Experiment 9 Title Objective : Preparation of Dibenzalacetone : Preparation of dibenzalacetoneDate: 28 March 2006 Chemical Equation and Structural Formula of Organic Substances Involved : Cyclohexanol Molecular weight (gmol-1) Weight (g) Moles# Limiting reagent Theoretical yield 100 10 0.1 / Sodium dichromate 262 20 0.076 Sulphuric acid 98 17.31* 0.177 Cyclohexanone 98 9.8 g (0.1 mol)@ * Given density of Concentrated sulphuric acid (H2SO4) Mass of sulphuric acid = 1.84 g/mL = Volume × Density × Mass concentration = 9.6 mL × 1.84 g solution/mL × 98 g H2SO4/100 g solution = 17.31 g H2SO4 = 10 g / 100 gmol-1 = 0.1 mol = 20 g / 262 gmol-1 = 0.076 mol = 17.31 g / 98 gmol-1 = 0.177 mol 3 mol cyclohexanone 98 g cyclohexanone # Moles of cyclohexanone Moles of sodium dichromate Moles of sulphuric acid @ Theoretical yield = 0.1 mol cyclohexanol × 3 mol cyclohexanol × mol cyclohexanone = 9.8 g cyclohexanone Apparatus : Round-bottomed flask (50 mL, 250 mL), measuring cylinder (50 mL), beaker (100 mL, 250 mL), conical flask (50 mL, 250 mL), glass rod, dropper, spatula, cork stand, thermometer, thermometer adapter, Liebig condenser, distillation adapter, still distillation head, separatory funnel, funnel supporting ring, retort stand, retort clamps, electrothermal heater, hot plate, fluted filter paper, oven, glass funnel, stemless glass funnel, electronic weigh, glass vial Chemicals : Dehydrated sodium dichromate (20 g), concentrated sulphuric acid (9.6 mL), distilled water (100 mL + 80 mL), cyclohexanol (10.6 mL), common salt, ether (25 mL), anhydrous magnesium sulphate, ice, boiling chips Procedure : i. while the upper organic layer is drained into a conical flask and mixed with the organic layer from the first extraction. clean 50 mL round-bottomed flask. The funnel is closed using its stopper before being shaken gently. The stopper is opened and the stopcock tap is turned to a vertical position to drain the lower aqueous layer into a clean. The mixture is cooled to about 20°C in an ice bath. The still head. Concentrated sulphuric acid (9. Distillate produced in the range of 150°C – 156°C is collected into the vial. a hissing sound of gas released is heard. Liebig condenser and the exiting adapter are dismantled and their internal walls are washed with acetone before being put into an oven. the mixture is transferred into the round-bottomed flask using a glass funnel. Any distillate produced before 150°C is collected into an empty beaker. After the organic and aqueous layers have formed. Water is run through the condenser from the pipe with the inlet near the exiting adapter and the outlet near the side-arm of the still head. assembled for a second distillation.e.6 mL) is measured using the measuring cylinder and a dropper and mixed with the dichromate solution in portions and stirred. Heating is stopped and the heater is taken away immediately after 80 mL distillate is collected. The funnel is then inverted with finger over stopper and the stopcock tap is opened to release the pressure inside the stopper. A spatula of common salt is added to the distillate and stirred to dissolve it.Dehydrated sodium dichromate (Na2Cr2O7. The funnel is put back into the ring and left for a certain time period to let the organic and aqueous layers to form. The distillate–salt solution is poured into the separatory funnel with the stopcock tap closed. with the top of the thermometer bulb aligned with the bottom of the side-arm of the still head. An empty glass vial together with is cap is weighed and its mass is recorded. Three to four boiling chips are added into the flask. A 100 mL beaker is placed below the exiting adapter to collect the distillate. Meanwhile. The shaking and venting of the funnel is repeated a few times until the audible hiss is no longer heard. After the cooling time has elapsed. A spatula of anhydrous magnesium sulphate is poured into the flask and left for about 5 minutes. Distilled water (80 mL) is measured and added into the flask. The distilland (contents of the flask) is heated. During the process. which is shaken and vented a few times. The mixture is then filtered via gravity filtration using the fluted filter paper in a stemless glass funnel into an empty. The . The upper organic layer is drained into another empty 50 mL beaker. The distillation components are taken out of the oven and together with the flask. a piece of filter paper is folded into fluted form. The thermometer together with its adapter is inserted into the still head. Three to four