- A student attempts to reduce acetophenone to 1-phenylethanol. The reaction mixture is analyzed by TLC against a pure standard of acetophenone. Both the reaction mixture lane and the standard lane show a single spot at exactly the same Rf value. What is the most reasonable conclusion?
Answer: The reaction failed, and the reaction mixture consists only of unreacted acetophenone.
The reduction of a ketone (acetophenone) to an alcohol (1-phenylethanol) involves converting a C=O group to a C-OH group. This change significantly increases the compound's polarity due to the addition of a hydrogen-bond donating hydroxyl group. Therefore, the product should have a much lower Rf value than the starting material on a silica gel plate. Since the TLC shows only a spot at the starting material's Rf, the most logical conclusion is that the reaction did not occur.
- The Fischer esterification of benzoic acid with methanol produces methyl benzoate. The reaction is monitored by TLC on silica gel using 25% EtOAc/hexanes. A sample of pure benzoic acid shows an Rf of 0.3 in this system. After 30 minutes, a sample from the reaction mixture shows a spot at Rf = 0.3 and a new spot at Rf = 0.7. What is the most accurate conclusion?
Answer: The reaction is proceeding but is incomplete; methyl benzoate is the product with Rf = 0.7.
The starting material is benzoic acid, a carboxylic acid. The product is methyl benzoate, an ester. Carboxylic acids are significantly more polar than esters due to their ability to hydrogen bond (both as a donor and acceptor) and form dimers. Therefore, benzoic acid will have a lower Rf on silica gel than methyl benzoate. The spot at Rf = 0.3 corresponds to the starting material, benzoic acid. The new, higher Rf spot at 0.7 must be the less polar product, methyl benzoate. Since the spot for the starting material is still present, the reaction is incomplete. - An analysis of a compound by TLC on silica gel using 30% ethyl acetate in hexanes results in an Rf value of 0.80. To achieve a more suitable Rf value for preparative column chromatography (ideally Rf ≈ 0.3), how should the eluent be modified?
Answer: Decrease the proportion of ethyl acetate to 10% to increase the compound's interaction with the stationary phase.
An Rf value of 0.80 indicates that the compound is moving too quickly up the plate, meaning the eluent is too polar. To decrease the Rf value, the polarity of the eluent must be reduced. This will cause the compound to spend more time adsorbed to the polar stationary phase (silica gel) and less time dissolved in the mobile phase. Decreasing the proportion of the polar component (ethyl acetate) from 30% to 10% will make the eluent less polar, thus lowering the Rf. Increasing the polarity (options A and C) would increase the Rf value even further. Adding triethylamine (D) is typically used to prevent streaking of basic compounds, not primarily to adjust Rf in this way. - A student analyzes a pure, solid compound by TLC on a silica gel plate. Instead of a compact spot, the student observes a long, vertical streak extending from the baseline. Which of the following is the most probable cause for this observation?
Answer: The initial spot of the sample applied to the baseline was too concentrated, overloading the stationary phase.
A long vertical streak on a TLC plate is a classic sign of overloading. When the initial spot is too concentrated, the stationary phase in that area becomes saturated. As the eluent moves up, it cannot establish a proper equilibrium, causing the compound to smear or 'streak' up the plate. If the compound were irreversibly bound (D), it would remain as a spot at the baseline. An unsaturated chamber (B) typically leads to a curved solvent front and inconsistent Rf values, not a single vertical streak. If the compound were not UV active (A), nothing would be seen under UV light. - When preparing to run a column, a student dissolves their entire crude sample in 20 mL of the eluent and carefully pipettes this solution onto the top of the packed silica gel column. Which of the following is the most likely result of this sample loading technique?
Answer: The large volume of solvent will cause the sample to start as a very broad band, leading to poor resolution and overlapping fractions.
For optimal separation in column chromatography, the sample should be loaded onto the column in the smallest possible volume of solvent. This ensures that all the molecules of the sample start their journey down the column from a very narrow band. Loading the sample in a large volume (20 mL is large in this context) means the starting band is already very wide. As the components travel down the column and separate, their individual bands will also broaden. If the starting band is too wide, the individual component bands will overlap significantly before they have a chance to separate, resulting in poor resolution and impure fractions.
- A student attempts to find a suitable mobile phase for the column chromatography of a crude reaction mixture on silica gel. A TLC run in 100% hexanes shows all components at the baseline (Rf ≈ 0). A second TLC run in 100% ethyl acetate shows all components at the solvent front (Rf ≈ 1). What is the most logical and efficient next step to find an appropriate eluent for separation?
Answer: Run a new TLC plate using a solvent system of intermediate polarity, such as 20% ethyl acetate in hexanes.
The initial tests show that 100% hexanes is too nonpolar (no movement) and 100% ethyl acetate is too polar (no retention). The logical next step is to test a mobile phase of intermediate polarity. A mixture like 20% ethyl acetate in hexanes is a standard starting point to see if the components can be resolved with Rf values in the useful range (ideally 0.2-0.5). Option A is a guess; it's better to test the mixture on TLC first. Option C changes the stationary phase, which is a more drastic step and not the immediate next logical one. Option D will not work; if there is no retention on TLC, there will be no separation on the column, regardless of flow rate. - A student performs a Grignard reaction between phenylmagnesium bromide and acetone, followed by an aqueous workup. The goal is to synthesize 2-phenyl-2-propanol. A common side reaction is the coupling of the Grignard reagent with unreacted aryl halide to form biphenyl. How would the product and this byproduct appear on a silica TLC plate?
