Relative exothermicities

Reaction with the ester is relatively exothermic. Sensitivity of the substrate-product to heating varies and should be considered for each particular compound. Phenyl benzoate can be methylenated at room temperature with no significant decrease in product yield. By contrast, dihydrocoumarin (Preparation B) gives no product under these conditions and must be methylenated at -78°C to obtain a good yield. For most substrates it is satisfactory to carry out reactions at 0°C. 

The data of Fig 1 can be depicted as material temp vs time plots. This is done in Fig 3. Note that, as expected, Ti reaches asymptotic values for T0 208°> whereas T increases in an expl fashion for T0 > 208°. Also note that the times at which Tx starts to increase rapidly decrease as T0 increases. The righthand ordinate of Fig 3 depicts the decompn rate of the reacting material. Note that this rate is quite low. Indeed for reactions that are relatively exothermic and have a fairly high energy of activation (see Vol 7 article on Kinetics ), most materials rarely undergo more than a few percent decompn before they expkT. It is also noteworthy (and indeed characteristic of most thermal explns) that the inflection points (the temp at which the curves pause before the rapid rise in temp) are quite close to the critical temp... 

Finally in this section, we should note the relatively exothermic electron attachment energies of Cu, Ag and Au —118, —126 and... 

In an attempt to determine whether ground state molecular orbital parameters could correlate a relatively exothermic... 

The electronic configuration of each halogen is one electron less than that of a noble gas, and it is not surprising therefore, that all the halogens can accept electrons to form X" ions. Indeed, the reactions X(g) + e - X (g), are all exothermic and the values (see Table 11.1), though small relative to the ionisation energies, are all larger than the electron affinity of any other atom. 

The use of dimethylformamide (b.p. 153°) as a solvent and diluent often increases the yield materially. The vigour of the exothermic reaction which occurs with a relatively reactive aryl hahde is moderated and, furthermore, the dimethylformamide is easily removed from the reaction product since it is water soluble. Aryl hahdes which are inert under the usual Ullmann conditions do not react in the presence of dimethylformamide. 

In the flask were placed 0.20 mol of the acetylenic alcohol, 0.24 mol of tosyl chloride and 350 ml of diethyl ether. The mixture was stirred at room temperature, until the solid had passed into solution and then cooled at -5 to -10 c in a bath of dry-ice and acetone. Machine-powdered KOH (130 g) was added with vigorous stirring, initially in relatively small portions [oa. 5 g), at intervals of 2 min. The reaction was strongly exothermic at first, and efficient cooling was necessary in order to maintain the temperature of the reaction mixture between -5 and O C... 

You have seen that measurements of heats of reaction such as heats of combustion can pro vide quantitative information concerning the relative stability of constitutional isomers (Section 2 18) and stereoisomers (Section 3 11) The box in Section 2 18 described how heats of reaction can be manipulated arithmetically to generate heats of formation (AH ) for many molecules The following material shows how two different sources of thermo chemical information heats of formation and bond dissociation energies (see Table 4 3) can reveal whether a particular reaction is exothermic or en dothermic and by how much... 

Adsorption is invariably an exothermic process, so that, provided equilibrium has been established, the amount adsorbed at a given relative pressure must diminish as the temperature increases. It not infrequently happens, however, that the isotherm at a given temperature Tj actually lies above the isotherm for a lower temperature Ti. Anomalous behaviour of this kind is characteristic of a system which is not in equilibrium, and represents the combined effects of temperature on the rate of approach to equilibrium and on the position of equilibrium itself. It points to a process which is activated in the reaction-kinetic sense and which therefore occurs more rapidly as temperature is increased. 

Acrylate and methacrylate polymerizations are accompanied by the Hberation of a considerable amount of heat and a substantial decrease in volume. Both of these factors strongly influence most manufacturing processes. Excess heat must be dissipated to avoid uncontrolled exothermic polymerizations. In general, the percentage of shrinkage decreases as the size of the alcohol substituent increases on a molar basis, the shrinkage is relatively constant (77). 

The relative effectiveness of nucleating agents in a polymer can be determined by measuring recrystallization exotherms of samples molded at different temperatures (105). The effect of catalyst concentration and filler content has been determined on unsaturated polyesters by using dynamic thermal techniques (124). Effects of formulation change on the heat of mbber vulcanization can be determined by dsc pressurized cells may be needed to reduce volatilization during the cure process (125). 

Relatively high concentrations of organic peroxide or azo initiators are needed to obtain complete polymerization. After the reaction peak exotherm, polymerization slows down. Initiator concentrations must be high enough to complete conversion. Polymerization is inhibited by oxygen and copper, lead, and sulfur compounds (11). 

Chemiluminescence. Chemiluminescence (262—265) is the emission of light duting an exothermic chemical reaction, generaUy as fluorescence. It often occurs ia oxidation processes, and enzyme-mediated bioluminescence has important analytical appHcations (241,262). Chemiluminescence analysis is highly specific and can reach ppb detection limits with relatively simple iastmmentation. Nitric oxide has been so analyzed from reaction with ozone (266—268), and ozone can be detected by the emission at 585 nm from reaction with ethylene. 

It is accepted that, at normal pressures, mtile is the thermodynamically stable form of titanium dioxide at all temperatures. Calorimetric studies have demonstrated that mtile is more stable than anatase and that brookite and Ti02 (ii) have intermediate stabiHties, although the relative stabiHties of brookite and Ti02(ii) have not yet been defined. The transformation of anatase to mtile is exothermic, eg, 12.6 KJ/mol (9), although lower figures have also been reported (63). The rate of transformation is critically dependent on the detailed environment and may be either promoted or retarded by the presence of other substances. For example, phosphoms inhibits the transformation of anatase to mtile (64). 

Once initiated, zirconium and carbon powders react exothermically in a vacuum or inert atmosphere to form zirconium carbide. With the greater availabiHty of relatively pure metal powders, this technique is coming into common use for the production of several refractory carbides. Zirconium carbide is not a fixed stoichiometric compound, but a defect compound with a single-phase composition ranging from ZrCQ to ZrCQ at 2400°C. 

The rate of chlorination of toluene relative to that of ben2ene is about 345 (61). Usually, chlorination is carried out at temperatures below 70°C with the reaction proceeding at a profitable rate even at 0°C. The reaction is exothermic with ca 139 kj (33 kcal) of heat produced per mole of monochlorotoluene formed. Chlorine efficiency is high, and toluene conversion to monochlorotoluene can be carried to about 90% with the formation of only a few percent of dichlorotoluenes. In most catalyst systems, decreasing temperatures favor formation of increasing amounts of -chlorotoluene. Concentrations of requited catalysts are low, generally on the order of several tenths of a percent or less. 

---