pieces of boiling chips are also put into the flask. a 250 mL round-bottomed flask is clamped to a retort stand with an electrothermal heater beneath it. The mixture is allowed to cool for 45 minutes. The distilland is heated and the temperature is observed. The still distillation head is connected to a Liebig condenser and its exiting adapter. The aqueous layer is poured back into the funnel with the stopcock closed. The temperature of the mixture in the flask is ensured to be between 55°C and 60°C by either heating it using a hot plate or cooling it using the ice bath. the aqueous layer is drained off into an empty beaker. The distillation apparatus is then assembled and set up. Meanwhile. Ether (25 mL) is measured and poured into the funnel. the temperature that reaches to a point where it stops increasing within the 150°C – 156°C range is taken as the boiling point temperature. Distilled water (100 mL) is measured using a graduated measuring cylinder and poured into the beaker containing the dichromate and stirred using a glass rod to dissolve it thoroughly. The chromic acid mixture is poured into the flask in portions and swirled. The vial is capped as soon as possible after the temperature increases past 156°C and the electrothermal heater is removed. which are clamped to another retort stand at a lower height.6 mL) is measured into a 250 mL conical flask. The separatory funnel is suspended using a supporting ring clamped to a retort stand. Cyclohexanol (10. empty 50 mL beaker.2H2O) (20 g) is weighed using an electronic weigh and a beaker. 24 – 16.e.20 g = 20. etc.0°C (reference: 150.04 g : Colourless liquid : 155°C Distillation of crude cyclohexanone gave pure cyclohexanone (4.24 g = (20.0°C).20) g = 4.vial is weighed to obtain the net mass of the distillate and labelled with details of the experimental results. . net mass.0 – 156. i. Chromic acid mixed with cyclohexanol produces a blackish-green solution.22 %) as colourless liquid with boiling point of 155. Observation : Dehydrated sodium dichromate when dissolved in water and sulphuric acid produces an orange solution (chromic acid). 41. boiling point.04 g. Mass of empty vial + cap Mass of empty vial + cap + cyclohexanone Net mass of cyclohexanone Characteristics of cyclohexanone Experimental boiling point = 16. written on it. Step 4: Removal of proton from α-carbon of β-hydroxyketone by base. does not have any hydrogen atom in the α-position and thus cannot give simple aldol condensation. forming an enolate ion. since it is dibenzalacetone (IUPAC name: 1. Step 8: Protonation of alkoxide by ethanol (CH3CH2OH). β-unsaturated ketone and a separate water molecule. aldol condensation is the condensation of two molecules of the same aldehyde or ketone. Step 10: Elimination of hydroxide produces dibenzalacetone. The hydroxide ion of the base extracts the α-hydrogen from acetone and resonance stabilizes the carbanion intermediate before reacting further with benzaldehyde as a strong nucleophile. However. In cases where acetone is excessive. In the presence of base. an aliphatic ketone is used. the mono-condensation product. since benzaldehyde. benzaldehyde is used in excess instead. as in this case. Step 7: Enolate ion adds to carbonyl group in benzaldehyde. Step 2: Enolate ion adds to carbonyl group in benzaldehyde. The mechanism continues after formation of benzalacetone. This crossed aldol condensation is called Claisen–Schmidt condensation. Step 5: Elimination of hydroxide produces benzalacetone. forming an alkoxide. is obtained. In this experiment. The mechanism of the aldol condensation between benzaldehyde and acetone is depicted below: Step 1: Base (NaOH) removes proton (H+) from α-carbon of acetone. Step 6: Base (NaOH) removes proton (H+) from α-carbon of benzalacetone. forming an alkoxide.Discussion : This experiment incorporates the use of aldol condensation reaction.5-diphenyl-1. Aldol condensations are usually catalyzed by acids or bases but basic catalysts are generally preferred. resulting in the formation of a new carbon–carbon bond (C–C) joining the carbonyl carbon of one molecule to the α-positioned carbon of the other molecule. Step 3: Protonation of alkoxide by ethanol (CH3CH2OH) produces β-hydroxyketone.4pentadien-3-one) that is wanted in this experiment. benzaldehyde is capable of participating in mixed (crossed) aldol condensation with aliphatic aldehydes or ketones that can form carbanions. acetone. so that 2 moles of benzaldehyde react with 1 mole of acetone to produce 1 mole of