Enthalpy-controlled reaction

In general, cycloadditions catalyzed by Lewis acids proceed at significantly lower temperatures and with higher selectivities than their uncatalyzed counterparts. Factors that contribute to the increased selectivity of the catalyzed reactions include lower temperatures and more organized transition states. For enthalpy-controlled reactions, lowering temperatures increases selectivity (recall Section 1.4, equation 1.5). Coordination of a Lewis acid to the enone carbonyl not only activates the enone by electron withdrawal, it also restricts conformational motion and thereby reduces the number of competing transition states. Figure 6.12 illustrates several chiral auxiliaries for dienophile modification that have been used in the Diels-Alder reaction. 

Two extreme situations should be noted. If p =0, then 8AG = — 78A5, and the reaction series is entirely entropy controlled it is said to be isoenthalpic. If I/p = 0, then 8AG = 8A//, and the series is enthalpy controlled, or isoentropic. All of these relationships apply also to equilibria, but we will be concerned with kinetic quantities. 

Kodama and Brydon [631] identify the dehydroxylation of microcrystalline mica as a diffusion-controlled reaction. It is suggested that the large difference between the value of E (222 kJ mole-1) and the enthalpy of reaction (43 kJ mole-1) could arise from the production of an amorphous transition layer during reaction (though none was detected) or an energy barrier to the interaction of hydroxyl groups. Water vapour reduced the rate of water release from montmorillonite and from illite and... 

Sfi f-Test 6.13B Methanol is a clean-burning liquid fuel proposed as a replacement for gasoline. Suppose it could be produced by the controlled reaction of the oxygen in air with methane. Find the standard reaction enthalpy for the formation of I mol CHjOH(l) from methane and oxygen, given the following information ... 

The input and output terms of equation 1.5-1 may each have more than one contribution. The input of a species may be by convective (bulk) flow, by diffusion of some kind across the entry point(s), and by formation by chemical reaction(s) within the control volume. The output of a species may include consumption by reaction(s) within the control volume. There are also corresponding terms in the energy balance (e.g., generation or consumption of enthalpy by reaction), and in addition there is heat transfer (2), which does not involve material flow. The accumulation term on the right side of equation 1.5-1 is the net result of the inputs and outputs for steady-state operation, it is zero, and for unsteady-state operation, it is nonzero. 

For a continuous-flow reactor, such as a CSTR, the energy balance is an enthalpy (H) balance, if we neglect any differences in kinetic and potential energy of the flowing stream, and any shaft work between inlet and outlet. However, in comparison with a BR, the balance must include the input and output of H by the flowing stream, in addition to any heat transfer to or from the control volume, and generation or loss of enthalpy by reaction within the control volume. Then the energy (enthalpy) equation in words is... 

The effect of a runaway follows from Equation (3-4). It shows that a system is more hazardous if it has a high reaction rate, a large inventory, or a high enthalpy of reaction and/or decomposition. If one of these three parameters is reduced and controlled, q may be kept under control as indicated in Figure 3-5. 

The RC1 reactor system temperature control can be operated in three different modes isothermal (temperature of the reactor contents is constant), isoperibolic (temperature of the jacket is constant), or adiabatic (reactor contents temperature equals the jacket temperature). Critical operational parameters can then be evaluated under conditions comparable to those used in practice on a large scale, and relationships can be made relative to enthalpies of reaction, reaction rate constants, product purity, and physical properties. Such information is meaningful provided effective heat transfer exists. The heat generation rate, qr, resulting from the chemical reactions and/or physical characteristic changes of the reactor contents, is obtained from the transferred and accumulated heats as represented by Equation (3-17) ... 

Vapor venting simulations were performed. Several parameters were varied in the simulation, such as up to a tenfold increase in the reaction rate and a doubling of the enthalpy of reaction. Failures of the control system and of the operators were simulated as well. It was concluded that the system can vent successfully, and that the rate of decomposition is not sufficiently rapid to allow for significant self-acceleration. 

