Chapter 2 Flashcards
In normal-phase column chromatography, which solvent has more eluting power, petroleum ether or dichloromethane? In what way is the eluting power of a solvent related to its polarity?
Dichloromethane has more eluting power in normal-phase (i.e with polar stationary phase) colum chromatography because it is more polar than petroleum ether.
In normal-phase chromatography, eluting power increases with increasing polarity of the solvent. Polar solvents can more readily compete with the polar stationary phase for the solute, thereby inceasing the time the solute spends in the mobile phase.
In reverse-phase column chromatography, which phase is more polar: the stationary phase or the mobile phase?
In reverse-phase chromatography, mobile phase is more polar.
How does the principle of “like dissolves like” explain the affinity of a compound for the mobile phase relative to the stationary phase?
The more alike the mobile phase is to the solute (i.e. the more similar in polarity), the greater their mutual affinity. If a solute has greater affinity toward the mobile phase than the stationary phase, it will spend more time in the mobile phase (and vice versa). The more time a solute spends in the mobile phase, the faster it elutes, leading to a shorter retention time (column chromatography) or larger Rf value (TLC).
State two major differences between TLC and column chromatography
- TLC elutes from the bottom → up; Columns elute from the top → down
- TLC accomodates small sample sizes only; Columns can separate much larger (gram scale) samples
- TLC isn’t usually used for compound isolation; Column chromatography is used for compound isolation
- TLC plates have the stationary phase bound to a solid backing; Columns are packed with loose powder of the stationary phase
- TLC is very quick; Colum chromatography usually takes significantly more time to perform.
If you had 5.0g of material that needed to be purified, would you opt for using TLC or column chromatography to purify your material? Explain why.
You would opt for column chromatography. TLC is primarily an analytical method used to separate very small quantities (~10-9 moles) of a sample, and is used more fore detectin of compounds than isolation of material.
Why should care be exercised in the preparation of a column to prevent air bubbles from being trapped in adsorbent?
Air bubbles in the adsorbent cause poor separations b/c they disrupt the solute equilibrium b/w the stationary and mobile phases. This phenomenon is often called “channeling” or “cracking”. As a result, components elute as broad rather than sharp bands and are often not fully separated.
Why is a layer of sand placed above the cotton plug prior to the addition of the column packing material?
Sand is placed above the cotton plug to provide a level base for the stationary phase, and to prevent any stationary phase from clogging the cotton plug.
Does fluorene or 9-fluorenone move faster down the column when petroleum ether is used as the eluant? Why?
In normal-phase chromatography, fluorene would move faster down the column using petroleum ether than 9-flurorenone. This is because fluorene, like petroleum ether, is not very polar and therefore has much greater affinity to the eluent. 9-Fluorenone, on the other hand, is polar and therefore more similar to the stationary phase.
Consider the structures of fluorene (5) and 9-fluorenone (6)
Other than the aromatic ring, specify what functional group, if any, is present in 6.
9-Fluorenone contains a ketone
Predict which compound is more polar, Fluorene or 9-Fluorenone, and explain why.
9-Fluorenone is more polar because of the carbonyl group of the ketone. Oxygen is much more electronegative than carbon, giving the C=O bond a large dipole moment. Since the rest of the molecule is largely non-polar this C=O bond dipole contributes strongly to the overall molecular dipole of 9-fluorenone.
Which compound, fluorene or 9-fluorenone, would you expect to elute from a normal-phase column first and why?
We would expect fluorene to elute first from a normal-phase column, because it would have less affinity for the polar stationary phase than 9-fluorenone and therefore spend more time in the mobile phase.
In the separation of fluorenen and fluorenone by column chromatography, what is stationary phase? The mobile phase?
The stationary phase is alumina (Al2O3) and the mobile phase is a mixture of petroleum ether and dichloromethane.
