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. However, keep in mind that you will need the practice solving NMR problems as well (HINT!).

Review the following topics: recrystallization theory, vacuum filtration, chromatography

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


2. Experiment

In the initial stage of the experiment, you will need to use absolute ethanol as solvent.

Dissolve both ketones first and bring the solution to a gently boil before you add the ethanolic potassium hydroxide solution. The color should change to dark purple right away and a dark precipitate is formed. If this does not occur after one minute, add a little bit more of ethanolic potassium hydroxide solution. Reflux teh mixture for additional ten minutes. If the color remains brown, remove the mixture from the hot plate and discard the setup after cooling. Start over making sure that you use all the correct chemicals e.g. absolute ethanol as solvent. Do not add the entire KOH solution at the same time. This will most likely cause a pretty violent reaction!

The solution for the UV-Vis part has to be fairly diluted (light pink). Try to estimate the molar extinction coefficients based on the concentration and the absorbance values. Discard the halogenated solvent in the appropriate waste container.



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 to obtain the IR spectrum of your final product?

c. How do you explain the difference in R_f values in the TLC experiment?

d. Why do you use a solvent mixture to purify the crude product? What would happen if you would use either ethanol or toluene alone?

e. What type of transitions do you see in the UV-Vis spectrum of your product?

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