You want to separate compound A from compound B by flash chromatography, which i

Question

You want to separate compound A from compound B by flash chromatography, which is similar to the thin layer chromatography you did in the laboratory. The following are the results of running these two compounds in different hexanes/ethyl acetate systems.

a. Provide the Lewis dot structures of hexane and ethyl acetate. Comment on the OVERALL polarity of each of these compounds.

b. Describe the trend observed in the tlc plates. Include a description of how the polarity of the solvent system affects the separation of the two solutes and use the observations to reach a conclusion about which compound is more polar (A or B).

BONUS: (3 points) A student loaded a mixture onto a small flash chromatography column in 1 mL of methylene chloride, then proceeded to elute with hexanes/ethyl acetate 10:1. He/she found that all of the mixture came off in the first 4 fractions, with no separation. What technical mistake did the student make?

Explanation / Answer

I've submitted a rendering of eight (8) different styles of structural diagrams for ethyl acetate . Most of them are different types of Lewis diagrams; however, those that lack dots (symbolizing electrons) around the oxygen atoms are technically not full Lewis diagrams, they might be better called molecular structure diagrams or something along these lines. The whole point of a Lewis dot diagram (also called a Lewis structure) is to show not only where the electrons are in relation to the structure of the molecule, but to place an emphasis on showing the ones that are not implicit in the lines that comprise the bonds of a molecular structure diagram. I've included them all because I had the file already created "in stock" in my personal library from a previous question, and because it gives you something to compare between the two aforementioned types. http://www.webanswers.com/post-images/5/52/5CF723CD-7ACA-4B16-8E81838DCE683A10.jpg It appears that the last three on the right column re not technically Lewis structures, merely molecular structural renderings of different types. (Some include terminal carbons and some are just stick or line structural drawings. The first one on the left is used mostly in teaching to help show where the electrons are VISUALLY, after this is learned, lines are substituted to shared pairs of electrons. The top one on the right column is three dimensional as well.) One styles that IS missing but is common for the sake of speed is where lone electron pairs are designated by lines perpendicular (or tangential) to the surface of the atom that the electrons occupy. Single electrons still require dots with that methodology though. (for a hypothetical element ?X? that has one lone pair of electrons and a bond to ?Y? it might normally be drawn like this ( :X-Y ), but in the shorthand style I just mentioned it would appear like this ( |X-Y ) where you can see (| = :). the lewis dot structure for hexane is as follw:- .C . H . H . H . C . H . H . C . H . H . C . H . H . C . H . H . C . H . H . H . . . . . . All chromatographic systems rely on the fact that a substance placed in contact with two immiscible phases, one moving and one stationary, will equilibrate between them. A reproducible fraction will partition into each phase, depending on the relative affinity of the substance for each phase. A substance which has affinity for the moving or mobile phase will be moved rapidly through the system. A material which has a stronger affinity for the stationary phase, on the other hand, will spend more time immobilized in that phase, and will take a longer time to pass through the system. Therefore, it will be separated from the first substance so the polarity of B is more than A

Related Questions

Navigate

• Browse (All)
• Browse Y
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.