Reactions sensitivities

Also, the rates of the propagation steps are equal to one another (see Problem 8-4). This observation is no surprise The rates of all the steps are the same in any ordinary reaction sequence to which the steady-state approximation applies, since each is governed by the same rate-controlling step. The form of the rate law for chain reactions is greatly influenced by the initiation and termination reactions. But the chemistry that converts reactant to product, and is presumably the matter of greatest importance, resides in the propagation reactions. Sensitivity to trace impurities, deliberate or adventitious, is one signal that a chain mechanism is operative. 

The use of glass apparatus to carry out reactions sensitive to light causes accidents. 

There are so many different examples of photochemical dimerizations and cross-cycloadditions 8-11,13-17) 0f olefinic compounds that one is not surprised to find several variations of mechanistic patterns. Simple olefins, including dienes and strained small ring, bicyclic olefins and styrene derivatives form a class of compounds that undergo such reactions sensitized by triplet energy donors. Some examples axe given in Eqs. 19—23, where only cyclobutane products are depicted. Theory... 

Anything that breaks the chain by converting the active chain-carrying species into an ordinary uncreactive molecule inhibits the reaction, and since the chains are often long an inhibitor may be effective in very small traces. The chain-starting catalysts may also be effective in very small amounts provided that no inhibitor is also present. The fact that a reaction is a chain reaction sensitive to small amounts of catalysts and inhibitors does not necesssarily mean that it is a radical chain, but the nature of the substances effective as catalysts or inhibitors will usually differentiate a radical chain from an ionic one. An example of an ionic chain reaction is the polymerization of an olefin-Lewis acid system when water is added as a co-catalyst. Water is so very effective that it is suspected that the polymerization observed in some cases with the driest obtainable reaction mixtures is due to the presence of minute and unavoidable amounts of water. 

Schmittel cyclization, 4f 30, see also Cycloaromatization reactions Schreiner cyclization, f 30, see also Cycloaromatization reactions Sensitizers, singlet oxygen, 233-234, 235-236... 

Since detailed chemical kinetic mechanisms involve the participation of a large number of species in a large number of elementary reactions, sensitivity and reaction path analyses are also essential elements of DCKM. Sensitivity analysis provides a means to assess the limits of confidence we must put on our model predictions in view of uncertainties that exist in reaction rate parameters and thermochemical and thermophysical data utilized, as well as the initial and boundary conditions used in the modeling work. Through... 

The above results lead to the conclusion that rearrangement of 124 to the cyclopropane 125, occurring in the reactions sensitized by DMA, must take place via radical-anion intermediates. Considering that the C—double bond in compounds 101, 117, 118, and 119 should be a better electron acceptor than the diphenylvinyl unit in 124, it is logical to assume that the rearrangement of the 1-azadienes also takes place via the same types of radical-ion intermediates. 

Splashes of TDI liquid in the eye cause severe conjunctival irritation and lacrimation. On the skin, the liquid produces a marked inflammatory reaction. Sensitization of the skin occurs but is uncommon because of proper work practices. There seems to be little relation between skin sensitivity and respiratory sensitivity to TDI. ... 

Hypersensitivity reactions Sensitivity reactions are more likely to occur on patients with a history of allergy, asthma, hay fever, or urticaria. 

I. Anthracene and Its Derivatives. Liu234 has convincingly demonstrated that several reactions sensitized by anthracene and its derivatives proceed through the second excited triplet state, T2. The first case studied was the photosensitized rearrangement of 54 to yield the products shown below. A break in the efficiency of triplet sensitized reaction was found between... 

Fig. 13. Summary of the literature data on various reactions (%40) studied on Ni/Cu alloys. Relative activity as a function of the bulk alloy composition. All data can be subdivided into two groups of reactions sensitive (II) or insensitive (I) to alloying. The surface Ni concentration follows approximately the lower limit of the band for reactions of group I. From V. Ponec, Jnt. J. Quantum Chem. 12, Suppl. 2, 1-12 (1977).

Notes A very strong base (pKa = 21.1) with poor nucleophilicity due to steric effects. This makes it a useful organic base for reactions sensitive to most other Lewis bases. 

Ethylene glycol in the presence of an acid catalyst readily reacts with aldehydes and ketones to form cyclic acetals and ketals (60). 1,3-Dioxolane [646-06-0] is the product of condensing formaldehyde and ethylene glycol. Applications for 1,3-dioxolane are as a solvent replacement for methylene chloride, 1,2-dichloroethane, 1,1,1-trichloroethane, and methyl ethyl ketone as a solvent for polymers as an inhibitor in 1,1,1-trichloroethane as a polymer or matrix interaction product for metal working and electroplating in lithium batteries and in the electronics industry (61). 1,3-Dioxolane can also be used in the formation of polyacetals, both for homopolymerization and as a comonomer with formaldehyde. Cyclic acetals and ketals are used as protecting groups for reaction-sensitive aldehydes and ketones in natural product synthesis and pharmaceuticals (62). 

