Instability, thermal

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

Catalyst Effectiveness. Even at steady-state, isothermal conditions, consideration must be given to the possible loss in catalyst activity resulting from gradients. The loss is usually calculated based on the effectiveness factor, which is the diffusion-limited reaction rate within catalyst pores divided by the reaction rate at catalyst surface conditions (50). The effectiveness factor E, in turn, is related to the Thiele modulus,

first-order rate constant, a the internal surface area, and the effective diffusivity. It is desirable for E to be as close as possible to its maximum value of unity. Various formulas have been developed for E, which are particularly usehil for analyzing reactors that are potentially subject to thermal instabilities, such as hot spots and temperature mnaways (1,48,51). 

Diffuse-thermal instabilities involve the relative diffusion reactants and heat within a laminar flame. These are the smaHest-scale instabilities (11). 

Although all the rings in Figure 1 contain six tt-electrons, the accumulation of electronegative nitrogen atoms in the polyaza structures leads to hydrolytic as well as thermal instability. This is noticeable in pyrimidine, and marked in the triazines and tetrazine. Some stability can be conferred by appropriate substitution, as we shall outline later. 

The maximum attainable production was sought that did not cause thermal runaway. By gradually increasing the temperature of the water, boiling under pressure in the reactor jacket, the condition was found for the incipient onset of thermal instability. Runaway set in at 485.2 to 485.5 K for the 12 m reactor and at 435.0 to 435.5 K for the shorter, 1.2 m reactor. The smaller reactor reached its maximum operation limit at 50 K lower than the larger reactor. The large reactor produced 33 times more methanol, instead of the 10 times more expected from the sizes. This... 

Thermal instability of reactants, reactant mixtures, isolated intermediates, and products. 

When designing ventilation systems with piston flow, one should also be aware of the effeas of thermal instabilities. Figure 8.27 shows a room with air... 

H2Se (like H2O and H2S) can be made by direct combination of the elements (above 350°), but H2Te and H2P0 cannot be made in this way because of their thermal instability. H2Se is a colourless, offensive-smelling poisonous gas which can be made by hydrolysis of Al2Se3, the action of dilute mineral acids on FeSe or the surface-catalysed reaction of gaseous Se and H2 ... 

In general the compounds have properties intermediate between those of the parent halogens, though a combination of aggressive chemical reactivity and/or thermal instability militates against the determination of physical properties such as mp, bp, etc., in some instances. However, even for such highly dissociated species as BrCl, precise molecular (as distinct from bulk) properties can be determined by spectroscopic techniques. Table 17.12 summarizes some of the more important physical properties of the... 

A similar thermal instability afflicts NH4Br03 (d —5°) and NH4IO3 (d 100°). 

Several other reactions have been studied but the overall picture is one of thermal instability,... 

Trichloro- and dichloromethane, ether, dioxane, benzene, toluene, chlorobenzene, acetonitrile, or even pyridine itself has been employed to carry out the one-pot syntheses. Tliese solvents allow straightforward preparation of the salts. The temperature range between 0° and 20°C is usually employed and the salts formed are sufficiently soluble. In the case of slow reactions, selection of a solvent with a higher boiling point is prohtable since thermal instability of the A -(l-haloalkyl)heteroarylium halides has not been reported. Addition of water or an aqueous solution of sodium acetate does not cause a rapid decomposition of the salts so that this constitutes a useful step in the optimization of some procedures. 

Tlie chemistry of 1,2-dithietanes is still emerging. Isolable and well-characterized 1,2-dithietanes are limited to only two compounds, 3,4-diethyl-l,2-dithietane 1,1-dioxide (77) and dithiatopazine (73).The synthesis of 1,2-dithietanes has been overshadowed by their thermal instability, which arises most probably from repulsive interactions between the lone-pair electrons on the sulfur atoms, as we have already seen in the chemistry of dithiiranes. 

Since it is well known that chloroalkenes are often much less stable than the corresponding alkanes, olefinic unsaturation may be an important source of thermal instability in PVC. Chain-end unsaturation could arise by disproportionation during bimolecular reaction of polymer radicals Eq. (2). 

From the results obtained by thermal decomposition of both low-molecular weight vicinal dichlorides in the gas phase [74,75] and of the copolymers of vinyl chloride and /rhead structures. Crawley and McNeill [76] chlorinated m-1,4-polybutadiene in methylene chloride, leading to a head-to-head, and a tail-to-tail PVC. They found, for powder samples under programmed heating conditions, that head-to-head polymers had a lower threshold temperature of degradation than normal PVC, but reached its maximum rate of degradation at higher temperatures. 

The interesting feature of the plot is the markedly discontinuous increase in thermal instability of the polymer with increase in the acetate content. The polymer was greatly destabilized at very low levels of acetate content, but further increase in instability with increasing acetate content is much more gradual and occurs at... 