dibenzalacetone. an aromatic aldehyde. thus the resonance-stability. the β-hydroxyketone produced initially undergoes dehydration spontaneously to form the resonance-stabilized α. forming a resonance-stabilized enolate. For this experiment. Generally defined in the simplest way. However. Step 9: Removal of proton from α-carbon by base. . benzalacetone. aldol condensation is complicated by the possibility of mono or disubtitution. forming a resonancestabilized enolate. The unsaturated ketone has a double bond conjugated to both the aromatic ring and the carbonyl group. one of those effective catalysts being ethanolic sodium hydroxide (CH 3CH2OH + NaOH). The reaction requires the molecule to possess a reactive carbonyl group and able to lose a proton (or hydrogen) from the αposition to form an enolate ion or carbanion. forming an enolate ion. As acetone has two methylene groups. Furthermore.During the preparation process. a water condenser is set up to cool down most of the ethanol vapour back to liquid form. Heating of the crystals is then initiated as the temperature reading of the apparatus is being observed. By folding the paper into fluted form. For that purpose. to allow the mixture to cool and more crystals to form. which is later passed through a long glass tube so that the crystals are collected at the bottom of the closed end of the capillary tube when it hits onto the surface below the long tube repeatedly. the crystal – liquid mixture is passed through the Hirsch funnel which is connected to the aspirator. the rapidity of filtration is increased as almost the entire surface area of the filter paper is being used. It also clogs up any possible tiny pores in the filter paper so that the results of filtration are maximized. Under cooler conditions. Any leftover crystals in the conical flask are washed with distilled water to facilitate its flow into the Hirsch funnel and increase the yield. Addition of boiling chips into the round-bottom flask before the heating process is to minimize the explosiveness of sudden boiling should the ethanol is heated to above its boiling point. Having allowed the crystals to be sucked dry for about 10 minutes. distilled water is able to dissolve any impurities without affecting the product. The internal walls of the conical flask are scratched using the glass rod to hasten crystallization. Filtration of the mixture while still hot also serves to remove any insoluble solid impurities while the mixture is still in liquid form. together with its specific filter paper. thus increasing the efficiency of filtration. While sucking the crystals dry using vacuum filtration. The capillary tube is then inserted into the melting point apparatus and observed from the magnifying lens of the apparatus. Insoluble impurities are excluded from the growing crystals. cool ethanol is used to rinse any leftover crystals off from inside the beaker’s walls and also the funnel to maximize the yield. The mixture initially turns pale yellow. Scratching however produced pale yellow crystals. thus preventing any loss of material. the filtrate is allowed to cool slowly at room temperature. Furthermore. a small portion of the crystals is grinded into powdery form using a capillary tube and about 2 mm height of crystals is collected inside the tube. Using a small conical flask prevents any of the reagents or catalysts from escaping in vapour form. Filtering the mixture immediately after dissolving of crude dibenzalacetone by gravity through a preheated stemless glass funnel with fluted filter paper inside into a beaker hinders premature crystallization of dibenzalacetone in the filter paper or funnel. This is done to ensure that hardly any dissolved dibenzalacetone in ethanol escapes in vapour form and also to maintain the temperature throughout the heating process. dibenzalacetone is no longer soluble in ethanol and crystallizes. After filtration. The temperature when the crystals are noticed to start to melt or the height of the collected crystals starts to drop suddenly is being recorded as Ti and the temperature . to suck dry the crystals via vacuum filtration. The hot mixture should be poured onto the upper portion of the filter paper as the larger surface area at this portion enables the mixture to be filtered more rapidly. ethanol evaporates easily during the heating process and therefore. It is only after about 15 minutes when the mixture turns red. the crystals are transferred into a round-bottomed flask for recrystallization. In the