If we make the assumption that the reverse of reaction 15.5 is diffusion-controlled and assume that the activation enthalpy for the acyl radicals recombination is 8 kJ mol-1, the enthalpy of reaction 15.5 will be equal to (121 - 8) = 113 kJ mol-1. This conclusion helps us derive other useful data. Assuming that the thermal correction to 298.15 K is small and that the solvation enthalpies of the peroxide and the acyl radicals approximately cancel, we can accept that the enthalpy of reaction 15.5 in the gas phase is equal to 113 kJ mol-1 with an estimated uncertainty of, say, 15 kJ mol-1. Therefore, as the standard enthalpy of formation of gaseous PhC(0)00(0)CPh is available (-271.7 5.2 kJ mol-1 [59]), we can derive the standard enthalpy of formation of the acyl radical Af//°[PhC(0)0, g] -79 8 kJ mol-1. This value can finally be used, together with the standard enthalpy of formation of benzoic acid in the gas phase (-294.0 2.2 kJ mol-1 [59]), to obtain the O-H bond dissociation enthalpy in PhC(0)0H DH° [PhC(0)0-H] = 433 8 kJ mol-1. 

Three-membered ring-forming processes involving X-CH2-CH2-F or CH2-C(Y)-CH2F (X = CH2, O, or S and Y = O or S) in the gas phase have been treated by the ab initio MO method with a 6-31+G basis set." When electron correlation effects were considered, the activation (AG ) and reaction (AG°) free energies were lowered by about lOkcal mol indicating the importance of electron correlation in these reactions. The contribution of entropy of activation -TAS ) at 298 K to AG is very small the reactions are enthalpy controlled. 

The chemical process gives the enthalpy of reaction, the flow rate, the reaction time, and the required reaction temperature. The first step in the sizing procedure is to calculate the required number of channels for the heat exchanger. Then the pass arrangement is selected in order to achieve the highest possible Reynolds number within an acceptable pressure drop. For example, if the total number of channels is fixed by the residence time channels in series will induce high velocities and high pressure drop channels in parallel will induce low velocities and low pressure drop. The second step is to estimate the heat transfer coefficient and to check that the heat flux can effectively be controlled by the secondary fluid (the lower heat transfer coefficient should be on the reaction side). 

When you meet the new reactions awaiting you in the rest of the book you should reflect that each is controlled by an energy difference. If it is an equilibrium, A G° must be favourable, if a kinetically controlled reaction, AG must be favourable, and either of these could be dominated by enthalpy or entropy and could be modified by temperature control or by choice of solvent. 

In most chemical reactors, temperature is a critical variable that must be controlled. Cooling water circulates in the reactor jacket, removing the enthalpy of reaction. To control the reaction temperature, the cooling-water flow rate to the jacket is controlled. Set the desired tenqjerature on the tenqjerature-indicator-controller (TIC), which is measured by a temperature sensor installed in the reactor. The control valve automatically corrects any deviations from the desired temperature by adjusting the cooling-water flow rate into the jacket. 

Intramolecular nucleophilic substitution to form thiiranes was studied by means of ab initio MO computations based on the 6-31G basis set <1997JCC1773>. Systems studied included the anions SCH2CH2F and CH2C(=S)CH2F which would afford thiirane and 2-methylenethiirane, respectively (Equations Z and 3). It was important to include electron correlation which was done with the frozen-core approximation at the second-order Moller-Plesset perturbation level. Optimized structures were confirmed by means of vibrational frequency calculations. The main conclusions were that electron correlation is important in lowering AG and AG°, that the displacements are enthalpy controlled, and that reaction energies are strongly dependent on reactant stabilities. 

The overall rate resulting fi om the occurrence of more than a single process at the phase boundary may control the values of A and so that they are not necessarily directly proportional to the active area of the solid, or equal to the enthalpy of formation of the reactant, respectively. Moreover, when oxygen elimination is accompanied by defect generation, resulting in a change in composition of a non-stoichiometric phase, the enthalpy of reaction cannot in general be expected to be... 

For large samples of pure tungsten at temperatures >2200 °C (0.65 7] ), the stress exponent n = 5 and A// = 585kJ-mol . This activation enthalpy is similar to that of self-diffusion in tungsten. It was therefore assumed that under these conditions the deformation of the grains by dislocation climb or glide processes is the rate-controlling reaction [1.63]. 

---