Question

Determine the yield limiting reagent and the theoretical yield for the following multistep synthesis from the reaction of ethyl acetoacetate. Draw the chemical complete chemical reaction and mechanism of Product C (determined to be 4-hydroxy-4 4-diphenylbutan-2-one) to Product E provided the procedure below.

Procedure

Step 4B [Product C to Product E]
   Place product [C] (0.5 g) in a 50 mL round bottomed flask, add acetone (15 mL), and concentrated HCl (2.5 mL). Boil under reflux on a water bath for 30 minutes. The solution should become pinkish-orange. Add water (30 mL), with swirling, then extract with ether (2 x 10 mL). Wash the combined ether layers with saturated sodium bicarbonate (3 x 10 mL), then water (2 x 10 mL). Dry the ether solution with anhydrous sodium sulfate, filter, and evaporate the ether to yield a yellow oil. This crude product [E] may be purified by rapid column chromatography.

Obtain a column with stopcock. If it does not have a sintered glass disc at the bottom, place a plug of cotton wool in the constriction. Clamp the column vertically, add 15 g of silica gel to the column, tap with rubber tubing to settle the packing, then place a 1 cm layer of sand on the top of the silica gel. Prepare 100 mL of 50/50 diethyl ether/petroleum ether (b.p. 40-60 °C). This mixture is referred to as “eluent” below. Keep the prepared mixture in a covered Erlenmeyer flask until needed. The successive steps must follow each other quickly, so prepare everything in advance.

1. Prewashing of column: Add eluent to the column, without disturbing the top of the packing. Open the stopcock, adding more eluent until the packing is saturated and liquid drips from the bottom. Tapping the column with rubber tubing will displace large bubbles, but if you delay here, more bubbles will form, especially on a warm day. When the column is prepared, allow it to run until liquid is just above the sand, then close the tap. Proceed immediately to the next step.

2. Application of the sample: Dissolve your crude product [E] in about 1 mL of eluent, transfer this solution by Pasteur pipette to the top of the packing. Rinse the flask with a further 2 mL of eluent, and transfer the rinsings to the column. As with all types of chromatography, your aim is to apply the sample in a narrow, concentrated band.

3. Elution of the product: Open the stopcock to allow the sample to flow on to the packing. Add more eluent to avoid the column drying out. The product will travel rapidly as a yellow band. Collect this band in a clean, dry, pre-weighed 100 mL round bottomed flask. When the yellow band has eluted, allow the column to run dry. Dispose of the used silica gel as instructed.

Evaporate the solvent to leave the pure product [E] as a yellow oil. Leave the flask on the rotary evaporator in the hot water for several minutes after all solvent appears to have gone, to make sure the last traces of solvent are removed. Measure the yield, and record the IR and ¹H and ¹³C NMR spectra. The product may discolor on standing, so show an Instructor the fresh material for checking.

Answer 1

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