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How to use temperature manipulation reaction selectivity? Check out these amazing experimental results!
In chemical reactions, the difference between thermodynamic control and kinetic control determines the composition of the product combination.When competition pathways lead to different products, reaction conditions affect selectivity and stereoselectivity.This difference is particularly important especially when the generation speed of reaction A is faster than that of reaction B, because the activation energy of generating A is lower than that of producing B, but B is a more stable product.Therefore, A is a dynamic product suitable for kinetic control, while B is a thermodynamic product suitable for thermodynamic control.The conditions of the reaction, such as temperature, pressure or solvent, will determine which reaction pathway is more advantageous.
"The choice of kinetic control or thermodynamic control will affect the composition of the final product, as these competitive reaction paths lead to different results."
In some cases, the advantage of thermodynamic control depends on the equilibrium state of the reaction.In particular, when the kinetic products form faster during the reaction time, the initial progress of the reaction may cause imbalance in the formation of these reactions due to rapid reactions.Thermodynamic control requires that the reaction can reach an equilibrium state before it can produce thermodynamic stable products.
Application in Diels-Alder reaction
The Diels-Alder reaction is a classic example of a chemical reaction that can produce two isomer products under different conditions.At room temperature, kinetic reaction control predominates, and the more unstable endo isomers become the main reaction product.However, when the temperature rises to 81 °C and lasts for a longer period of time, chemical equilibrium can tend toward a thermodynamically more stable exo isomer 1.
"At low temperature, the reaction mainly produces an expanded [4+2] cyclized adduct, while at high temperatures, the formation of chain adducts is observed."
In addition, a complete example of kinetic and thermodynamic reaction control was achieved in the combined Diels-Alder reaction of bisaryldiene with hexafluoro-2-butyne or dimethylacetylene dicarboxylate discovered in 2018.The reaction at low temperatures showed chemical selectivity and completely formed pincer-[4+2] cycloadduct, but dominoadduct was specially formed at high temperatures.These results not only demonstrate the critical role of temperature, but also reveal the complexity of different pathways competing with different stability.
Effects in ketone and enol chemistry
In the process of enol negative ion protons, the kinetic product is enol, while the thermodynamic product is ketone or aldehyde.In the deprotonation of asymmetric ketones, the kinetic product is the enol from the most proton-detonated α-H, while the thermodynamic product has a more highly substituted enol group.When low temperature and large structure alkalis are used, kinetic selectivity will be increased.This process reflects how to control the product using reaction conditions.
"Thermodynamic control is still a possible strategy even if the reaction is in a kinetic control state."
In electroaffinity addition reaction, temperature also affects product selection.When the reaction of hydrobromic acid and 1,3-butadiene rise above room temperature, most of them produce the thermodynamic more stable 1,4 adduct 1-bromo-2-butene, but if the temperature is lowered below room temperature, the kinetic 1,2 adduct 3-bromo-1-butene is preferred.
Characteristics of Reaction Control
In principle, each reaction is a continuum between pure kinetic control and pure thermodynamic control.These terms depend on a specific temperature and time range.When the temperature is low and the reaction time is short, the reaction is close to pure kinetic control.For a long enough time frame, each reaction ends up approaching pure thermodynamic control.In short, the primary product of the reaction is the easiest to generate, and thermodynamic control requires that the reaction achieves a balance between reversibility and product.In kinetic reaction control, one or two forward reactions lead to a rate of possible products significantly faster than the rate of chemical equilibrium.
"The characteristics of the reaction reflect the profound effect of temperature and time on selectivity."
Finally, it can be said that using temperature manipulation reaction selectivity is a powerful chemical tool.By adjusting the reaction conditions, scientists can not only affect the rate of reaction, but also manipulate the generated products.As our understanding of these processes deepens, what new discoveries may be led to in future experiments?
