experiment 2

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CHEM 324 Organic Chemistry III Experiment 2 Kinetic vs Thermodynamic Control in Competing Reactions Jiani He 26380107 Performed on Thursday, September 14 th and Thursday, September 28 th , 2023. Submitted on Thursday October 5 th 2023

Introduction In experiment 2, a series of condensation reactions were performed under various conditions in order to determine which product is under thermodynamic control and which is under kinetic control. The condensation reactions performed with semicarbazide and the ketone cyclohexanone forms cyclohexanone semicarbazone and the aldehyde furfural forms furfural semicarbazone as seen in Figures 1 and 2 respectively. Thus, in the first week, Parts A and B were performed in order to yield products A1, A2, B1 and B2. In the second week, Part C was performed to yield products C1 and C2 as well as take the melting points of each product and mixtures of products described in the results section. In Part A, cyclohexanone semicarbazide (A1) and furfural semicarbazide (A2) were formed as can be seen in Figure 1 and 2 respectively. In Part B, both cyclohexanone and furfural were mixed with semicarbazide under different conditions. Part B1 was performed under normal conditions, while part B2 was performed under high temperature and long reaction time (1.5 hours). As such, B1 should form cyclohexanone semicarbazone under kinetically controlled conditions while B2 should form furfural semicarbazone under thermodynamically controlled conditions. In addition, in Part B, when equivalent amounts of semicarbazide, cyclohexanone and furfural are mixed together, according to a study, under these conditions, the final product is majorly furfural semicarbazone when product is studied after a few hours of the start of the reaction while the final product is majorly cyclohexanone semicarbazide when it is studied after a few minutes from the start of the reaction (Conant, 1932). In Part C, the products from Part A were used in conjunction with either cyclohexanone or furfural. C1 was obtained by mixing A1 (cyclohexanone semicarbazide) with furfural for 1.5 hours to attempt to obtain furfural semicarbazone and cyclohexanone while C2 was obtained by mixing A2 (furfural semicarbazone) with cyclohexanone for 1 hour to attempt to obtain cyclohexanone semicarbazone and furfural.

Figure 1 - Reaction performed in Part A1. Figure 2 - Reaction Performed in Part A2. Figure 3 - The forward reaction preceded in Part C2 while the reverse in Part C1. The laboratory techniques used throughout the experiment were water baths in order to heat the reactants evenly, vacuum filtration in order to isolate the product that was recrystallized, recrystallization in order to undissolve the product from the solution, and melting point temperature in order to elucidate if what was made corresponds to the theoretical melting point as well as identify the product and determine if it is controlled kinetically or thermodynamically.

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Experimental Data Melting Points (°C) Observations on melting points Colour observations at melting point A1 166.5-168.9 a bit higher than cyclohexanone semicarbazone clear A2 N/A N/A black/brown B1 168.2-172.3 higher yet close to cyclohexanone semicarbazone yellow B2 147.6-150.8 A bit lower than both cyclohexanone semicarbazone and furfural semicarbazone black/brown C1 200.3-204.5 higher than both cyclohexanone semicarbazone and furfural semicarbazone clear C2 203.5-207.8 higher than cyclohexanone semicarbazone, yet close to furfural semicarbazone brown A1B1 161.5-170.5 A bit lower than B1 yellow A2B1 153.1-158.8 lower than B1 black/brown A1B2 149.2 around same as B2 (a bit lower) black/brown A2B2 136.4 lower than B2 black/brown B1B2 146.8-148 Lower than B1 and B2 B1C1 129.3 lower than B1 and C1 clear B2C1 139.2 lower than C1 and B2 clear B1C2 170.3-172.8 lower than B1 and C2 clear B2C2 181.3-201.3 a bit higher than B2 and lower than C2 brown Figure 4 - Observations of the colour of the mixtures and products at the melting point. colour of solution colour of precipitate colour of powder colour of powder at melting point A 1 NA NA NA clear A 2 yellow iridescent yellow/white iridescent white black/brown B1 NA NA white yellow B2 orange/brown tint yellow NA orange-brown black/brown C1 yellow yellow iridescent gold clear C2 yellow yellow sparkly yellow (like A2) brown Figure 5 - Observations of the colour of the product at various stages of the reactions. Results and Calculations Determination of the limiting reagent: Since the same number of moles of cyclohexanone and semicarbazide were used, this means that the limiting reagent is either of the reactants. Since the ratio of reagents to products is 1:1, the number of moles of reagents is equal to the number of moles of product. In

