Endothermicity 448 Subject

The authors of [99] proposed a calorimetric method for determining the degree of the polymer-filler interaction the exothermal effect manifests itself in the high energy of the polymer-filler adhesion, the endothermal effect is indicative of a poor, if any, adhesion. The method was used to assess the strength of the PVC-Aerosil interaction with Aerosil surface subjected to different pre-treatments... 

Propene at 955 bar and 327°C was being subjected to further rapid compression. At 4.86 kbar explosive decomposition occurred, causing a pressure surge to 10 kbar or above. Decomposition to carbon, hydrogen and methane must have occurred to account for this pressure. Ethylene behaves similarly at much lower pressure, and cyclopentadiene, cyclohexadiene, acetylene and a few aromatic hydrocarbons have been decomposed explosively [1], It is mildly endothermic (AH°f (g) +20.4 kJ/mol, 0.49 kJ/g) and a minor constituent of MAPP gas [2],... 

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

The minimum design temperature should be the lowest temperature to which the tank will be subjected, taking into consideration the minimum temperature of material entering the tank, the minimum temperature to which the material may be autorefrigerated by rapid evaporation of low-boiling liquids or mechanically refrigerated, the minimum ambient temperature of the area where the tank is located, and any possible temperature reduction by endothermic physical processes or chemical reactions involving the stored material. API 620 provides for installations in areas where the lowest recorded 1-day mean temperature is 50°F (10°C). 

Proton and C-nmr, ESCA, and Raman studies provide a wealth of information which unfortunately is not subject to a unique interpretation. The main conclusion to be drawn therefore is that the structure of the solvent stabilized cation is still unproven. Gas phase estimates of the heat of formation of the norbomyl cation imply a rather marked stability of the stmcture relative to other secondary ions (Kaplan et al., 1970). When combined with other estimates of the heat of formation of the t-butyl cation, however, these data suggest that hydride transfer from isobutane to the norbomyl ion will be endothermic by 6 to 15 kcal mole . This is contrary to experience in the liquid phase behaviour of the ion, and the author s conclusion that their observation of enhanced stability is evidence of stabilization by bridging deserves further scmtiny. 

An important question regarding SN2 reactions in the gas phase concerns the stereochemistry and the extent to which a Walden inversion occurs at the reaction site. Since the experimental techniques monitor exclusively ion concentration, the actual nature of the neutrals produced in the reaction is subject to some doubt. An indirect method to ascertain the nature of the products is to assess the thermochemistry of other possible reaction channels. In the case of methyl derivatives, the alternatives are few and result in highly endothermic reactions, as exemplified in (22) and (23). For more complicated systems, this argument may not be satisfactory or may not yield an unequivocal answer. 

Addition reactions of carbon radicals to C—O and C—N multiple bonds are much less-favored than additions to C—C bonds because of the higher ir-bond strengths of the carbon-heteroatom multiple bonds. This reduction in exothermicity (additions to carbonyls can even be endothermic) often reduces the rate below the useful level for bimolecular additions. Thus, acetonitrile and acetone are useful solvents because they are not subject to rapid radical additions. However, entropically favored cyclizations to C—N and C—O bonds are very useful, as are fragmentations (see Chapter 4.2, this volume). 

Differential Thermal Analysis (DTA) — A procedure for recording the difference in temp between a substance and a reference material, against either time or temp as the two specimens are subjected to identical temp regimes in an environment heated or cooled at a controlled rate. The record is the differential thermal or DTA curve the temp difference (A T) is usually plotted on the ordinate with endothermic reactions downward and time or temp on the abscissa increasing from left to right Differential Scanning Calorimetry (DSC) —... 

Differential thermal analysis was used as a further measure of thermal stability. The polymer powder was pressed at 200°C in air to form a film. The film sample was heated at 10°C/minute from 25°-500°C in a nitrogen atmosphere. The the temperature at the onset of decomposition— i.e.y dehydrochlorination—as well as the peak endothermic temperature of the polymer which had already been subjected to a temperature of 200°C during film formation, were obtained from the DTA plot (Figures 3 and 4). 

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