In most conversions where two (or more) different products form, the predominant

Question

In most conversions where two (or more) different products form, the predominant product is the kinetic and thermodynamic product. Sometimes, though, the kinetic product differs from the thermodynamic product. Show the reaction energy diagram of two competing reactions, A ? B and A ? C, for which the kinetic product (C) is different from the thermodynamic product (B). Both reactions follow a one-step mechanism. How would you run the reaction to obtain predominantly kinetic product? Under which conditions would you obtain mostly thermodynamic product? Please explain.

Explanation / Answer

follow this The purpose of this experiment is to test kinetic theory by running several reactions and determine which product are formed under kinetic conditions and which products are formed under thermodynamic conditions. Kinetically controlled products are those which have the lowest transition states between competitive reactions; this would be represented by TS1 in the above diagram. At low temperatures, this makes it easier for the reaction to overcome the activation barrier ensuring that the kinetic product is formed faster. Thermodynamically controlled products are those which have a more stable product (P2) due to the product energy level being lowest between competitive reactions. Therefore at high temperatures, the thermodynamic product is dominant because it is possible to reach the higher transition state. It is in essence easier to reach the activation energy barrier. In Part A of the experiment, two reactions are carried out to synthesize semicarbazones to be used in Part C of the experiment. The first reaction being the preparation of cyclohexanone semicarbazone; The second being the preparation of furfural semicarbazone; These reactions will be further analyzed under thermo conditions and kinetic conditions in part B of this experiment. To test this theory experimentally, in Part B of the experiment, two competing reactions are carried out; Cyclohexanone, furfural and semicarbazide are reacted, initially without heat and then again this reaction is carried out with heat. The significance of this process is to determine which product is formed under kinetic conditions (forms faster at low temperatures) and which product is formed under thermodynamic conditions (forms slower but is the major product at high temperatures). Experimental The purpose of Part A of the experiment is to furfural and cyclohexanone semicarbazones for part C of the experiment. 50 mmol of semicarbazide hydrochloride along with 10.0g of sodium bicarbonate are mixed in 125 ml of deionized water in an Erlenmeyer flask. Once effervescence ceases, 50 mmol of cyclohexanone is added and swirled in the flask. Once a precipitate forms, the mixture is then vacuum filtered and the product isolated. The product is then recrystallized from deionized water. The melting and yield is then recorded This procedure is then repeated for preparation of the furfural semicarbazone (50 mmol of furfuraldehyde is used instead of 50 mmol of cyclohexanone). Part B of the experiment is to truly demonstrate the fundamental theory behind competitive reactions and thermo vs kinetic control. 10 mmol of semicarbazide hydrochloride is mixed with 2.0g of sodium bicarbonate and 25 ml of deionized water in a 125 ml Erlenmeyer flask. Once effervescence has ceased, 10 mmol cyclohexanone and 10 mmol of furfuraldehyde are added to the solution. Once all reactants are properly mixed and a precipitate forms, the mixture is then vacuum filtered to isolate a product. This product is the recrystallized from deionized water. The product is then verified by measuring the melting point and mixed melting point. This procedure is then repeated under heating for about 1 hour. Part C of the experiment is to verify kinetic vs thermo control of part B through interconversion reactions. 10 mmol of cyclohexanone semicarbazone (obtained from Part A) is mixed with 10 mmol of furfuraldehyde in 20 ml of deionized water in a 125 ml Erlenmeyer flask. This mixture is placed in a heat bath for roughly 1 hour. The mixture is then cooled to room temperature and the precipitate is then vacuum filtered and recrystallized from deionized water. The product is then identified using melting point and mixed melting point.

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