Substitution reactions threshold

The vertical ionization potential for a solvated chemical species can be the measure of its reactivity in the solution phase, especially for a single electron transfer reaction. It has been reported that the ionization potentials of anions in solution are conelated with the kinetic parameter for nucleophilic substitution reaction. This implies that an important aspect of the activation process of the reaction is a single electron transfer from anion to substrate. The ionization potential for solvated species has been available as the threshold energy E by photoeiectron emission spectroscopy for solution (PEES). This spectroscopic technique is able to provide the , values of almost any solvated species, such as organic, inorganic, cations, anions and neutral molecules in aqueous and nonaqueous solutions. 

The 1.7-eV CFs F excitation threshold follows from extrapolation of low-pressure data for hot H-for-H and F-for-F substitution reactions. The other excitation enemes represent lower-bound estimates obtained from standard heats of formation. Cf. References 25 and 45 ... 

Let us consider the dynamical significance of the present results for CFs F produced from the CH3CF3 F-for-CH3 primary process (Reaction 56). Based on the intrinsic imimolecular properties of 5-atom molecules, collisional deactivation of excited hot atom activated methanes should require large (P/Z). This behavior has been demonstrated for CH3 H, CH3 F, CH2F F, and CF3 F activated via energetic substitution reactions (11,47- 0,74). The results showm in Table V thus indicate that the nascent CF3 F from Reaction 56 contains little internal excitation in excess of the decomposition threshold value. 

Valencich and Bunker, using a diffraent approach, devised an empirical CH, surface which was adjusted to fit some of the known experimental data for reactions (53)—(56), including the reaction thresholds and relative cross-sections at 2.8 eV, and used trajectories on the surface to explore other features of the reactions such as cross-sections at other energies. Substitution in CH (53) at energies near... 

Threshold energy for hydrogen substitution reaction is more difficult to measure. Chou and Rowland (1969) used photolysis of TBr to obtain hot tritium atoms for the substitution reaction. They compared the yield of substituted CD3T in methane to the DT abstraction yield whose threshold energy is known to be smaller. The ratio substitution/abstraction was extrapolated to zero at which the threshold value of energy for substitution was determined to be 1.5 eV. 

Wolfgang and his coworker (Menzinger and Wolfgang 1969) studied threshold energy of substitution products in T -i-C6Hi2 (solid cyclohexane) reaction using a tritium beam accelerator. For simple substitution, the threshold energy is about 0.5 eV. However, for the products (n-hexane-T and hexane-T) for which C-C bond rupture is required, it is about 4.5 eV. 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

The formalism for treating primary isotope effects on unimolecular processes follows analogously to the development above, once due account is taken of the difference in zero point energies on isotope substitution at the reaction site (which is reflected in an isotopic difference in the threshold energy Eo). For thermal activation the rate ratio in the high pressure limit is straightforwardly obtained from Equation 14.25. For H/D effects... 

Two important conclusions can be drawn from the simunary of the symmetry analysis of Ar/CO collisions in Table 6. First, no SIKIE is predicted for C substitution because the symmetry of the system is independent of the isotope of carbon involved. Second, because the predicted a based symmetry restrictions for Ar COj cluster formation are identical to those predicted for (002)2, dependence of the magnitude of observed 0 SIKIE on the conditions of CO2 formation is expected. However, the e/f parity label state propensities for El-produced COJ, inferred from 0 SIKIE in (COj) formation, are not sufficient to predict the magnitude of 0 SIKIE in Ar-COj formation because, for above the threshold for Ar formation, COj ions are also produced by the charge-transfer reaction,... 

The intensity of undesirable sensory notes has been positively correlated with the content of carbonyl compounds formed through lipid autoxidation reactions. In general, the carbonyl compounds present have the greatest impact on flavor owing to their low flavor thresholds in comparison with hydrocarbons, substituted furans, and alcohols. Aldehydes are major contributors to the loss of desirable flavor in meats because of their rate of formation during lipid oxidation and low flavor threshold. Thus, an alternative approach for monitoring the extent of lipid oxidation in fats and oils is to measure... 

For the 7HQ-A3 cluster, excitation of ammonia-wire vibrations induces the photoreaction at a threshold of about 200 cm-1 [52], The reaction proceeds by tunnelling, as shown by deuteration of the wire (ND3). It has been found that substitution of NH3 by one, two or three H20 molecules in the wire increases the threshold with each additional water molecule, up to about 2000cur1 in the 7HQ-W3 cluster [15,55],... 

A major distinction for nucleophilic reactions with ambident anions is whether they proceed with kinetic or thermodynamic control.80 N-Substituted saccharins (10) should be thermodynamically more stable because of amide character than the isomeric pseudosaccharin (3) of imidate structure. In fact 3 may be rearranged thermally to 10 in an irreversible reaction.96 The threshold for thermodynamic control appears to be lowered for electrophiles with multiple bonds, e.g., formaldehyde, reactive derivatives of carboxylic acids, but also quaternary salts of N-heterocyclic compounds.80 It will be seen that in those cases substitution indeed occurs at the nitrogen, not necessarily through thermodynamic control. 

Figure 17.3 shows the reaction scheme for the breakdown of phenol to odorless low molecular weight decomposition products usiug HOCl. The threshold odor concentrations of the various chloride substituted pheuoUc compouuds are also indicated in brackets. Note that the worst offenders are 2-monochlorophenol and 2,4-dichlorophenol, which have an odor threshold of 2.0 /xg/L. In order to effect these breakdown reactions, superchlorination would be uecessary, which would also mean that the odor had increased before it disappeared. 

In a recent series of impact tests the plastic deformation rates required to initiate chemical reaction were measured at the reaction site for 10 different explosive materials ranging from the sensitive explosives RDX and HMX to the insensitive explosives PBXN-128 and PBX 9502 [21]. The measured plastic deformation rate ranged from Ay/At =. 7 x lO" s for RDX or HMX to Ay/At > 3 x 10 s PBXN-128 to PBX-9502. Substituting these measured plastic deformation rates into the expression for the calculated plastic deformation rate in the previous paragraph implies that first reaction occurs in the impact tested materials when 10 < T(x,U)pc < 10 . As shown above, T(r, U)pc = 10 predicts the observed plastic strain in RDX crystals just prior to the initiation threshold. 

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