Difficulty: Intermediate | Prerequisites: Basic organic functional groups, polarity concepts
Column chromatography is one of the workhorse separation techniques in organic chemistry. This experiment uses it alongside thin-layer chromatography (TLC) to separate a mixture of ferrocene and acetylferrocene, two organometallic compounds with different polarities. If you understand polarity and how molecules interact with stationary and mobile phases, the rest follows.
Thin-layer chromatography (TLC) is used first to scout the best solvent system for separating ferrocene from acetylferrocene, then column chromatography scales that separation up. The less polar compound (ferrocene) elutes first in a silica-gel column with a low-polarity solvent such as hexane, while the more polar compound (acetylferrocene) sticks longer and needs a more polar eluent to come off. The whole experiment hinges on the polarity difference between the two compounds and how that polarity interacts with the stationary phase.
Thin-layer chromatography (TLC)
A quick analytical technique where a small spot of mixture is placed on a coated plate, developed in a solvent, and the distance each component travels is measured. Think of it as a rapid screening tool to find the right solvent before committing to a full column.
Column chromatography
A preparative separation technique in which a mixture is loaded onto a column packed with an adsorbent (usually silica gel or alumina) and components are washed through with solvent, collected in separate fractions. In simple terms, this is the scaled-up version of TLC where you collect enough material to weigh and analyse.
Stationary phase
The fixed adsorbent material packed into the column or coated on the TLC plate (typically silica gel, SiO2). Polar compounds stick to it more strongly. Think of it as the surface everything competes to cling to.
Mobile phase (eluent)
The solvent or solvent mixture that flows through the stationary phase and carries components along. In simple terms, this is what washes compounds off the column.
Rf value (retention factor)
The ratio of the distance a compound travels to the distance the solvent front travels on a TLC plate. It ranges from 0 to 1 and is specific to a given compound in a given solvent system. Think of it as a compound's fingerprint for a particular solvent combination.
Elution
The process of washing a compound through a chromatography column with solvent. In simple terms, it is the act of flushing a compound off the column and into a collection flask.
Polarity
A measure of how unevenly electrons are distributed in a molecule. More polar molecules interact more strongly with polar stationary phases like silica gel, causing them to move more slowly through the column.
Fraction
A portion of eluent collected as it exits the column. Each fraction may contain a single compound, a mixture, or nothing at all. In simple terms, fractions are the individual test tubes or flasks you collect as the column runs.
Ethyl acetate/hexane solvent system
A common binary eluent mixture for silica-gel chromatography. Hexane is non-polar and ethyl acetate is moderately polar, so adjusting their ratio tunes the eluting power. More ethyl acetate means a more polar mobile phase, which pushes polar compounds off the column faster.
TLC plates are coated with silica gel (polar stationary phase).
A small spot of the ferrocene/acetylferrocene mixture is applied near the bottom of the plate.
The plate is developed in a chamber containing an ethyl acetate/hexane mixture.
After development, the solvent front is marked and Rf values are calculated for each spot.
Different ratios of ethyl acetate to hexane are tested to find the mixture that gives the best separation (well-spaced spots) and Rf values in the ideal range (roughly 0.2 to 0.5).
Spots are visualised under UV light or by exposure to iodine vapour. Ferrocene typically appears yellow/orange; acetylferrocene may appear darker or show different behaviour under UV.
A solvent system is "good" for column chromatography when TLC shows two well-resolved spots with Rf values between about 0.2 and 0.5.
If Rf values are too high (spots near the solvent front), the eluent is too polar and compounds will rush through the column together.
If Rf values are too low (spots near the baseline), the eluent is too non-polar and compounds will take forever to come off, producing broad, overlapping bands.
The solvent ratio chosen on TLC is used as the starting eluent for the column.
The column is packed with silica gel (the stationary phase) and a layer of sand on top to protect the surface.
The ferrocene/acetylferrocene mixture is dissolved in a small volume of solvent and carefully loaded onto the top of the column.
Eluent (the chosen ethyl acetate/hexane mix) is added and allowed to flow through by gravity.
Compounds separate into coloured bands as they move through the column at different rates.
Fraction 1: The first eluent that comes through before any coloured band. Usually discarded or checked.
Fraction 2 (Band 1): The first coloured band to elute. Because it moves fastest through the polar silica, this is the less polar compound, ferrocene (yellow/orange).
Fraction 3: Material collected between the two main bands. This may contain a small amount of mixed material.
Fraction 4 (Band 2): The second coloured band. This is the more polar compound, acetylferrocene (darker orange/red). A solvent polarity increase (more ethyl acetate) may be needed to push it off.
After collection, solvents are evaporated to yield solid samples that are weighed and characterised by melting point.
Ferrocene is symmetric and non-polar. It interacts weakly with the polar silica surface.
Acetylferrocene has an acetyl group (C=O), which is polar. The carbonyl oxygen hydrogen-bonds or dipole-interacts with silanol groups on silica, so it is retained longer.
On a silica column: less polar = faster = first out.
R_f = \frac{\text{distance travelled by compound}}{\text{distance travelled by solvent front}}Both distances are measured from the origin (the baseline where the spot was applied). The Rf value is dimensionless, always between 0 and 1.
