Acid-Base Extraction

Introduction Extraction involves dissolving a compound or compounds either from a solid into a solvent or from a solution into another solvent. A familiar example of the first case is making a cup of tea or coffee. The soluble flavour and odour chemicals, and caffeine are extracted from the solid tea leaves or ground coffee beans into hot water. Insoluble plant material is left behind in the tea bag or coffee filter. An example of the second case is an experiment that is often done in an introductory organic lab when an organic solvent is used to extract the caffeine from an aqueous tea or coffee solution, leaving the more water-soluble compounds behind in the aqueous solution. This is, in fact, how decaffeinated tea and coffee were industrially produced for many years. An acid-base extraction is a modification of case that is, a solvent-solvent extraction. Before looking at what makes it an acid-base extraction, first consider solvent-solvent extractions in general. A requirement of a solvent-solvent extraction is that the two solvents be mostly immiscible. That is, a mixture of the two solvents will separate into two layers of thin of oil and water. It is important to note that the upper layer is that which is less dense. For example, a mixture of tert-butyl methyl ether and water will separate into two layers, with the ether layer that has density = 0.74 g/mL being on top of the water layer that has density = 1.0 g/mL. Note that in almost every case, one of the solvents is water or some other aqueous solution. The other solvent is an organic solvent that is mostly immiscible with water. Examples include tert-butyl methyl ether, hexanes, and dichloromethane. The organic solvent must also be volatile and easily-evaporated so it can be easily removed by evaporation at the end. In addition, an acid-base extraction operates on the same principle but can provide a further level of fine tuning. If one or more of the compounds in the mixture to be separated is acidic or basic, the solubility of these acidic and basic components can be manipulated to the advantage by applying simple acid-base reactions. Using such a manipulation, an acidic or basic compound that may be ether soluble and water insoluble can be changed to be ether insoluble and water soluble by carrying out an acid-base reaction. The solubility of acidic and basic compounds can thus be changed at will. Once the layers are separated as before, the neutral acid or base can be regenerated by yet another acid-base reaction. Objectives 1. To separate a mixture of benzoic acid and a neutral compound into its components by extraction. 2. To determine the yields and melting points for both products. Procedure A. Extraction of NaOH 1. 0.24 g of Triphenylmethanol and 0.12 g benzoic acid were weighed and placed in a test tube. 10 ml ether was added to the mixture and the test tube was shook to dissolve the mixture. 2. The solution was transferred to the separatory funnel and 5 ml of 1M NaOH was added, tightly capped it and the layers were mixed by swirling and shaking gently for 30 seconds. 3. Pressure was vented by titling the funnel and opening the stop cork. The stop cork was closed, shook and vented again. 4. The mixture was let stood to allow the layers to separate. The top layer was ether layer and the bottom was the aqueous layer. 5. The aqueous layer was drained into an Erlenmeyer flask and labelled as first extraction of NaOH. 6. Another 5 ml of the 1M NaOH solution was added to the ether layer in the funnel and step 1 until 4 were repeated for the second extraction step. The aqueous solution was transferred to another Erlenmeyer flask and labelled as second extraction NaOH. B. Determination of Benzoic Acid 1. The flask was taken to the hood and slowly, with stirring using glass rod, 3.9 ml of 6M HCl was added into each Erlenmeyer flask and tested the acidity of solution using blue litmus paper. 2. The solution was cooled along with a few ml of water in a separate beaker in an ice bath for 15 minutes. 3. Solid benzoic acid was collected using a Buchner funnel and filter paper. The solid benzoic acid was washed with 2-3 ml of chilled water and suction was allowed to continue for five minutes to get dry solid. 4. The dry solid was gently scraped into a watch glass and the drying process was completed in the oven. 5. The solid was weighed and melting point was obtained. The percent recovery of benzoic acid was calculated. C. Determination of Unknown Compound 1. 5 ml of saturated solution of NaCl was added to the ether layer in the separatory funnel and capped, mixed, vented and let the layers separated as before. 2. The aqueous layer was discarded. The ether layer was transferred into an Erlenmeyer flask and 1 g of anhydrous Na2SO4 was added to the ether layer. The flask was closed, swirled and let stood for 10 minutes. 3. A small dry beaker was weighed and the dried ether layer was transferred to the beaker. 