Organic Chemistry Fall Experiment #4
Chromatography
Prelaboratory Questions
1. What is the purpose of the stationary phase and the mobile phase in chromatography?
2. Chromatographic methods depend on distributing the substances being separated between the two phases discussed in question #1. Explain what interactions (in terms of intermolecular forces) occur between the compounds being separated and the two phases (stationary and mobile). Explain how these interactions enable compounds to be separated.
3. Consider the TLC plate shown below that was done on silica gel using a solvent mixture of 4:1 hexane/EtOAc as the eluent.
(a) Which of the two organic compounds labeled A and B is less polar?
(b) Calculate the Rf value of compound B.
(c) How would the following changes affect the Rf values of A and B?
i. Change to a more polar solvent mixture – Draw the resulting TLC chromatogram using the blank plate provided on the left above. Also, write in your eluent on the line provided above the chromatogram.
ii. Change to a less polar solvent mixture
iii. Add more sample to the TLC slide before it is eluted
iv. Allow more time for the solvent front to move further
4. Of the following compounds, what will be the order of elution (1st to last) from a silica gel column using 80% hexane and 20% ethyl acetate as your eluent?
5. Here’s an example procedure. Calculate the amount of each underlined reagent needed to perform this synthetic transformation based on the amount of benzoic acid that you are starting with.
To a 50 mL round-bottom flask, was added 2.5 g of benzoic acid and 100 mL of ethyl acetate as a solvent. The reaction mixture was cooled in an ice bath and 1.4 molar equivalents of acetic anhydride were added all at once. After 20 min of reaction time, 1.2 molar equivalents of copper sulfate were added and the ice bath was removed.
NOTES:
1. The idea of the equivalent is to allow you to scale a reaction to whatever amount you deem necessary. So, for example, if you know that you have 1.0 spoonful of sugar and you know you need 1.3 equivalents of salt, then you are really saying that you need (1.3 x 1.0 spoonfuls) or a total of 1.3 spoonfuls of salt. Of course, this assumes that the reaction of salt and sugar is 1 to 1. Simply use the term “equivalents” as a scaling factor.
2. You should use this information as a guide to set-up calculations for the amounts needed in this week’s experiment. The only reagent you will not be able to calculate beforehand is the unknown alcohol. You will receive the information for your unknown alcohol when you arrive in lab.