An electrocatalytic reaction is an electrode reaction sensitive to the properties of the electrode surface. An electrocatalyst participates in promoting or suppressing an electrode reaction or reaction path without itself being transformed. For example, oxygen reduction electrode kinetics are enhanced by some five orders of magnitude from iron to platinum in alkaline solutions or from bare carbon to carbon electrodes modified with Fe phthalocyanines or phenylporphyrins. For a comprehensive discussion of the subject, the reader is referred to refs. (76, 95, and 132-136). 

Since all photooxidation reactions were carried out under similar experimental conditions in solution both for oxidations sensitized by RB and reactions sensitized by P -RB, the ratio of the rate of photooxidation with RB and the rate of photooxidation with P-RB is equal to the ratio of the quantum yield for singlet oxygen formation with RB to that of the quantum yield for singlet oxygen formation with P-RB (7). 

Reactions sensitized by monatomic gases have been very extensively studied. The majority of the cases involve mercury vapor. Direct reaction of excited mercury atoms with other gases has also received considerable attention of late. 

The possibility of the intermediacy of the triplet state of benzene itself has been discussed by Atkins et al. [108], Photoaddition of alkenes to arenes is often accompanied by the formation of dimers of the alkene, a reaction sensitized by triplet benzene. With methyl acrylate and methyl vinyl ketone, however, it was found that the ratio of ortho cycloadducts to alkene dimers increased with the concentration of benzene. Because the yield of T, benzene increases with benzene concentration, these results might indicate that ortho photocycloaddition of aery-... 

Gives blue coloration when added to neutral or acid solution of cobalt, due to a complex formation (Vogel s reaction) sensitivity, 0.5 pg... 

Many years ago Taylor1 noted that the amount of a surface which is catalytically active is determined by the reaction catalyzed . More recently, Boudart2 proposed dividing reactions catalyzed by metals into two groups -structure sensitive and structure insensitive reactions. Sensitive reactions were those which for a particular metal showed a marked variation in activity with method of preparation. (Earlier Boudart et al had used the terms facile and demanding .)... 

Gas-induced morphological changes have been reported, and there is growing evidence that this may be a common occurrence with supported metal catalysts. There is further evidence that a small metal particle may consists of a solid core having a fluid-like surface layer of metal atoms. This raises the possibility that in addition to having catalytic reactions which are structure sensitive it may be necessary to allow that structures are sensitive to catalytic reactions, i.e., reaction-sensitive structures. It is possible that during the initial adsorption of the reactants a small particle will change its surface structure into one which best suits those particular reactants. 

In the study, a certain type of chemical reaction, sensitive to the charge state of a catalyst, was chosen. As test reactions for copper and nickel films, the following chlorohydrocarbons conversions were selected [30,34,39] ... 

There are many potential applications for the liquefaction and dissolution of chemically modified wood. Examples include the fractionation of modified wood components, the preparation of solvent-sensitive and/or reaction-sensitive wood-based adhesives [4,9,11,19], the preparation of resinified wood-based moldings such as the foam type [4], and the preparation of wood-based fibers and their conversion to carbon fibers [20]. 

The devices were applied not only to simply determine the refractive index of a liquid but also to detect a particular analyte molecule by an immunosensing reaction. Sensitivities down to zeptomole [50] quantities were reported. 

It has been suggested that the first step of reaction (6) may be the formation of a carboxylic species COOHads. Carboxyl radicals have indeed been observed by Zhu et al." for potentials lower than 0.65 V using Fourier Transform infrared Reflectance Absorption Spectroscopy with the Attenuated Total Reflection mode (ATR-FTtR). Moreover Anderson et al." made numerical simulation which indicated that the formation of an adsorbed carboxylic species was energetically more favorable. Here, it has to be noted that the electro-oxidation of CO being a stracture sensitive reaction (sensitive to the superficial stracture symmehy" and to the presence of surface defects) this species can be used to study the activity of a catalyst but also as a molecular probe to characterize the catalytic surface. ... 

Quantitative investigations on the reactions sensitized by azomethane , bi-acetyF and di-t-butyl peroxide , as well as those on the iodide-catalyzed decomposition , all gave an order of with respect to the sensitizer and that of unity with respect to the acetaldehyde. The agreement between the overall reaction orders of the thermal decomposition of acetaldehyde and of the sensitized decompositions seems to support the suggestion that both involve chain mechanisms. 

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