Answer: Product (alcohol) would have a low Rf; byproduct (biphenyl) would have a high Rf.
The product, 2-phenyl-2-propanol, is a tertiary alcohol. The hydroxyl group makes it quite polar and capable of hydrogen bonding, so it will adsorb strongly to silica gel and have a low Rf value. The byproduct, biphenyl, is a nonpolar hydrocarbon. It will have very weak interactions with the silica gel and will be readily eluted by the mobile phase, resulting in a very high Rf value. Therefore, the polar product will have a low Rf and the nonpolar byproduct will have a high Rf.
- Four compounds are spotted on a single silica gel TLC plate: butanal, 1-butanol, pentane, and diethyl ether. After development, what is the correct order of the compounds from the baseline (lowest Rf) to the solvent front (highest Rf)?
Answer: 1-butanol, butanal, diethyl ether, pentane
The order on a silica gel TLC plate from lowest Rf to highest Rf corresponds to an order from most polar to least polar. We must rank the compounds by polarity based on their intermolecular forces. 1-butanol is most polar due to its ability to hydrogen bond (donor and acceptor). Butanal (an aldehyde) is next, with a strong dipole-dipole interaction. Diethyl ether has a weaker dipole and can only act as a hydrogen bond acceptor. Pentane is a nonpolar alkane with only weak van der Waals forces. Therefore, the order of increasing polarity is pentane < diethyl ether < butanal < 1-butanol. The order of Rf values will be the reverse, so the order from lowest Rf to highest Rf is 1-butanol, butanal, diethyl ether, pentane. - A reaction is expected to convert 4-methoxybenzoic acid into methyl 4-methoxybenzoate. How should the relative TLC Rf values on a silica plate change upon successful conversion?
Answer: The Rf value will increase significantly because an ester is much less polar than a carboxylic acid.
Polarity is the primary determinant of Rf on a silica gel plate. Carboxylic acids are very polar due to their ability to donate a hydrogen bond and form dimers. Esters lack the acidic proton and are significantly less polar. Therefore, converting the carboxylic acid (starting material) to an ester (product) will dramatically decrease its interaction with the polar silica gel, causing it to travel much further with the eluent and have a significantly higher Rf value.
- A researcher is separating a mixture of a relatively non-polar compound A (Rf = 0.8 in 25% EtOAc/hexane) and a very polar compound B (Rf = 0.05 in 25% EtOAc/hexane). Which chromatographic strategy would be most efficient for separating these two compounds on a silica column?
Answer: Start with a low-polarity eluent (e.g., 10% EtOAc/hexane) to elute compound A, then increase the eluent polarity (e.g., to 50% EtOAc/hexane) to elute compound B.
This situation is ideal for gradient elution. Compound A has a high Rf, and compound B has a very low Rf in the same solvent. Using a single isocratic eluent would be inefficient: an eluent weak enough to give A a good Rf for separation would take an extremely long time to elute B. The best strategy is to start with a less polar eluent to cleanly elute the less polar compound A. Once A is off the column, the eluent polarity is increased to quickly elute the more strongly-adsorbed, polar compound B. This is called a step gradient. Option C is incorrect as starting with a high polarity eluent would cause both compounds to elute together at the beginning. Option D sacrifices resolution for speed. - An incomplete oxidation of benzyl alcohol with PCC yields a crude mixture containing unreacted benzyl alcohol, the product benzaldehyde, and a non-polar hydrocarbon impurity. What will be the order of elution for these three compounds from a silica gel column, from first to last?
Answer: Hydrocarbon impurity, benzaldehyde, benzyl alcohol
Elution order from a silica gel column is from least polar to most polar. We must rank the polarity of the three components. Benzyl alcohol is the most polar due to its hydroxyl (-OH) group, which can hydrogen bond with the silica gel. Benzaldehyde is of intermediate polarity due to its polar carbonyl (C=O) group. The hydrocarbon impurity is non-polar. Therefore, the non-polar hydrocarbon will elute first, followed by the moderately polar benzaldehyde, and finally the most polar benzyl alcohol will elute last.
- A reaction is run to convert reactant R into product P. A TLC is performed on silica gel with three lanes: 'R' (pure reactant), 'Mix' (the reaction mixture), and 'Co' (a co-spot of R and the mixture). In the 'Mix' lane, two spots are observed. In the 'Co' lane, the lower of the two spots from the 'Mix' lane appears significantly more intense, while the upper spot is unchanged. What is the most valid conclusion?
Answer: The reactant R is more polar than the product P, and the reaction has not gone to completion.
The co-spot lane is used to definitively identify spots. Since adding more reactant R to the mixture in the 'Co' lane intensifies the lower spot, the lower spot must be the unreacted starting material, R. This means the reaction is incomplete. Because R is the lower spot, it has a lower Rf value and is therefore more polar than the substance creating the upper spot. The upper spot, which only appears in the 'Mix' lane, must be the product, P. Therefore, the reactant R is more polar than the product P, and the reaction is incomplete.
