Sensitivity toluene effect

Nitrations can be performed in homogeneous media, using tetramethylene sulfone or nitromethane (nitroethane) as solvent. A large variety of aromatic compounds have been nitrated with nitronium salts in excellent yields in nonaqueous media. Sensitive compounds, otherwise easily hydroly2ed or oxidized by nitric acid, can be nitrated without secondary effects. Nitration of aromatic compounds is considered an irreversible reaction. However, the reversibihty of the reaction has been demonstrated in some cases, eg, 9-nitroanthracene, as well as pentamethylnitrobenzene transnitrate benzene, toluene, and mesitylene in the presence of superacids (158) (see Nitration). 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

In this section, we present a unified picture of the different electronic effects that combine to determine methyl rotor potentials in the S0, Sp and D0 electronic states of different substituted toluenes. Our approach is based on analysis of ab initio wavefunctions using the natural bond orbitals (NBOs)33 of Weinhold and cowork-ers. We will attempt to decompose the methyl torsional potential into two dominant contributions. The first is repulsive steric interactions, which are important only when an ortho substituent is present. The second is attractive donor-acceptor interactions between CH bond pairs and empty antibonding orbitals vicinal to the CH bonds. In the NBO basis, these attractive interactions dominate the barrier in ethane (1025 cm-1) and in 2-methylpropene (1010 cm-1) see Figure 3. By analogy, donor-acceptor attractions are important in toluenes whenever there is a substantial difference in bond order between the two ring CC bonds adjacent to the C-CH3 bond. Viewed the other way around, we can use the measured methyl rotor potential as a sensitive probe of local ring geometry. 

The pincer-type palladacycle (120) (R = 1Pr), which is actually a derivative of a dialkylphos-phinous acid (themselves excellent ligands see Section 9.6.3.4.6) was shown to allow the crosscoupling of aryl chlorides with terminal acetylenes ((120), ZnCl2, Cs2C03, dioxane, 160 °C). However the high reaction temperature may be prohibitive for the actual application of this catalytic system, as acetylenes are known to be thermally sensitive.433 The same palladacycle (R = Ph) is effective in the Suzuki-Miyaura reaction with aryl bromides and activated aryl chlorides (K2C03, toluene, 130 °C). 

The sPS has been exploited as sensitive material for fiber optic chemical sensors based on reflectance measurements and aimed to detection of chloroform and toluene in water and air environments48 50. The refractive index of sPS thin films is estimated to be about 1.578. The effect of the analyte sorption in the crystalline domain was modeled as an increase in the material density, which in turn leads to an increase in the refractive index according to the Lorentz-Lorenz law ... 

P/P0), the colloidal crystal film exhibited an additional increase in the response sensitivity likely due to the swelling of the core of the core/shell submicron particles. This effect was more pronounced upon exposure to toluene. 

Numerous aromatic nitro compounds have explosive properties, and thus it is important to understand the role that enthalpy of formation has on the sensitivity and long-term stability of these compounds. We will examine three nitro-substituted aromatic families for which thermochemical data can be found in the literature2,84 derivatives of nitrobenzene, aniline and toluene. The choice of these three families allows us to compare the various electronic effects exerted by the parent functional group. The parent compounds differ electronically with respect to the aromatic ring in that ... 

It has been recently reported that, contrary to previous assumptions, primary steric effects due to a branching in the amine do not produce a large decrease in the reaction rate when the first step is rate-determining82. In Sat Ar reactions of amines with fluoronitroben-zene, it is generally accepted that the second step of the mechanism depicted in Scheme 1 is rate-determining base catalysis is frequently found and the observed rate constants obey equation 2. Nevertheless, the reaction of o-fluoronitrobenzene with n- and iso-propylamine in toluene and in DMSO are only slightly sensitive to the nucleophile concentration. The... 

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