1. When preparing a mixture to be added to a column for chromatographic separati
ID: 1008285 • Letter: 1
Question
1. When preparing a mixture to be added to a column for chromatographic separation, it is important to add the mixture as a very concentrated solution. Why? (be sure your answer includes a discussion of what will happen if the solution is too dilute when it is added to the column)
2. Predict in which the following compounds will elute from a chromatography column: acetophenone, 1,3- dichlorobenzene, 4-aminobenxoic acid, and benzoic acid?
3. In looking through some references to experimental procedures, a student finds two references for purifying a compound from a reaction mixture. In one procedure, pure product is obtained by carrying out column chromatography using a solvent mixture of hexane and ethyl acetate. In the second procedure, methylene chloride is used as the mobile phase. Assuming the time required and the volumes of solvents are equal, what are the advantages and disadvantages of each of these solvents systems?
Explanation / Answer
1. When preparing a mixture to be added to a column for chromatographic separation, it is important to add the mixture as a very concentrated solution. Why?
Usually the solutions prepared to add over a column of silica gel for chromatographic separation are more polar in order for the entire mixture to dissolve. When this polar and concentrated solution added to the column it immediately starts eluting (travels down the column) and while doing so it carries along with it all the compounds in the mixture. Since it is a polar mixture there will be little separation of components. Hence the mixture is dissolved in small amount of solvent and prepared a very concentrated solution.
2. Predict in which the following compounds will elute from a chromatography column: acetophenone, 1,3- dichlorobenzene, 4-aminobenxoic acid, and benzoic acid?
The order of elution of compounds depends on polarity of component mixture. If the above mixture of compounds added on a chromatography column then their eluting pattern will be as follows:
1,3-dichlorobenzene..................least polar
acetophenone...........................polar
benzoic acid..............................polar
4-aminobenxoic acid,................more polar and forms an internal salt.
3. In looking through some references to experimental procedures, a student finds two references for purifying a compound from a reaction mixture. In one procedure, pure product is obtained by carrying out column chromatography using a solvent mixture of hexane and ethyl acetate. In the second procedure, methylene chloride is used as the mobile phase. Assuming the time required and the volumes of solvents are equal, what are the advantages and disadvantages of each of these solvents systems?
Hexane and ethyl acetate Advantages: This mixture can be graded from least polar (100% hexane) to higly polar (100%) depending on the requirement of student because it is a combination of non-polar solvent and polar solvent. Since it's a mixture of solvents with different polarities the polarity of eluting mixture can be changed rapidly which allows rapid elution of purifying This is known as gradient elution system.
Hexane and ethyl acetate Disadvantage: Finding right eluting mixture for solvents in order to separate the purifying compound may be difficult time consuming and laborious.
Methylene chloride advantage: If only methylene chloride is used as the mobile phase then the eluting solvent polarity is uniform throughout the column duration and no need to make an eluting mixture. This kind of elution is known as isocratic elution.
Methylene chloride Disadvantage: The separation of purifying compound may be slow and takes long time. However the purifying compound may be eluted in a more pure manner because of single solvent system for elution.
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