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18 Dec 2019

We performed an Aldol condensation of acetone with benzaldehyde to form dibenzalacetone. NaOH was used as base, in ethanol solvent. In the work-up, we used water to remove acetone and NaOH and the water was removed by a syringe to practice decanting. We, then, added ethanol while the reaction vial was in an ice bath and let it stir for 2 minutes. We removed the ethanol and added more ethanol but the reaction vial was put in a hot bath (about 90 degrees celcius) under reflux until the solution became clear (1-2 minutes). Then we set it down and let it cool to room temp and let the crystals form. We then removed the ethanol and added cold 0.5mL ethanol and gently agitated the mixture to wash the crystals in an ice bath.

So, I know that putting in the warm bath under reflux until clear and letting it cool down is the recrystallization portion and I know why recrystallization for this is a good purification method. What I don't understand is why we had room temp ethanol wash before that and cold ethanol wash afterwards. Thank you in advance!

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Deanna Hettinger
Deanna HettingerLv2
31 Dec 2019

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Related questions

Calculate the mmol used for each of the reactants in the procedure

Hi, so I am unsure if this could be answered or not, but for my Organic Chemistry laboratory, I performed an aldol Condensation. The reaction used 0.25g of 2,4-dimethoxybenaldehyd and 0.20g of 1-indanone. I added 0.5g of NaOH. The reaction stood for 15 minutes and began to harden. I then added 2ml of HCL to the tested the basicity of the solution. I then vacuum filtrated the product and rinsed with HCl. Then I began the recrystallization of the product from 90% warm ethanol. The solution cooled and then was transferred to an ice bath. During this time, I was unable to obatain any crystals from my product. I am having trouble figuring out where I went wrong in my experiement to not get an crystals.

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Preparation of acetyl salicyclic acid (Aspirin) post-lab questions

1.)At the end of the reaction, you were asked to add ice-cold water to the reaction mixture. What do you think is the reason for this? Explain with chemical equations.

2.)How would the % yield of aspirin be changed if the final rinse of the solid product on the filter paper was done with warm water instead of ice-cold water?

3.)If you run a thin-layer chromatography for a mixture containing salicylic acid and aspirin using a 1:1 solvent mixture of ethyl acetate and hexane, which compound do you think would have more Rf value. Explain why? Bring in the structural features in your explanation.

summary Procedure of lab we did:

- Place approximately 0.300 g of salicylic acid in a tared empty 5 mL conical reaction vial. Record the actual weight of salicylic acid used. Using automatic pipet, add 0.8 mL of acetic anhydride (acetic anhydride vapors are very irritating-handle it carefully, in the hood). Add 2-3 drops of concentrated sulfuric acid using a dropper.

-Clamp the assembly so that the vial is partially submerged in the hot water. Turn on the magnetic stirrer. Once the water bath starts boiling, start timing the reaction. When the mixture has been allowed to react at 100 oC (the temperature of boiling water) for 15 minutes, you can consider the reaction to be complete. During the heating time, take about 20 mL of deionized water in a flask and chill it in an ice-water bath.

- After the reaction has gone on for 15 minutes, add very cautiously, drop by drop, 1 mL of deionized water to the reaction vial. At this point you should have a solution in your reaction vial. Remove the reaction vial form the hot water bath, add another 1-2 mL of DI water and let it stand at room temperature for few minutes. Then, detach the air condenser carefully and remove spin vane with forceps. Place the vial in a beaker and cool down to RT. During this time, the aspirin may begin crystallizing. If not, scratch the walls of the vial with a glass rod. Then, cool the vial in an ice-water bath.

- When the vial appears full of crystals, add about 3-4 mL of ice-cold DI water. Filter the aspirin using vacuum filtration (use Hirsch funnel). Leave the filtration setup in place for few minutes-this will draw air through the sample and will help dry aspirin.

Recrystallization of Aspirin:

- Transfer the rest of the crude aspirin to a 25 mL Erlenmeyer flask and do recrystallization using ethanol.

Purity of Aspirin:

- Using the melting point apparatus, determine the melting point of your crude aspirin and the recrystallized aspirin.

- To do FeCl3 test, get 4 test tubes. Place 1 mL of ethanol and 2 drops of 1% FeCl3 solution in each test tube. Add few crystals of salicylic acid to one test tube. Add few crystals of your crude aspirin product to the second test tube. In the thirst test tube, place a few crystals of your recrystallized aspirin. The fourth test tube will be your “blank”. Shake each of the test tubes and record your observations.

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