Slower reacting systems

This conclusion is in agreement with Lewis who stated that slower reacting systems tend to show a greater effect under microwave radiation than faster reacting ones [54] . 

Highly reactive epoxy adhesives are generally more brittle than slower-reacting systems, and this results in lower peel and impact strengths. 

This conclusion is in agreement with a remark of Lewis who stated that slower reacting systems tend to show a greater effect under microwave radiation than faster reacting ones [82]. In this way, during solvent-free Wittig olefination with phosphoranes, it was shown that the benefit of MW irradiation increases with less reactive systems. The best stabilized phosphoranes do not react at all in the solid state with aldehydes or ketones under conventional heating but necessitate MW irradiation [83]. 

Slower reacting systems, necessitating a high energy of activation and consequently with late product-like transition states along the reaction coordinates, in agreement with the Hammond postulate. 

Once the adhesive system is applied, water reacts preferentially with the more reactive ketimine, instead of with the slower reacting isocyanate. In the presence of water, the ketimine unblocks to reform the ketone and diamine. Once formed, the diamine will react quickly with the isocyanate to form a polyurea. 

This sort of analysis could be extended to any metal-catalyzed chemistry in which a large runaway chiral excess is induced in the product by way of a small chiral excess of the molecules that serve as ligands to the metal. It is only necessary that the D,L-metal center be kinetically slower and thermodynamically more stable than the d,d- or L,L-complexes in order that any small e.e. of a chiral ligand be translated into chiral dominance of the reaction product. That the initial e.e. resulting in chiral takeover within a reacting system can be induced by asymmetric mineral surfaces indicates that a general chemical route to the asymmetry of life may exist. 

The intermediates which play a role in a cycle of a homogeneous catcilyst can be characterized by various spectroscopic techniques such as NMR, IR, Raman spectroscopy, and UV-vis spectroscopy. Also, intermediates may crystallize from a reaction mixture and the structure can then be solved with a single-crystal X-ray determination. Only on rare occasions do intermediates crystallize from the reacting systems since their concentrations are low. Often one turns to model compounds of the actual catalyst by changing the ligand or the metal. For example, iridium complexes show the same catalytic behaviour as the rhodium complexes. Since they are often much slower as catalysts the intermediates can be intercepted (see below). Another common approach is the synthesis of a ligand that simultaneously contains the substrate of the catalytic reaction this may also lead to the isolation of likely intermediates. 

This brief survey shows that there are many options for measuring phase equilibria in reacting systems, which allow to carry out such studies for a wide range of systems and conditions. The main limitation for experimental investigations of reactive vapor-liquid equilibria is related to the velocity of the reaction itself if phase equilibrium measurements of solutions are needed, which are not in chemical equilibrium, the reaction must be considerably slower than the characteristic time constant of the phase equilibrium experiment. Apparatus are available, where that time constant is distinctly below one minute. For systems with reactions too fast to be studied in such apparatuses, it should in many cases be possible to treat the reaction as an equilibrium reaction, so that the information on the phase equilibrium in mixtures, which are not chemically equilibrated is not needed. 

Synergistic effects of tin and amine catalysts are of technical importance and are widely studied principally because of the differences in reactivity. Metal catalysts are usually employed in systems based on the slower reacting aliphatic isocyanate adducts [27]. 

In natural fractures, the ratio of surface area to acid volume is much less, and deeper treatment is possible. In fracture acidizing, the ratio of rock surface area to acid volume is even lower. Very deep stimulation in fracturing applications is therefore possible, espedally with slower-reacting, low-leak-off add systems. 

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleo diilic substitution reactions. They are primary and do not form caibocation intermediates, but the /-butyl substituent efiTectively hinders back-side attack. The rate of reaction of neopent>i bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, tiie ner rentyl system reacts with rearrangement to the /-pentyl system, aldiough use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary beiuyl system in entry 3 exhibits high, but not complete, inversiotL This is attributed to racemization of the reactant by ionization and internal return. 

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

For fused heterocyclic systems too, we can often make predictions based on the above principles, though many exceptions are known. Thus, indole is chiefly substituted in the pyrrole ring (at position 3) and reacts faster than benzene, while quinoline generally reacts in the benzene ring, at the 5 and 8 positions, and slower than benzene, though faster than pyridine. 

---