Benzil is a fairly bright yellow solid; it looks similar to the benzoin but a brighter color. Collect the benzil crystals by vacuum filtration. You may rinse the crystals and the container with distilled water if necessary. Allow the crystals to dry in the Bchner funnel, with air passing over them, for at least five minutes.
Then transfer the crystals to a pre-weighed beaker or watch glass, and dry them in the oven for about 10 minutes. Test for the Presence of Unoxidized Benzoin This is a simple test to determine if your reaction is complete, and if your product is pure.
This is a qualitative test, so it isnt necessary to measure the quantities exactly; they can be approximated. In a test tube, dissolve about 0. If unreacted benzoin is present, the solution soon acquires a dark brown or black color owing a complex of benzil with a product of auto-oxidation of benzoin. This reaction may also produce a muddy-looking precipitate. If no dark color develops in minutes, and the sample remains a yellow or amber color, this indicates that the sample is free from benzoin in other words, the oxidation of benzoin was complete.
Sometimes students are confused as to the results of this test because you have never seen what a positive result looks like. You can observe a positive result if you add a small amount of benzoin; observe the color that develops. The aqueous filtrate is non-toxic, and should be neutral or slightly basic. Flush the waste down the drain, with plenty of water.
Report Record the yield, color and melting point of the purified benzil. Depending on the procedure that you followed last week, you may need to measure the melting point of last weeks product, also.
Check with your instructor to see if this is required. This corresponds to the melting point of the crude product concluding that purification failed. The final product of benzoin contained 13C NMR peaks at Finally, a peak at Regarding the 1H NMR spectra, four multiplet peaks appeared in the range of 7. A peak at 5. A peak at 3. An impurity of ethanol appeared at 4.
Finally, the IR spectra displayed a peak at cm-1 representing the C-H stretches, a peak at cm-1, accounting for the alcohol group, and a strong peak at cm-1 representing the carbonyl group. Overall, the spectra confirmed the condensation of benzoin. When this mixture was heated and refluxed, a strong red color appeared in the reflux condenser, proving the presence of nitric gas.
A total of 1. The 13C NMR produced a peak at Four peaks appeared between The 1H NMR displayed three multiplet peaks at 7.
This data proved the success of the oxidation of benzoin to produce benzil. For the final reaction, once benzil and aqueous potassium hydroxide were combined, the reaction turned from black to brown.
This intermediate step produced potassium benzilate. After workup, a total of 0. The 13C NMR spectra displayed a weak peak at Four peaks at The 1H NMR spectrum produced a peak at 7. A peak at 2. A peak did not appear at 12 ppm that would have represented the hydrogen of the carboxylic group, which means the reaction was not carried to completion. Under no circumstances should you remove the flask from the hood while the reaction is going on! DO NOT remove the reaction mixture from the hood until the instructions indicate that you should do so.
Although nitrogen oxides are not absorbed through the skin, they will stain your skin yellow or brown -- you should wear gloves during this procedure. You only need to reserve a very small scoop for the last part; about 0. If your yield from last week is less than 4 grams, your instructor can give you some extra for this procedure. Place the flask in a boiling water bath for 10 minutes -- this also must be performed in the hood.
It is best to perform this reaction in a mL Erlenmeyer flask, to minimize the possibility of splattering. The mixture will begin as a slurry mixture of solid and liquid; as it heats up to the temperature of the hot water, it should liquefy.
You will also see the formation of dark orange or brown NO and NO2 fumes as the reaction proceeds. After the 10 minute heating period, slowly add 75 mL of water to the reaction mixture, cool to room temperature, and swirl for a minute or two to coagulate the precipitated product; make sure that there are no nitrogen oxide fumes remaining in the flask.
After this the reaction mixture may be removed from the hood. You can do the rest of the lab at your desk. Note: At this point you may notice one or more globules of an oily substance in your flask. This is your product, but it may not solidify in acid solution. Use litmus or pH paper to determine if the mixture is basic; if it is still acidic, keep adding NaOH, about 4 - 5 mL at a time, until the pH is basic. The product should begin to crystallize as you add the NaOH.
Once the solution is basic, cool the mixture in an ice bath for a few minutes.
The pure product of benzoin showed the following physical characteristics: 2. A peak at 5.
Dissolve the product from the reaction above in a minimum amount of hot ethanol; remember that the mixture should be kept hot on the hot plate 4 until all of the solid has dissolved. It causes severe burns to the skin! Which signal would be most de-shielded in this spectrum? At this point you should check with your instructor to determine if the product needs to be recrystallized.
If your yield from last week is less than 4 grams, your instructor can give you some extra for this procedure. After the 10 minute heating period, slowly add 75 mL of water to the reaction mixture, cool to room temperature, and swirl for a minute or two to coagulate the precipitated product; make sure that there are no nitrogen oxide fumes remaining in the flask. Disadvantages include time-consuming experiments, error within intermediate steps, or the presence of side reactions. At this point you should check with your instructor to determine if the product needs to be recrystallized. In this reaction, the two aldehydes serve different purposes; one aldehyde donates a proton and one aldehyde accepts a proton.
One the solid is completely dissolved, add water dropwise until the solution becomes cloudy, and set aside to crystallize. Record the mass in your notebook.
Rearrangement of the intermediate results in polarity reversal of the carbonyl group, which then adds to the second carbonyl group in a second nucleophilic addition. You can do the rest of the lab at your desk.