Chromatography 1. Explain how functional groups are a strong indicator of Rf val

Question

Chromatography

1. Explain how functional groups are a strong indicator of Rf values?

2. A sample of 2-naphthol and naphthalene are separated using chromatographic techniques. Which sample would have a higher Rf value on a TLC plate? Which sample would come off first in a column?

3. A 1.50G sample of 2-naphthol and naphthalene was diluted with 2.00mL of methanol and loaded into a column packed with silica gel sludge in hexanes. Upon development, it was noted that 1.50gram sample came in the first 3 fractions with no separations. Determine the technical mistake that was made and how to rectify the situation?

Explanation / Answer

In Chromatographic separation

Substrate interacts with stationary phase which dictates the rate of separation

stronger the interaction, more slowly the substance moves

polar substrates move more slowly than non-polar ones

The rate at which a compound moves defines the Rf of the compound in the chromatographic separation.

TLC Data can be quantified using "ratio to front" or Rf values

Rf = Distance traveled by compound/ Distance traveled by solvent

So this value is a fraction.

Based on the Rf you can estimate if the following intermolecular interactions between the stationary phase and functional group exist

1. Hydrogen Bonding - alcohols R-OH carboxylic acids

2. Dipole-Dipole (Electrostatic) Interactions - aldehydes ketones

3. Coordination - amines R-NH2

4. Van der Waals interactions - hydrocarbons R-CH3

2. A sample of 2-naphthol and naphthalene are separated using chromatographic techniques.

Since the intermolecular interactions between 2-naphthol and the stationery phase will be higher the Rf of 2-naphthol will be lower which means it will move slower and so naphthalene will move faster.

3) In a Chromatographic separation the compound needs to be loaded on the top of a well packed silica gel column, which is having just enough solvent on top of it to prevent it from drying. This column is then allowed to absorb the compound first with the minimum of spread and then allowed to separate into bands. This will then help in separation.

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