Most detailed answer gets the points! After running the following reaction you a

Question

Most detailed answer gets the points!

After running the following reaction you analyze your crude product and observe that it is a mixture of the product, diphenyl methanol, and the starting material, benzophenone. To purify your product you choose to run a column. Describe in detail how to separate these two compounds using a silica gel column and ethyl acetate as your eluent. Be sure to include loading your sample, eluting your sample, analyzing the fractions, collecting the pure product, and verifying the purity.

Explanation / Answer

Your alcohol will elute first because it is more polar than the diphenyl ketone. The hexane will push the alcohol out, while the ketone will have an affinity for the low polar silica gel stationary phase. The acetone solvent will remove the ketone at the end. You can run a gas chromatography and do peak integration to see the percent composition of fractions. You can also run an IR spectra to simply see which functional group is present in your sample. Peaks broad and strong at >3200 1/cm indicate alcohol in fraction. If you don't see this, but you do see a carbonyl peak at 1700 1/cm then you have a ketone in the fraction.

BELOW IS THE DETAILED PROCEDURE.

The column construction is the same for both macro-scale and mini-scale experiments. Columns are always packed the day they are to be used and a new column is packed for each subsequent trial. Generally, micro-scale column chromatography uses a Pasteur pipet to separate mixtures of 10-100 mg. The column is filled with a small piece of cotton at the bottom to act as a plug. Next, a thin layer of sand is placed on top of the cotton. The next layer in the column consists of the adsorbent material (i.e. the stationary phase). While the two most commonly used adsorbents are silica gel and alumina (Al2O3), silica gel is usually the medium of choice for sensitive natural compounds. Finally, another thin layer of sand is placed on top of the adsorbent layer. Room is left at the top of the column for the solvent and sample to be applied. Normally, the force of gravity alone is sufficient to elute the components within the sample. However, air pressure can be applied to speed up the flow of solvent, dramatically decreasing the time needed to purify the sample. This technique is referred to as flash chromatography.

Preparation of the column

Set up a ring stand and a mini finger clamp to support the column to be

packed.

Secure the Pasteur pipet with the finger clamp. Do not over-tighten the

finger clamp as the Pasteur pipet may easily crack or break.

Insert a small piece of cotton into the column.

Use a wooden applicator to move it down to the bottom of the column (do

not compact the cotton too much). The cotton serves to prevent the sand and

silica from falling through the bottom of the column.

With a spatula, add a thin layer of sand (not more than 0.5 cm) on top of the

cotton.

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Carefully fill the column with 5-6 cm of silica as shown by your instructor. Be sure to tap the side of the Pasteur pipet gently with a spatula so that the silica settles. Once the silica settles, you may need to add a little more.

Now, add a second layer of sand (no more than 0.5 cm) on top of the silica.

The column should now have about 1.5 cm of free space.

Rinsing the column

Microcolumns run very quickly, so it is important that you have everything prepared (solutions,

pipets, and receiving containers) before starting the column. In six labeled test tubes deliver the following solvent volumes (these solvents will be used in succession to run the column):

a) 2 mL hexanes

b) 2 mL hexanes

c) 2 mL hexanes

d) 2 mL 10% acetone in hexanes*

e) 2 mL 10% acetone in hexanes*

f) 2 mL 10% acetone in hexanes*

*Prepare this mixture fresh to guarantee the correct proportion of acetone: hexane

Label six 5 mL vials 1-6. These vials will be used to collect the column fractions.

Before any sample can be added to the column, all materials inside the column have to be thoroughly

wetted with the eluting solvent, hexanes. This helps to keep air bubbles out of the column.

Add hexanes (mobile phase) to the free space on top of the column until it is almost filled. Add the

solvent slowly, being sure to not disturb the sand or stationary phase.

Let the solvent flow on its own until it reaches about half way through the column, but continue to

add more hexanes as to not let the column run dry.

Attach the tubing to the tip of the Pasteur pipet. This is done to elongate the tip of the column and to

make the end of the pipet flexible.

Insert the syringe into the tubing and suction the solvent through the column. This will get rid of air

bubbles trapped in the column. Have extra hexanes available to replenish the solvent at the top of the column. DO NOT allow the solvent to drop below the top of the sand layer, otherwise, air bubbles will form and interfere with the functioning of your column. Always keep the column wet!

Remove the tubing and allow the solvent to flow through the column.

Continue adding solvent until the adsorbent packing is thoroughly wet and the solvent is dripping into

a waste beaker placed below the column.

When the solvent elutes to a point just above the sand layer, place 1 drop of the spearmint oil in the

column.

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Without allowing the column to run dry and after the sample has penetrated the column (the liquid level should be just above the sand), add the first solvent from the list above a little bit at a time being sure to not disturb the sand or stationary phase (use a disposable pipet to do this).

Reattach the tubing and the syringe to the tip of the Pasteur pipet.

Very slowly, suction the eluent into the syringe collecting at a rate of approximately 0.5 mL per

minute. Collect a total of 2 mL of eluent in the syringe. Stop suctioning and remove the syringe. After

the syringe has been removed, quickly turn the tubing upward to prevent leaking.

Transfer the 2 mL collected into the vial labeled

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