Reactivity, behavioural

Activity studies give the most variable results, but there is the suggestion that exposure to lead causes an increase in behavioural reactivity. Activity measurements are a global representation of brain function, and therefore altered activity levels are at best a non-specific indication of nervous system dysfunction. There is the suggestion that increased locomotor activity in young animals occurs at moderately high exposures of lead, but this tendency... 

It is important to realise that mass spectrometric measurements in TG-MS are not performed directly on the polymer but only evolved gases are detected and identified. Factors influencing component loss from polymeric matrices are volatility, rate of diffusion, solubility in the polymeric matrix, flow-rate, temperature, AT, sample thickness, etc. Therefore, information about the polymeric matrix is obtained in an indirect way, and concerns especially the thermal stability, degradation mechanism and kinetics, performance behaviour, reactivity, and analysis of volatile additives, residuals, monomer occlusions... 

The authors suggest that increased behavioural reactivity may be the explanation for these differential effects. An alternative explanation is that the improvement on the two-way active avoidance task is due to damage to the hippocampus, or related structures, as this is known to improve performance on this task. This is thought to be because of deficits in spatial ability, so that the animal loses the sense of place, and does not remember, and therefore does not fear, the previously dangerous place. 

Several reactivity trends are worth noting. Reactions that are rapid frequently stay rapid as the temperature or centre-of-mass kinetic energy of the reactants is varied. Slow exothenuic reactions almost always show behaviour such tliat... 

In addition to the many applications of SERS, Raman spectroscopy is, in general, a usefiil analytical tool having many applications in surface science. One interesting example is that of carbon surfaces which do not support SERS. Raman spectroscopy of carbon surfaces provides insight into two important aspects. First, Raman spectral features correlate with the electrochemical reactivity of carbon surfaces this allows one to study surface oxidation [155]. Second, Raman spectroscopy can probe species at carbon surfaces which may account for the highly variable behaviour of carbon materials [155]. Another application to surfaces is the use... 

The second aspect, predicting reaction dynamics, including the quantum behaviour of protons, still has some way to go There are really two separate problems the simulation of a slow activated event, and the quantum-dynamical aspects of a reactive transition. Only fast reactions, occurring on the pico- to nanosecond time scale, can be probed by direct simulation an interesting example is the simulation by ab initio MD of metallocene-catalysed ethylene polymerisation by Meier et al. [93]. 

Unfortunately, insufficient data make it impossible to know whether the activity coefficients of all aromatic compounds vary slightly, or whether certain compounds, or groups of compounds, show unusual behaviour. However, it seems that slight variations in relative rates might arise from these differences, and that comparisons of reactivity are less sound in relatively concentrated solutions. 

The phenomenon was established firmly by determining the rates of reaction in 68-3 % sulphuric acid and 61-05 % perchloric acid of a series of compounds which, from their behaviour in other reactions, and from predictions made using the additivity principle ( 9.2), might be expected to be very reactive in nitration. The second-order rate coefficients for nitration of these compounds, their rates relative to that of benzene and, where possible, an estimate of their expected relative rates are listed in table 2.6. 

The behaviour of benzene is the datum from which any discussion of aromatic compoimds must start the reactivity of an aromatic compound is its rate of reaction relative to that of benzene when both are taking part in reactions occurring under the same conditions and proceeding by the same mechanism. 

There are certain limitations to the usefulness of nitration in aqueous sulphuric acid. Because of the behaviour of the rate profile for benzene, comparisons should strictly be made below 68% sulphuric acid ( 2.5 fig. 2.5) rates relative to benzene vary in the range 68-80% sulphuric acid, and at the higher end of this range are not entirely measures of relative reactivity. For deactivated compounds this limitation is not very important, but for activated compounds it is linked with a fundamental limit to the significance of the concept of aromatic reactivity as already discussed ( 2.5), nitration in sulphuric acid cannot differentiate amongst compounds not less than about 38 times more reactive than benzene. At this point differentiation disappears because reactions occur at the encounter rate. 

For deactivated compounds this limitation does not exist, and nitration in sulphuric acid is an excellent method for comparing the reactivities of such compounds. For these, however, there remains the practical difficulty of following slow reactions and the possibility that with such reactions secondary processes might become important. With deactivated compounds, comparisons of reactivities can be made using nitration in concentrated sulphuric acid such comparisons are not accurate because of the behaviour of rate profiles at high acidities ( 2.3.2 figs. 2.1, 2.3). 

An interesting aspect of this reaction is the contrasting stereoselective behaviour of the dimethyisulfonium and dimethyloxosuifonium methylides in reactions with cyclic ketones (E.J. Corey, 1963 B, 1965 A C.E. Cook, 1968). The small, reactive dimethyisulfonium ylide prefers axial attack, but with the larger, less reactive oxosulfonium ylide only the thermodynamically favored equatorial addition is observed. 

In the case of alkyl radicals [e.g., methyl radical (197, 198) and cyclohexyl radical (198)], their nucleophilic behaviour enhances the reactivity of the 2-position. Here it is necessary to have full protonation of the nitrogen atom and to use specific solvents and radical sources. 

The pyrazole molecule resembles both pyridine (the N(2)—C(3) part) and pyrrole (the N(l)—C(5)—C(4) part) and its reactivity reflects also this duality of behaviour. The pyridinic N-2 atom is susceptible to electrophilic attack (Section 4.04.2.1.3) and the pyrrolic N-1 atom is unreactive, but the N-1 proton can be removed by nucleophiles. However, N-2 is less nucleophilic than the pyridine nitrogen atom and N(1)H more acidic than the corresponding pyrrolic NH group. Electrophilic attack on C-4 is generally preferred, contrary to pyrrole which reacts often on C-2 (a attack). When position 3 is unsubstituted, powerful nucleophiles can abstract the proton with a concomitant ring opening of the anion. 

The behaviour of pyrazoles towards nitrosation is similar to their behaviour described above towards diazo coupling, i.e. aminopyrazoles and pyrazolones readily react with nitrosation agents, like alkyl nitrites (81FES1019), to afford stable nitroso derivatives. Some simple nitrosopyrazoles have been isolated, for example the blue-green 3,5-dimethyl-4-nitrosopyrazole, and many others have been proposed as reactive intermediates in the direct conversion of pyrazoles into diazonium or diazo derivatives (Scheme 25) (B-76MI40402). 

Boranes are extremely reactive compounds and several are spontaneously flammable in air. Arac/tno-boranes tend to be more reactive (and less stable to thermal decomposition) than niiio-boranes and reactivity also diminishes with increasing mol wt. C/oio-borane anions are exceptionally stable and their general chemical behaviour has suggested the term three-dimensional aromaticity . 

Environments are considered in detail in Chapter 2, but some examples of the behaviour of normally reactive and non-reactive metals in simple chemical solutions will be considered here to illustrate the fact that corrosion is dependent on the nature of the environment the thermodynamics of the systems and the kinetic factors involved are considered in Sections 1.4 and 1.9. 

The corrosion behaviour of different constituents of an alloy is well known, since the etching techniques used in metallography eu e essentially corrosion processes which take advantage of the different corrosion rates of phases as a means of identification, e.g. the grain boundaries are usually etched more rapidly than the rest of the grain owing to the greater reactivity of the disarrayed metal see Sections 1.3 and 20.4). 

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