this case, the number of moles in A1 is 50mmol. Therefore, the theoretical yield of product A1 depends on the number of moles of the reagents. Determination of the theoretical yield of A1 (cyclohexanone semicarbazone): 50.00 mmol reagent = 50.00 mmol A 1 ∗ mol 1000 mmol ∗ 155.2 g mol = 7.76 gof cyclohexanone semicarbazone Determination of the %yield of A1 (cyclohexanone semicarbazone): %yield = Experimentalyield ( g ) Theoreticalyield ( g ) ∗ 100 = 4.436 7.76 ∗ 100 = 57.51% Likewise, for A2 (furfural semicarbazone), the theoretical yield is determined by performing the same calculation except multiplying by furfural semicarbazone’s molecular weight (153.2g/mol) rather than cyclohexanone semicarbazone’s molecular weight (155.2). Results can be seen in Figure 6. identity Actual yield (g) Theoretical yield (g) %yield A1 Cyclohexanone semicarbazone 4.436 7.76 57.51 A2 Furfural semicarbazone 0.577 7.66 7.53 B1 Cyclohexanone semicarbazone 0.538 1.552 34.66 B2 Furfural semicarbazone 1.136 1.532 74.15 C1 Furfural semicarbazone 2.428 1.532 158.4 C2 Furfural semicarbazone 0.951 1.532 62.08 Figure 6 - The Percent yield of the products formed in the kinetically controlled and thermodynamically controlled reactions of A1 and A2 respectively. Discussion As the Hammond-Postulate says, the structure the transition state most resembles in energy is the most stable structure (ChemLibretexts, 2015). As such, under kinetic control, the transition state resembles the starting material the most while under thermodynamic control the transition state resembles the product’s structure the most. This means, that although the kinetic product is formed quicker than the thermodynamic product it is not the most stable

product. The product formed under thermodynamic conditions is the most stable. Thus, in Part A, both reactions from figures 1 and 2 were performed separately under equal conditions in order to obtain a pure product. This was achieved as evidence from Figure 4 shows that the experimental melting point of cyclohexanone semicarbazone (166.5-168.9°C) is quite close to the theoretical melting point (166°C) and that of furfural semicarbazone which the melting point couldn’t be determined because of the texture. Also, the low theoretical yield of A2 shows that the reaction required more time to produce more product or required heating in order to increase the rate of reaction. In Part B, both the ketone and the aldehyde were mixed in order to react in the same solution under equal conditions in order to see which product would be favored under which conditions. In Part B1, the cyclohexanone semicarbazide was favored because there was no heating and no waiting, meaning the reaction occurred under kinetic control. Furthermore, the melting point of B1 (168.2-172.3°C) shows that, although it is higher than cyclohexanone semicarbazone’s melting point (167.2°C) it is closer to the latter rather than to furfural semicarbazone’s melting point (196.8°C). For B2 (147.6-150.8°C) whose melting point is closer in proximity to furfural semicarbazone’s and cyclohexanone semicarbazone’s melting point. In Part C, the interconversion of cyclohexanone semicarbazone and furfural semicarbazone was looked at. In C1, the starting material consisted of A1 and furfural under thermodynamic conditions (high temperature and long reaction time). According to the melting point, the product obtained was furfural semicarbazone and as such an interconversion did happen. In Part C2, product A2 and cyclohexanone were reacted at high temperature but for an hour. Theoretically, the product shouldn’t turn into cyclohexanone semicarbazone because the most stable product is already formed. Instead, more furfural semicarbazone should form. According to the melting point of C2, there was an increase of melting point from A2 which indicates that more furfural semicarbazone was formed. In other words, in Parts C1 and C2, the formation of the most stable product took precedent under thermodynamic conditions. The melting points of the mixtures show the following. The mixture of A1 and B1 shows a similar melting to those of A1 and B1 separately. This means that cyclohexanone

