Catalytic and Solvophobic Promotion of High Pressure Addition Reactions

Catalytic and Solvophobic Promotion of High Pressure Addition Reactions 

That pressure is a fundamental physical parameter has been known since the origin of mankind. On a time scale however, high pressure chemistry is a relatively recent field, the first measurement of a reaction under pressure being reported in 1892 by Roentgen in the acid-catalyzed inversion of sucrose [1], Although high pressure techniques have now reached the maturity in many scientific fields, the application of high pressure for synthetic purposes has been confined almost exclusively within academic spheres. 

One of the main reasons is probably related to the small rate constant increase in the low pressure range (0-300 MPa) even for fairly pressure-dependent reactions such as pericyclic cycloadditions. The kinetic effect is derived from the relationship of Evans and Polanyi in the transition state theory as  

AV is the activation volume defined as the difference in partial molar volumes when the reaction progresses from initial to transition state. 

The estimation of the optimal pressure was previously discussed by taking into account the possible pressure dependence of [2] as well as the interrelation of pressure and temperature defined under isokinetic conditions [3], The relationship (Eq. (10.1)) underlines that the rate constant increases exponentially with pressure. The logarithmic behavior is illustrated in Fig. 10.1 which shows the variation of the rate constant ratio fep/ko with pressure at 25 °C. As an example, let us consider a pressure of 300 MPa which is usually an upper limit for large commercial pressure vessels. At that pressure the value of fep/fco approaches 10-40 for pressure- 

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In these cases the size of the activation volumes obviously depends not only on the effective packing probably caused by the restriction of vibrations and rotations in the transition state, but also on the transition-state polarization enhanced by the polar groups leading to a further decrease in volume which is not observed in the less polarized cycloadducts. Blake and Jorgensen [51] have assumed similar effects to explain the acceleration of Diels-Alder reactions in water. Catalytic and solvophobic promotion of high pressure addition reactions will be discussed in Chapter 11. 

I 70 Catalytic and Solvophobic Promotion of High Pressure Addition Reactions Tab. 10.15. Pressure dependence of the dielectric constant e (20 °C). 

J40 I 10 Catalytic and Solvophobic Promotion of High Pressure Addition Reactions... 

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