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 $\text{SiO}<em>2$ or $\text{Al}</em>2\text{O}_3$) 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: $R<em>f = \dfrac{\text{distance travelled by solute (center of spot)}}{\text{distance travelled by solvent front}}$, $0 \le R</em>f \le 1$ 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 $R<em>f = 1.80$ with solvent distance $6.35\,\text{cm}$. Because $R</em>f$ 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 $R<em>f$) disappears; the higher $R</em>f$ 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)

  • $P<em>A = X</em>A P^{\circ}<em>A$, $P</em>B = X<em>B P^{\circ}</em>B$ (Raoult).

  • $P<em>{\text{tot}} = P</em>A + P_B$ (Dalton).

  • Vapor composition: $Y<em>A = \dfrac{P</em>A}{P<em>{\text{tot}}}$, $Y</em>B = 1 - Y_A$.

  • Algebraic manipulation: $P<em>{\text{tot}} - P^{\circ}</em>B = X<em>A (P^{\circ}</em>A - P^{\circ}_B).$

• Worked example (Questions 12 & 13)

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

  • $X<em>A = \dfrac{P^{\circ}</em>B - P<em>{\text{tot}}}{P^{\circ}</em>B - P^{\circ}_A} = \dfrac{0.79-0.60}{0.79-0.52}=0.70.$

  • $X<em>B = 0.30;\; Y</em>B = \dfrac{X<em>B P^{\circ}</em>B}{P_{\text{tot}}}=\dfrac{0.30\times0.79}{0.60}=0.39.$

• 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 $x<em>L$ and vapor curve at $y</em>V$ .

  • 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 $k = \dfrac{n<em>s}{n</em>m}$, 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: $R<em>f = \dfrac{d</em>{\text{spot}}}{d_{\text{solvent}}}.$.

• Raoult’s Law: $P<em>i = X</em>i P_i^{\circ}.$.

• Dalton’s Law: $P<em>{\text{tot}} = \sum</em>i P<em>i = \sum</em>i Y<em>i P</em>{\text{tot}}.$.

• Vapor–liquid relation (binary): $P<em>{\text{tot}} - P</em>B^{\circ} = X<em>A \left(P</em>A^{\circ} - P_B^{\circ}\right).$

• GC retention factor: $k = \dfrac{t<em>R - t</em>M}{t<em>M}$ (where $t</em>M$ 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.