1. General

If you don’t have the chance to finish all the parts of the experiment this week, you will have to do so next week.

Review the following topics: recrystallization theory, vacuum filtration, chromatography, Beer-Lambert Law

Note for the teaching assistant: Dibenzylketone has a melting point close to room temperature. Place the bottle in warm water (not boiling water!) at the beginning of the lab section. Don’t forget to remove it at the end.


2. Experiment

The students should use their own benzil in order to perform the experiment.

In the initial stage of the experiment, absolute ethanol is used as solvent. The used glassware has to be dry.

Dissolve both ketones first and bring the solution to a gently boil before adding the ethanolic potassium hydroxide solution dropwise. The color should change to dark purple immediately and a dark precipitate should form a few minutes into the reaction. If this does not occur after five minutes, add a little bit more of ethanolic potassium hydroxide solution. Reflux the mixture for an additional ten minutes. If the color remains brown, remove the mixture from the hot plate and discard the mixture after cooling. Start over making sure that you use all the correct chemicals e.g. absolute ethanol as solvent.

WARNING: Do not add the entire KOH solution at the same time. This will most likely cause a pretty violent reaction and the suspension will shoot out of the condenser resulting in a purle mess! Make sure that the spin vane is correctly oriented (=point down). This will keep the solid in suspension and usually avoids bumping.

The solution for the TLC should be fairly diluted. Your TA is going to provide solutions for the two reagents. Make sure that you use a capillary spotter (your TA will show you how you can make it) when you spot the TLC plate. Once the TLC plate is in the jar, close the jar and let it sit undisturbed on the bench to develop the TLC.

The solution for the UV-Vis part has to be fairly diluted (very light pink). Try to estimate the molar extinction coefficients based on the concentration and the absorbance values using Beer's Law.


3. Things to think about

a. Why do you have to dissolve both ketones first before you add the ethanolic KOH solution?

b. What is the advantage of using a KBr pellet (compared to a solution in dichloromethane) to obtain the IR spectrum of your final product?

c. How do you explain the difference in R_f values in the TLC experiment? (Hint: Computer lab meeting 4)

d. Why do you use a solvent mixture to purify the crude product?

e. What do you observe in the UV-Vis spectrum of your product?

f. How would the ¹³C-NMR spectrum change if you lower the temperature of the solution.