Due to their thermal instability, this method cannot be applied to the preparation of benzo-thiepins. Although the ft-oxo sulfoxide moiety in precursors such as 5-methoxy-4-phenyl-l-benzothiepin-3(2/7)-one 1-oxide makes them candidates for a Pummerer reaction, treatment with acetic anhydride and triethylamine at - 30 C results in preferential enol acetylation to afford the corresponding 1-benzothiepin 1-oxide.14... 

Using this procedure, monocyclic thiepins were synthesized at — 30°C.76 77 Both the [2 + 2] cycloadduct and the resulting thiepins were observed in low-temperature ll NMR spectra however, due to the thermal instability and other reasons,78 it was not possible to isolate monocyclic thiepins prepared in this manner. 

The selective insertion of diphenylacetylene in the cyciopaiiadated sulfide complex 1 leads to the stable organometallic complex 2, which can be depalladated with silver(I) tetrafluoroborate to give a mixture of the dibenzothiepinium salt 3 and the dibenzo[Z>,z ]thiepin 4.91 Demethyla-tion of 3 to yield 4 is complete after refluxing overnight in chlorobenzene. The synthetic scope of this method for thiepin derivatives is limited due to their thermal instability, but the method is very suitable for the synthesis of 1//-2-benzothiopyrans.91... 

The reaction fails with the thermally generated nitrene on account of the thermal instability of the paracyclophane under the reaction conditions. 

Remarkably, in view of the general thermal instability and acid sensitivity of 1//-azepines, 2,3,4,5,6,7-hexafluoro-l//-azepine-l-carbonitrile(13) is hydrolyzed by hot concentrated sulfuric acid to 2,3,4,5,6,7-hexafluoro-l//-azepine-1-carboxamide (14).230... 

The activation barriers for the rearrangement depend on the nature of the TV-substituents and are higher for sulfonyl (S02R) than for acyl (COR) or alkyl groups. The parent system 1 (R = H) does not undergo a clean isomerization, probably due to the thermal instability of the product. 

Vapor phase brominations have given rise to varying products dependent on the reaction temperature. At 300°C bromine converted quinoline in the presence of pumice into 68 (25%) at 450°C 2-bromoquinoline (25%) became the major product at 500°C the yield of the 2-bromo isomer increased to 53%, but there was some dibrominated material [77HC(32-1)319]. The absence of 3-bromoquinoline at the higher temperatures could be accounted for in terms of radical attack, or it could be due to thermal instability of that isomer [59CI(L)1449]. 

Although orthophosphates are themselves passivating, anodic inhibitors (and also cathodic inhibitors, forming a calcium phosphate barrier film), the film strength is weak, even in simple HW systems and they are not used for this purpose. Nevertheless, despite the thermal instability of sodium hexametaphosphate and other polyphosphates, phosphates in general have several important properties that make them useful in boiler plant operations. These properties include ... 

Slagt et al. [134] have stated that because of their thermal instability and reactivity sultones could not be easily analyzed by gas chromatography. They studied the two methods published by Martinsson and Nilsson using a Carlo Erba Fractovap G1 equipped with a flame ionization detector and a glass column (length 0.65 m OD 1/4 in.) filled with 10% OV 1 on Chromosorb W-AW (80-100 mesh). The column temperature was 230°C and the injector/de-tector temperature 275°C. The gas flow rates were N2 25 ml/min (carrier gas), H2 25 ml/min, and air 250 ml/min. One microliter of sample was injected. 

An important feature of sulfation chemistry is the thermal instability of the acid sulfate, which breaks down to a mixture of products including the parent alcohol, the dialkyl sulfate (R0S020R), the dialkyl ether (ROR), isomeric alcohols, olefins (R CH=CH2), and esters (R0S03R). Because of the thermal instability of the acid sulfate it is necessary to avoid high sulfation temperatures and to neutralize the acid sulfation product soon after its formation. An aging time of about 1 min at 30-50°C is adequate for the second reaction whereby the desired alkyl hydrogen sulfate is formed. In practice the minimum sulfation feasible temperature is determined by the need for the feedstock and reaction mixture to be mobile liquids (Table 3). 

The thermal instability of 37 reduces its applicability with poorly reactive dienes such as vinylcyclohexene and its derivatives 38, unless high pressure (HP) is employed. Ultrasound is not only effective in promoting the cycloaddition of 37 with 38, but sometimes also improves the regioselectivity. Some data are illustrated in Table 4.8 and compared with cycloadditions in refluxing benzene and under high pressure. The reactions of 37 with reactive dienes such as cyclopentadiene and l-(trimethylsiloxy)-1,3-butadiene give a good yield of type D adducts under mild conditions, while with less reactive dienes, such as isoprene and butadiene, poor results are obtained. 

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