Question

2. Consider a sample that is a mixture composed of napthalene, benzoic acid, and phenol. The sample is spotted on a TLC plate and developed in a hexanes/ethyl acetate solvent mixture. Predict the relative R_f values for the three compounds in the sample.

Answer 1

TLC is done based on the polarity. The stationary phase present here is highly polar silica group that is negatively charged. So if our molecule has polar groups such as carboxyl group, hydroxyl, amide, amine etc. it will be more adsorbed on to the silica ie, their non covalent interaction will be strong compared to simple alkyl chain compounds and aromatic rings (non-polar).

Rf value is the ratio of distance moved by the molecule to the distance moved by the solvent front.

The more polar the molecule the less will be the Rf value.

The solvent system is selected in a way that the spots move and get seperated properly.

Here naphthalene doesn't have any polar group. Its only aromatic ring and so they will almost move along with solvent front. Thus it will have the Rf value almost near 1.

Benzoic acid will move the least as it has carboxylic acid group. It will have the least Rf value.

Phenol have hydroxyl group and will have Rf value in between of the other two.