Consider a sample that is a mixture composed of biphenyl, benzoic acid, and benzyl alcohol. Predict the order of elution of the components in this mixture on the TLC plate when solvent system a, b or c is used.
a)Cyclohexane: Methylenechloride:100%:0%
b)Cyclohexane: Methylenechloride:70%:30%
c)Cyclohexane: Methylenechloride:0%:100%
To predict the order of elution of the components in the mixture on the TLC plate, we need to consider the polarity of the components and the polarity of the solvent systems. The general rule for TLC is that more polar compounds will move more slowly up the plate compared to less polar compounds.
Now, let's analyze the polarity of the compounds in the mixture:
1. Biphenyl: Biphenyl is nonpolar because it consists of two benzene rings connected by a single bond. It is expected to have low polarity.
2. Benzoic acid: Benzoic acid contains a carboxylic acid group (-COOH), which makes it more polar than biphenyl. The presence of a polar functional group increases its polarity.
3. Benzyl alcohol: Benzyl alcohol contains a hydroxyl group (-OH), which makes it more polar compared to biphenyl but less polar than benzoic acid.
Now let's analyze the polarity of the solvent systems:
a) Cyclohexane: Methylenechloride: 100%: 0%:
This solvent system is nonpolar since it contains only cyclohexane. In this nonpolar solvent system, the elution order is expected to be biphenyl > benzyl alcohol > benzoic acid. The less polar compounds are expected to elute first.
b) Cyclohexane: Methylenechloride: 70%: 30%:
This solvent system is intermediate in polarity since it contains a mixture of cyclohexane and methylenechloride. In this case, the elution order could be benzyl alcohol > biphenyl > benzoic acid. The more polar compound, benzyl alcohol, is expected to elute first.
c) Cyclohexane: Methylenechloride: 0%: 100%:
This solvent system is polar since it contains only methylenechloride. In this polar solvent system, the elution order could be benzoic acid > benzyl alcohol > biphenyl. The most polar compound, benzoic acid, is expected to elute first.
Remember that the elution order prediction is based on the polarity of the compounds and the polarity of the solvent system but can vary depending on the interactions between the compounds and the stationary phase of the TLC plate. It is always a good idea to run a pilot experiment to confirm the elution order.