Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 APPARATUS 17parent molecule or the reactant concerned, although other molecules sometimesachieve this efficiency.Pressure effects also occur in adiabatic reactions, in particular for very exothermicreactions. The rate coefficient for the unimolecular decomposition of di-tert.-butyl peroxide (dtBP) increases with the pressure in a 500 mI spherical reactionvessel6'. Batt and Benson have shown that this increase can be directly associatedwith the production of a thermal gradient'. A similar increase in the rate coefficientoccurs in the case of di-tert.-amyl peroxide6' but here it may be difficult to separatethe exothermic effect from that due to sensitised decomposition. The thermal gradientmay be reduced or removed by the addition of diluents or by the use of anoctopus-shaped RV (see p. 12). The diluent may be an inert transfer agent or maybe taking part in the reaction. Inert transfer agents play a dual role in oxidationsystems, in particular the H2+02 system6'. At low pressures the global ratecoefficient is decreased by the addition of foreign gases due to enhanced gasphase termination; at higher pressures, the global rate increases due to gas phaseheating.TABLE 1RELATIVE EFFICIENCIES OF ENERGY TRANSFERIN UNIMOLECULAR REACTIONSReactant FxO NxO * Me,N, 0 Cyclopropane Cyclobutane * i-Pro fspecies 250°C 653°C 310°C 492" C 448" C radical1 .o0.40-0.82--1.011.131.13--0.88-1 .o0.660.470.20--0.240.23--1.321.50---1 .o0.07---0.460.21--0.030.250.460.201 .o0.05-0.070.12-0.07--0.08-0.70.24-1.101 .o0.070.120.210.10-0.21----0.440.38-1.121 .o-0.080.2-0.12--0.140.38-0.31--W. KABLITZ AND H. J. SCHUMACHER, Z. Physik. Chem., B, 20 (1933) 406.* M. VOLMER AND H. FROEHLICH, Z. Physik. Chem., B, 19 (1932) 85; AND M. BOGDAN, B, 21(1933) 257; AND H. BRISKE, B, 25 (1934) 81.D. v. SICKMAN AND 0. K. RICE, J. Chem. Phys., 4 (1936) 608.H. 0. PRITCHARD, R. G. SOWDEN AND A. F. TROTMAN-DICKENSON, Proc. Roy. SOC. (London),A, 217 (1953) 563.H. 0. PRITCHARD, R. G. SOWDEN AND A. F. TROTMAN-DICKENSON, IBID, A, 218 (1953) 416.D. L. COX, R. A. LIVERMORE AND L. PHILLIPS, J. Chem. SOC., B, (1966) 245.References pp, 104-1 I1