A higher Rf means the compound has less affinity for the stationary phase relative to the mobile phase. A lower Rf means stronger retention on silica.
Percent mass recovery from column chromatography:
\% \text{ recovery} = \frac{\text{combined mass of all collected fractions}}{\text{mass of original sample loaded}} \times 100Column chromatography is used in pharmaceutical manufacturing to purify drug compounds after synthesis. It is also the backbone of natural-product isolation, where chemists extract a single active compound from a complex plant or marine extract. TLC is routinely used in forensic labs to identify controlled substances and in food-safety testing to detect contaminants.
Students often think a higher Rf means a compound is more polar. It does not. A higher Rf on silica means the compound is less polar (it spends more time in the mobile phase and less time stuck to the polar stationary phase).
Students sometimes assume the solvent system used for TLC must be changed for the column. It does not need to be. The same ratio that gives good TLC separation is the starting point for the column eluent.
Students occasionally think increasing the percentage of ethyl acetate slows elution. It does the opposite. More ethyl acetate increases eluent polarity, which competes with the stationary phase for polar compounds and speeds their travel through the column.
Students confuse the order of elution with molecular weight. Elution order on silica is governed by polarity, not size. Ferrocene elutes first because it is less polar, not because it is smaller.
⚠️ Be able to calculate Rf from distance data and explain what a given Rf value tells you about compound polarity.
⚠️ Know why ferrocene elutes before acetylferrocene on a silica column (polarity difference, not molecular weight).
⚠️ Understand how changing the ethyl acetate/hexane ratio changes Rf values and elution behaviour.
⚠️ Be prepared to explain why you would choose one solvent system over another based on TLC results (target Rf range of 0.2 to 0.5, good spot separation).
⚠️ Know the purpose of the "waste" fraction collected between the two coloured bands and why it matters for purity.
⚠️ Percent mass recovery calculations appear frequently. Know the formula and be able to discuss what less-than-100% recovery means (compound left on the column, lost during transfer, incomplete evaporation).
True or false: On a silica-gel TLC plate, a compound with Rf = 0.8 is more polar than a compound with Rf = 0.3. (False. Higher Rf on silica means less polar.)
Fill in the blank: In column chromatography of ferrocene and acetylferrocene, the compound that elutes first is __________ because it is __________. (Ferrocene; less polar.)
True or false: Increasing the percentage of ethyl acetate in the eluent will decrease the Rf values on TLC. (False. More ethyl acetate increases eluent polarity, which increases Rf values on silica.)
Fill in the blank: The ideal Rf range for good column chromatography separation is approximately _____ to _____. (0.2 to 0.5.)
True or false: Percent mass recovery above 100% means the separation was unusually efficient. (False. Recovery above 100% suggests solvent was not fully evaporated or the sample was contaminated.)
Q: A student runs TLC of a ferrocene/acetylferrocene mixture in 30% ethyl acetate/hexane and gets Rf values of 0.9 and 0.7. What should they do before running the column, and why?
A: Decrease the percentage of ethyl acetate. Both Rf values are too high, meaning compounds will rush through the column with poor separation. A lower ethyl acetate percentage (e.g. 10 to 20%) would bring the Rf values into the 0.2 to 0.5 range.
Q: Explain why acetylferrocene is retained longer on a silica-gel column than ferrocene.
A: Acetylferrocene contains a carbonyl group (C=O), which is polar. This group interacts via dipole interactions and possible hydrogen bonding with the polar silanol (Si-OH) groups on the silica surface, causing acetylferrocene to be adsorbed more strongly and travel more slowly through the column.
Q: A student collects fractions from a column and finds the total mass of all fractions is 85% of the original sample mass. Provide two possible explanations.
A: Some compound may have remained adsorbed on the silica and never eluted. Material may also have been lost during transfer steps (loading the column, transferring fractions to evaporation flasks). Incomplete solvent evaporation is another possibility, though that would typically inflate rather than reduce apparent recovery.
Q: Why is it important to collect a fraction between the two main coloured bands rather than combining it with either band?
A: The intermediate fraction may contain a mixture of both compounds. Including it in either the ferrocene or acetylferrocene fraction would reduce the purity of that sample and lower the melting point of the resulting solid.
This connects to melting point determination because the purity of your column fractions is confirmed by comparing experimental melting ranges to literature values. A broad or depressed melting range signals impurity, which ties directly to the mixed-melting-point identity test covered later in the course.
The polarity concepts here are the same ones that govern extraction (liquid-liquid partitioning), solubility, and intermolecular forces. If you understand why acetylferrocene sticks to silica, you already understand a good portion of the logic behind solvent selection in recrystallisation.
Spectroscopy (NMR and IR) is used alongside chromatography to confirm the identity of separated fractions, covered in the companion study notes.
Column chromatography, thin-layer chromatography, TLC, Rf value, retention factor, silica gel, alumina, stationary phase, mobile phase, eluent, elution, ethyl acetate, hexane, binary solvent system, polarity, adsorption chromatography, ferrocene separation, acetylferrocene separation, CHEM 2311, organic chemistry lab, fraction collection, percent recovery, UV visualisation, iodine staining, chromatography worksheet