4. The ether layer was evaporated on a hot water bath, then the beaker was cooled at room temperature, dried and reweighed to determine the yield of unknown sample. 5. A small amount was scraped and melting point of the unknown compound was obtained. 6. The melting points obtained were compared and the percent recovery of the unknown neutral compound was calculated. Result Actual weight of benzoic acid Weight of benzoic acid recovered Percent recovery of benzoic acid = 0.12 g = 0.0751 g 0.0751 = 0.12 × 100 % = 62.58 % Observed melting point of benzoic acid = 139 – 141˚C Actual weight of unknown sample Weight of unknown sample recovered Percent recovery of unknown sample = 0.24 g = 0.1843 g 0.1843 = × 100 % 0.24 = 76.79 % Observed melting point of unknown sample = 162 – 169˚C The identity of unknown sample is Triphenylmethanol. Discussion Throughout the experiment, benzoic acid was removed from the solution by adding sodium hydroxide, NaOH. This compound reacted with the benzoic acid to form its conjugate base. The NaOH accepted a hydrogen from benzoic acid. When benzoic acid became its conjugate base, it favoured the aqueous layer which was created by the formation of water in the previous reaction over the ether layer. Thus, it moved to that layer, which was on the bottom because it is denser than the ether layer. This aqueous layer was then removed into a flask and once isolated, an acid, HCl, was added to interact with the conjugate base of benzoic acid. The HCl donated a hydrogen to the conjugate base to turn it back into benzoic acid. A base, NaCl, was then added to interact with the ether layer. The salt water works to pull the water from the organic layer to the water layer. This is because the concentrated salt solution wants to become more dilute and because salts have a stronger attraction to water that to organic solvents. Thus, the layers separated which the top was ether layer as the organic layer is less dense than the aqueous layer. This aqueous layer was then discarded. The final component was dried by sodium sulfate to absorb any water it contained and then evaporated the ether layer on the hot water bath. Furthermore, the theoretical melting point of benzoic acid is 123˚C and the melting point range of benzoic acid obtained was 139 – 141˚C. This cannot accurately confirm the identity of the compound because the melting point obtained is higher than the theoretical melting point. This means that the sample is not pure and there is an impurity present. Meanwhile, there was 62.58 % recovery for benzoic acid. This may be due to the loss of product on the filter during the drying process. The loss also may have occurred because the NaOH added did not fully react with the benzoic acid, resulting in some of the acid remaining in the aqueous layer. The melting point range of ether layer was 162 – 169˚C and it matched with the theoretical melting point of Triphenylmethanol which is 163˚C. This confirmed the identity of the unknown sample because the actual melting point is in the range of that sample which means the sample is most likely very pure. Meanwhile, the percent recovery of unknown sample is 76.79 %. This low recovery may be due to the loss of product during separation of aqueous layer after adding NaOH or NaCl. Conclusion In a nutshell, it was possible to separate a mixture of benzoic acid and neutral compound to its component by using the acid-base extraction. Each solid was extracted from the mixture during the process of utilization. Although the components of the mixture were separated, the percent recovery shows that there was some errors involved in the process. The melting point of the benzoic acid also did not match the theoretical melting point. The solid melted over a range of temperatures which means the products were not being pure compounds. Questions 1. List four water-immiscible liquids other than ether layer that could be used to extract organic compounds from aqueous solution. i. ii. iii. iv. Toluene Pentane Benzene Cyclohexane 2. Why is it wrong to leave a bottle of anhydrous sodium sulfate or calcium chloride open? Because the term anhydrous means it has no water so by leaving the bottle open will allow them to mix with the water from the atmosphere and become hydrated. 3. Draw the structure of the product when benzoic acid reacts with sodium hydroxide. Why is the product of this reaction easily extracted into H 2O while the original benzoic acid is not so easily extracted into H2O? This is because the more hydroxide ion, OH- that remove hydrogen, H+ to form a salt and the polar salt is soluble in aqueous solution where both OH- and benzoate are bases, the stronger base takes the H+ to form the weaker base. 4. Why does benzoic acid precipitate out when the aqueous layer is acidified with HCl? Because the benzoic ion is stronger bases than Cl-, so each one takes H+ from HCl which means that the acid forms are not soluble in water so it will precipitate out. 5. Develop a procedure for isolating a neutral compound from a mixture containing a basic impurity by drawing a flow chart as on lab manual page 20.