid curve at xL and vapor curve at yV .

    • Each theoretical plate moves horizontally then vertically to simulate equilibrium steps.

    • Number of plates available: \text{plates} = \dfrac{\text{column length}}{\text{HETP}}.

  • Example (Q14)- 24\,\text{cm} column, HETP 8\,\text{cm}
    \Rightarrow 3 plates + reboiler = 4 stages. Starting feed 70 % toluene (high-BP) / 30 % acetone. Step off 3 equilibrium stages toward vapor curve \rightarrow distillate composition corresponds to L3/L4 region (exact label depends on supplied diagram). (Conceptual method illustrated; pick the level whose third stepping meets vapor curve.)

Binary Mixture Problem (Q6 & 7)

  • After first drop shows Y_B = 0.20, one moves horizontally to liquid curve, down to the x-axis, etc. Reading the graph (not reproduced here) would give original \approx 60\%\,A and initial boiling \approx 67^{\circ} \text{C} .

Practical selections (Q22)

  • Fractional distillation is preferable for:- Determining BPs of volatile-liquid mixtures (A).

    • Separating 50:50 cyclohexane (80^{\circ} \text{C}) / toluene (111^{\circ} \text{C}) (C).

    • Simple distillation suffices for seawater desalination or ether removal from high-mp solids.

Gas Chromatography (GC)

  • Primary separation principle: partitioning of analytes between gaseous mobile phase and liquid stationary phase coated on inert solid support.

  • GC column architecture

    • Stationary phase: high-BP, non-volatile liquid film bonded or coated onto a finely divided support (packed) or onto capillary walls (WCOT/FOT).

    • Mobile phase: inert gas (He, N
      (2) , H (2) ) sweeps sample through column.

  • Retention metrics

    • Retention time t_R: interval from injection to peak apex (Q25).

    • Retention (capacity) factor k = \dfrac{ns}{nm}, the analyte distribution between stationary (s) and mobile (m) phases (Q23).

    • Chromatogram: graphical output of detector response vs time, comprising a series of peaks (Q24).

  • Influence of column parameters (Q17 & 18)

    • Increase column length
      \Rightarrow longer path
      \Rightarrow t_R increases.

    • Increase column temperature
      \Rightarrow faster elution (lower partitioning)
      \Rightarrow t_R decreases.

  • Example calculation set (Transesterification, Q19–21)

    1. Chart paper speed: 120\,\text{mm min}^{-1} = 2\,\text{mm s}^{-1}.

    • First peak (ethanol) at 7.81\,\text{cm} = 78.1\,\text{mm}
      \Rightarrow t_R = 39.1\,\text{s}.

    • Second peak (butyl acetate) at 14.88\,\text{cm} = 148.8\,\text{mm}
      \Rightarrow t_R = 74.4\,\text{s} (matches Q19).

    1. Corrected areas (area \times weight factor W_f ):

    • Butyl acetate =105\times1.10=115.5\,\text{mm}^2.

    • Ethanol =110\times0.82=90.2\,\text{mm}^2.

    • Fraction butyl acetate \text{=}\dfrac{115.5}{115.5+90.2}=0.562.

    • Mass in 3.20\,\text{g} total mixture =3.20\times0.562=1.80\,\text{g} (Q20).

    1. Reaction yield

    • Limiting reagent: ethyl acetate n=\dfrac{2.75}{88.11}=0.0312\,\text{mol}.

    • Theoretical butyl acetate m_{\max} = 0.0312\times116.16=3.62\,\text{g}.

    • \text{%}\,\text{yield}=\dfrac{1.80}{3.62}\times100\approx49.6\% (Q21).

Chromatography Vocabulary (GC/TLC)

  • Elution (A): process of washing analytes through a column with mobile phase.

  • Stationary phase (B): fixed phase inside column or on TLC plate that interacts selectively with analytes.

  • Chromatogram (C): detector output plot of signal vs time or distance.

  • Retention factor (D): ratio of analyte amounts in stationary vs mobile phase; describes equilibrium distribution.

  • Retention time (E): time between injection and analyte peak appearance.

Key Equations & Relationships

  • TLC: Rf = \dfrac{d{\text{spot}}}{d_{\text{solvent}}}..

  • Raoult’s Law: Pi = Xi P_i^{\circ}..

  • Dalton’s Law: P{\text{tot}} = \sumi Pi = \sumi Yi P{\text{tot}}..

  • Vapor–liquid relation (binary): P{\text{tot}} - PB^{\circ} = XA \left(PA^{\circ} - P_B^{\circ}\right).

  • GC retention factor: k = \dfrac{tR - tM}{tM} (where tM is unretained marker time).

  • Theoretical plates: N = 16\left(\dfrac{t_R}{W}\right)^2 (Gaussian peak width W at base).

  • Column efficiency: \text{HETP} = \dfrac{L}{N}; efficiency \uparrow as HETP \downarrow.

Ethical, Practical & Safety Considerations

  • TLC solvents may be volatile and toxic (e.g., diethyl ether, hexane); always work in a fume hood, wear gloves.

  • Diethyl ether forms peroxides; store properly and test before distillation.

  • GC carrier gases (H
    (_2) ) are flammable—use leak detectors; TCD cells are hot—avoid burns.

  • Distillation under reduced pressure is used to lower boiling points of temperature-sensitive compounds, minimizing decomposition.