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Chemistry Prelims question 2





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Chromatography

Column chromatography (and its various types) are the most used separation methods in chemistry and it relies on different relative solubilities. The column consists of a stationary phase and generally the solvent system makes the mobile phase. The solvent system is continuously
supplied into the column, while changing the composition, if necessary, to separate components of a mixture.

You have been asked to separate a mixture of toluene, benzyl alcohol, benzaldehyde and benzoic acid using a silica column (that is capable of extensive hydrogen bonding) and a solvent system of dichloromethane/methanol with continuously increasing fraction of
methanol with time.

1. How do you think this separation works, and what is the resulting order
of elution (the order in which the compounds come out of the column)?

2. Were you to use a single solvent – hexane for example – throughout the
process, what do you expect might happen (and therefore why were you
asked to use a time-gradient in concentrations)?

3. Columns can have different stationary phases depending on the need.
Say you replace the silica column by a C18-column where the stationary
phase is densely packed C18-alkyl chains. What would your solvent
system look like, and what would be the order of elution?

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