1. Predict the order of elution of a mixture of triphenylmethanol, biphenyl, benzoic acid, and methyl benzoate from an alumina column. You may need to examine the structural elements in each compound.
2. Once the chromatography column has been prepared, why is it important to allow the level of the liquid in the column to drop to the level of the alumina before applying the solution of the compounds to be separated?
3. A chemist carried out a column chromatography experiment and reached the point at which the two compounds being separated were about three-fourths of the way down the column. He stopped the column since he had tickets to an opera. He returned on the next morning and re-started the column. He found that the compounds eluted together off the column as a mixture. Speculate the reason for this.
1. To predict the order of elution of the mixture on an alumina column, you need to consider several factors: polarity, functional groups, and molecular size.
In general, compounds with higher polarity or more polar functional groups tend to elute later, while less polar compounds elute earlier. Similarly, larger molecules tend to elute later, while smaller molecules elute earlier.
Now let's analyze the structural elements of the compounds in the mixture:
- Triphenylmethanol: This compound has a hydroxyl group (-OH) attached to a phenyl group. It is relatively polar due to the presence of the hydroxyl group, so it will elute later.
- Biphenyl: Biphenyl consists of two phenyl groups bonded together. It is nonpolar and relatively small, so it will elute earlier.
- Benzoic acid: Benzoic acid has a carboxyl group (-COOH) attached to a phenyl group. The carboxylic acid functional group makes it more polar than biphenyl, but less polar than triphenylmethanol. It will likely elute between the two.
- Methyl benzoate: Methyl benzoate has a methyl group (-CH3) attached to a phenyl group. It is less polar than benzoic acid but more polar than biphenyl due to the ester functional group. It will likely elute between the two as well.
Based on this analysis, the predicted order of elution from an alumina column would be:
1. Biphenyl
2. Methyl benzoate
3. Benzoic acid
4. Triphenylmethanol
2. It is important to allow the level of the liquid in the column to drop to the level of the alumina before applying the solution of the compounds to be separated for several reasons:
a) To prevent distortion of the separation: When the liquid level is above the alumina, the compounds will travel through the liquid, which can result in uneven and distorted separation. Allowing the liquid level to drop ensures that the compounds are properly interacting with the stationary phase (alumina) for effective separation.
b) To avoid overloading the column: If the liquid level is too high, there is a risk of overloading the column with too much sample. This can lead to poor resolution and band broadening of the separated compounds. Allowing the liquid level to drop ensures a controlled application of the sample, preventing overloading.
c) To ensure efficient separation: By allowing the liquid level to drop, you create a sufficient distance between the applied sample and the stationary phase. This allows time for the compounds to interact with the alumina and separate effectively, leading to better resolution and purity of the eluted compounds.
3. The reason the compounds eluted together as a mixture when the chemist stopped the column three-fourths of the way down and restarted it on the next morning is likely due to an insufficient separation time for complete elution.
During the overnight break, the compounds in the column may have continued to interact with the stationary phase, resulting in slower elution. However, when the column was restarted, the compounds that were initially separated but not yet fully eluted would have had additional time to interact and mix together again.
Column chromatography relies on the differential interaction between the compounds and the stationary phase. If the separation process is interrupted before the compounds have completely eluted, there is a possibility of re-equilibration and mixing. In this case, the interruption led to insufficient separation, and when the column was restarted, the compounds eluted together as a mixture. To achieve complete separation, it is important to allow sufficient time for each compound to fully elute before stopping or interrupting the column chromatography process.