process of determining the melting point. Crude organic compounds in solid state at room temperature are purified via recrystallization whereby the compound is dissolved in a hot organic solvent. it is chemically inert towards dibenzalacetone. Using a stemless glass funnel is to allow the product come out in liquid form so that the product will not crystallize prematurely and block the flow of the filtrate. ethanol is used as it dissolves most polar and nonpolar substances. an electrothermal heater is used to heat the mixture instead of the Bunsen burner to avoid fire accidents. The mixture is left undisturbed for about a further 10 minutes. all four reagents and catalytic chemicals were mixed in a 25 mL conical flask. Having done this. As ethanol is a volatile solvent with a boiling point of 78°C. In addition. thus producing by-products. Kulaweic. the percentage yield of the crystalline product was found to be 34. Thus. The same occurs when transference from the funnel to the round-bottomed flask and from the watch glass to the vial. “Organic Chemistry Laboratory Manual” 2nd ed. it can be concluded that the product is sufficiently pure but not 100% pure. The acetone might have not reacted completely with the ethanolic sodium hydroxide catalyst thus the low number of carbanions present to attack benzaldehyde. Ti – Tf marks the range of the melting point of the dibenzalacetone crystals. The carbanion intermediate produced could have attacked other acetone molecules instead of benzaldehyde. Crystallization of the product might have been incomplete and that some of the product could have been left behind in liquid form. shiny flakes. If one wants to obtain the maximum results.88%) with a melting point of 105. 34. as according to Donald L. it is advised that the collection process be repeated as a second crop of crystals might be obtained. A small amount of the product had been left in the conical flask when transference to the Hirsch funnel.88 % Such a low percentage of yield could be due to several reasons listed below: a.35 g ×100% 3.6°C in the form of pale yellow. Robertson’s “Aldol Condensation: Preparation of Dibenzalacetone (1.3°C – 107.when the crystals have completely transformed into liquid form is recorded as Tf.88 %. crude dibenzalacetone should be transferred from the Hirsch funnel directly into the round-bottom flask used for heating. The reference melting point temperature. several steps of precaution need to be taken. the pure crystals need to be ensured to be dry enough and free of impurities so that the actual net mass of pure dibenzalacetone and a satisfactory melting point range can be obtained. as shown in the calculations below: Percentage yield = = Actual yield ×100% Theoretical yield 1. The product is not exactly 100% pure. Robert J. after vacuum filtration. due to the possibility of impurities in the product and also errors in techniques used during the experiment. To minimize the above errors. References : Svoronos. Paris. . Other than that. 243 – 245. c. United States of America: McGraw-Hill Company. d.35 g. b. In the process of determining the melting point.3°C – 107.6°C. Sarlo. (1997). During gravity filtration.87 g = 34.4-pentadien-3-one)” (2006). the range of the melting point obtained from the experiment was found to be 105. is 107°C.5Diphenyl-1. the mixture to be filtered is ensured to be still hot to avoid any premature crystallization on the fluted filter paper. Edward. Conclusion : Crossed Claisen–Schmidt aldol condensation between excessive benzaldehyde and acetone and dehydration of the aldol produced produces dibenzalacetone (1. which is located in that range.. First of all. Upon completion of the experiment. .. (2006). “The Organic Chem Lab Survival Manual: A Student’s Guide to Techniques” 3rd ed. Inc. Inc. from http://www..miracosta. United States of America: John Wiley & Sons. 121. United States of America. .. McBride. Zubrick.Wade. 2006. Mary F. Katherine. “Synthesis of Dibenzalacetone by the Aldol Condensation”.5Diphenyl-1.edu/home/dlr /211exp5. Wilcox. Charles F. 2006. United States of America: Prentice Hall.G. Wilcox. 397 – 399.htm. Robertson. 1056 – 1057. L.... “Aldol Condensation: Preparation of Dibenzalacetone (1. Jr.htm. “Experimental Organic Chemistry: A Small-Scale Approach” 2nd ed.edu/students/m/mcbride/organic%20lab%208. Retrieved April 2. Jr.4-pentadien-3-one)”. Donald L.susqu. James W. Pearson Education. (2005). (1995). (2005). (1992). “Organic Chemistry” 6th ed. Retrieved April 2. from http://www. 1060..