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A Separation and purification Scheme Essay


There are millions of different organic compounds. Most of them are found in mixtures and in order to achieve a pure form they need to be separated, isolated, and purified. However, there are endless numbers of possible mixtures, which make it impossible to have a pre-designed procedure for every mixture. So chemists often have to make their own procedures. The purpose of this experiment was to prepare the student to the real world by them designing their own procedure which will help them understand the techniques of separation and purification better. The goal was to extract two of the components of the mixture that consisted of 50% benzoic acid, 40% benzoin, and 10% dibromobenzene.

The benzoic acid was extracted by the use of base and extraction technique. Then a crystallization was performed in order to separate benzoin by the use of hexane a solvent, leaving the dibromobenzne in the mother liquor. The percent recovery for benzoic acid was 91.8% and 80.3% for the benzoin. The MP for benzoic acid was 119-122°C (1) and 135-136°C (2)for the benzoin. The high percent recovery and closeness of measured MP to the literature value indicate that the designed procedure was successful.

Results and Discussion:

The initial sample consist of:
Table 1:
Benzoic acid (50%)| Benzoin (40%)| Dibromo-benzene (10%)| (3)| (4)| (5)|


1) Dissolve 1.0 g of the mixture in methylene chloride in a centrifugal tube 2) Set up the separatory funnel. 3) In order to extract the benzoic acid add 5.0 ml of the 1 mol NaOH solution, then shake the funnel 1 time per second and open it while it is upside down to allow the pressure out. 4) Allow the solution to settle.

5) The NaOH layer will extract the benzoic acid . 6) Perform a second extraction on methylene chloride repeat the steps from 2-6. 7) Add 2 ml of the HCl to the beaker that contain the NaOH solution the following reaction 8) Set up a vacuum filtration .

9) Filter the ppt formed as result of step 7 from the newly acidic solution. Then wash the filtered ppt with distilled water.

Allow the extracted benzoic acid to dry.

10) Weigh the extracted benzoic acid and measure the MP. 11) Dry out the methylene chloride solution that contain the benzoin and the dibromobenzene by the use of anhydrous sodium sulfate. 12) Pipet the remaining methylene chloride to a new beaker. Place the flask in hot water bath, and boil the methylene chloride out, leaving the benzoin and the dibromobenzene . 15) to separate the benzoin and the dibromobenzene, a crystallization should be performed 16) Heat some of the hexane to boiling and place the solid to be crystallized into an Erlenmeyer flask. 17) Pour a small amount of the hot solvent into the flask containing the solid. 18) Swirl the flask to dissolve the solid.

19) Place the flask on the steam bath to keep the solution warm. 20) If the solid is still not dissolved, add a tiny amount of hexane and swirl again. 21) After all of the solid dissolves, move the flask from the hot plate and allow it cool to room temperature. After a while, crystals should appear in the flask. 22) place the flask in an ice bath to finish the crystallization. 23) Set up a vacuum filtration

24) Separate the formed crystals (benzoin) and allow them to dry, then take the MP. 25) Optionally evaporate the mother liquor from pre weighted flask .weigh the impurities in the mother liquor and measure the MP. Table 3:

The impurities from the mother liquor the dibromobenzne+ some of the benzoin| 0.129| 129% Which indicate some of the benzoin remained in it| 89-96 °C| *Percent recovery= (retained Mass/ theoratical) x 100% = % recovery Data analysis:

The procedure success is determined by the percent recovery (the higher the percent is the more successful the procedure is as long it does not exceed the 100%), and the more similar the MP of the separated substances and the literature value the more successful procedure. The melting points of the benzoin and the benzoic acid obtained in this experiment are close to the literature value, which indicates that the procedure was successful. The percent recovery is relatively high, but some of that mixture was lost throughout the experiment and some of the benzoin remained trapped in the mother liquor


(1) CRC: Handbook of chemistry and physics, 85th edition. Veazey, W.R.; Charles,D.H.; CRC Press: Cleveland, 2004, 40-3. (2) The Merck Index: An Encyclopedia of Chemicals and Drugs, 12th edition. Siegmund, O.H.; Merck and Co: NJ,137,220. (3) Pubchem. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=243. (4) Chemical Land. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=243. (5) Chemspider. http://www.chemspider.com/Chemical-Structure.8093.html.

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