Rupture chemical reaction mechanism

An example of the application of molecular mechanics in the investigation of chemical reactions is a study of the correlation between steric strain in a molecule and the ease of rupture of carbon-carbon bonds. For a series of hexasubstituted ethanes, it was found that there is a good correlation between the strain calculated by the molecular mechanics method and the rate of thermolysis. Some of the data are shown in Table 3.3. 

Acid and base catalysis of a chemical reaction involves the assistance by acid or base of a particular proton-transfer step in the reaction. Many enzyme catalysed reactions involve proton transfer from an oxygen or nitrogen centre at some stage in the mechanism, and often the role of the enzyme is to facilitate a proton transfer by acid or base catalysis. Proton transfer at one site in the substrate assists formation and/or rupture of chemical bonds at another site in the substrate. To understand these complex processes, it is necessary to understand the individual proton-transfer steps. The fundamental theory of simple proton transfers between oxygen and nitrogen acids and... 

Explosion An explosion is a rapid expansion of gases resulting in a rapidly moving pressure or shock wave. The expansion can be mechanical (by means of a sudden rupture of a pressurized vessel), or it can be the result of a rapid chemical reaction. Explosion damage is caused by the pressure or shock wave. 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

The theory stems from the writer s work on simple electron transfer reactions of conventional reactants (5). A simple electron transfer reaction is defined as one in which no bonds are broken or formed during the redox step such a reaction might be preceded or followed by bondbreaking or bond-forming steps in a several-step reaction mechanism. Other chemical reactions involve rupture or formation of one or several chemical bonds, and only a few coordinates suffice to establish their essential features. In simple electron transfers in solution, on the other hand, numerous coordinates play a role. One cannot then use the usual two-coordinate potential energy contour diagram (4) to visualize the... 

Overall, therefore, the available literature supports the generally held view that the durability of UF-bonded wood products is governed by the susceptibility of cured UF resin bonds to scission by both hydrolysis and swell/shrink stresses. Note, moreover, that in either case, the most likely product of scission will ultimately be formaldehyde and further that mechanical stress enhances the rates of many chemical reactions (37). In fact, simplistic calculations based on formaldehyde liberated from bond ruptures at least indicate the possibility that formaldehyde from swell/shrink stress rupture could contribute significantly to total emission. Assume, for example, that board failure occurs due to rupture of one chemical bond type which liberates one molecule of formaldehyde and consider two cases (a) a conservative one in which only 5 percent of those bonds rupture in 50 years, i.e., probable board durability greater that 50 years, and (b) a much less conservative case in which 30 percent of those bonds rupture in 20 years, i.e., probably failure in 20 years or less. Case (a) leads to a first order scission rate constant of 3.3 x 10 s and a hypothetical board emission rate (see Appendix 3a) that is below the maximum liberation rate permitted by the German E-1 standard (7). However, Case (b) leads to a first order scission rate constant of 5.7 x 10 s and a hypothetical board emission rate above that allowed by the HUD standard (8). (FormaIdehyde-wood interactions and diffusion effects would... 

The experimental approaches which have been developed to probe the rupture of single covalent bonds, which is the first step in a mechanically activated reaction, include single molecule AFM experiments and force clamp in which the molecule is pulled at a constant velocity from the substrate, or under constant force. In the first case the force increases as the molecule is pulled from the substrate and a rupture force is measured which is the maximum force that the molecule can sustain before rupture and a drop in the force signal. In the case of force clamp experiments, a constant force is applied to determine the impact of force on reaction pathways. Sonication has also been used as a means to mechanically activate chemical reactions in solution rests where long chain polymers containing mechan-ophores are pulled apart when a cavity formed in the sonication process collapses. Experimental approaches based on these methods form the basis for molecular scale examination of mechanically activated chemical reactions. First principles approaches to date have mainly concentrated on the pulling of molecules under constant velocity, as the measured rupture forces provide a convenient quantity to compare theoretical approaches with experiment. 

Fig. 12 Reaction mechanisms at two different temperature for the rupture of a PEG fragment in water. Reprinted with permission from D. Aktah and I. Frank, J. Am. Chem. Soc., 2002, 124, 3402-3406. Copyright 2002 American Chemical Society.

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