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semicarbazone is the identity of both products. The mixture of A2B1 shows that it is lower than B1 This means, that there was more cyclohexanone semicarbazone present than there was furfural semicarbazone (A2). Therefore, B1 most probably constituted of cyclohexanone semicarbazone with traces of furfural semicarbazone. The melting point of A1B2 is lower than that of A1 and slightly lower than that of B2. As such, since A1 is cyclohexanone semicarbazone, B2 is definitely and most probably furfural semicarbazone due to its proximity to furfural semicarbazone’s theoretical melting point. The melting point of A2B2 is lower than B2 since the melting point of A2 (furfural semicarbazone) cannot be determined. As such, B2 formed mostly furfural semicarbazone with trace amounts of cyclohexanone semicarbazone that might have contributed to the low mixture’s melting point. The melting point of B1C1 is lowerer than both B1 (cyclohexanone semicarbazone) and C1 meaning C1 is maybe cyclohexanone semicarbazone. But turns out the correct product is furfural semicarbazone. The possible reason that we obtained the lower melting point is the mixture of impurities, due to the fact of low melting point of impurities, we may have the melting point lower than both B1 and C1. When looking at B2C2’s melting point, it is observed that it is lower B2’s (furfural semicarbazone) and C1’s. This gives more evidence to the fact that C1 is furfural semicarbazone. As seen with B2B2, the melting point of B1C2 is lower than both B1 (cyclohexanone semicarbazone) and C2 separately. This probably means C2 is very pure furfural semicarbazone. When looking at B2C2’s melting point, it is higher than that of B2 and of C2 separately. This gives further support that C2 is furfural semicarbazone. Conclusion Thus, under kinetic control, short reaction time and room temperature, cyclohexanone semicarbazone was formed. On the contrary, under thermodynamic control, long reaction time and high temperature, furfural semicarbazone was formed. As expected, Part A yielded the desired products because they were performed seperately. Also, more A1 was formed than A2 because A1’s transition state resembles the starting materials’ structure more than A2’s transition state. In other words, since no additional energy was given, the kinetic product was

favored because less energy was required to make it while the thermodynamic product required more help. In Part B, the kinetically controlled reaction produced mostly cyclohexanone semicarbazone with traces of furfural semicarbazone in B1 while the thermodynamically controlled reaction yielded furfural semicarbazone with traces of cyclohexanone semicarbazone in B2. In Part C, under the same thermal conditions and same long reaction times, furfural semicarbazone was formed in both C1 and C2 because it is the most stable product and the energy required to allow the transition state to reach the least stable conformation was available. References  Conant, J.B.; Bartlett, P.D.; 1932. A quantitative Study of Semicarbazone Formation. https://pubs.acs.org/doi/pdf/10.1021/ja01346a030 , p.18. (accessed Oct 02, 2023).  Chemistry 324 Laboratory Manual Organic III: Organic Reactions ; Concordia University: Montreal, QC, August 2023; pp. 19-25.  ChemLibretexts. 2015. Hammond’s Postulate. https://chem.libretexts.org/Reference/Organic_Chemistry_Glossary/Hammond %E2%80%99s_Postulate (accessed Oct 02, 2023). Questions 1) Explain why product B in Figure 3(i) is neither kinetically nor thermodynamically controlled. Product B, on page 22 of the Lab manual, in Figure 3(i) is neither under kinetic or thermodynamic control because it does not fit the criteria needed. In other words, it does not have the smallest relative free energy of reaction (ΔG B >ΔG A ) in order to be thermodynamically controlled nor the smallest transition free energy (ΔG B ‡ > ΔG A ‡ ) in order to be kinetically controlled. 2) For the experiments in Part B and C, which product is kinetically controlled, and which is thermodynamically controlled? For Part B, process B1 was not heated while process B2 was for one and a half hours. This leads to believe that B1 is the kinetically controlled while B2 is the thermodynamically controlled products.

For Part C, C2 was heated for 1 hour instead of an hour and a half like C1 so this leads to believe that C2 is the kinetically controlled product while C1 is the thermodynamically controlled product. 3) What effect would the changes below have on the formation of semicarbazones in the competitive reactions (see Part B)? Would they affect the interpretation of the data? a. You used 2.00g of semicarbazide hydrochloride instead of 1.00g With 2.00g of semicarbazide instead of 1.00, there would need to be more sodium bicarbonate that would neutralize the Cl. Without it, the reaction that would be preferred to occur would not to its full potential. b. You doubled the volumes of the aldehyde and the ketone. The semicarbazide would become the limiting reagent due to the readily available ketone and aldehyde. 4) Draw an energy diagram for the competing reactions studied

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