Mechanisms intermolecular hydrogen

The various mechanical properties of polyamides may be traced in many instances to the possibility of intermolecular hydrogen bonding between the polymer molecules and to the relatively stiff chains these substances possess. The latter, in turn, may be understood by considering still another equilibrium, this one among resonance structures along the chain backbone ... 

There is quite some evidence for a mechanism as formulated above,especially for the six-membered transition state—the Barton reaction is observed only with starting materials of appropriate structure and geometry, while the photolysis of nitrite esters in general seldom leads to useful products formed by fragmentation, disproportionation or unselective intermolecular hydrogen abstraction. 

Hydrogen Abstraction Photoexcited ketone intermolecular hydrogen atom abstraction reactions are an interesting area of research becanse of their importance in organic chemistry and dne to the complex reaction mechanisms that may be possible for these kinds of reactions. Time resolved absorption spectroscopy has typically been nsed to follow the kinetics of these reactions but these experiments do not reveal mnch abont the strnctnre of the reactive intermediates. " Time resolved resonance Raman spectroscopy can be used to examine the structure and properties of the reactive intermediates associated with these reactions. Here, we will briefly describe TR experiments reported by Balakrishnan and Umapathy to study hydrogen atom abstraction reactions in the fluoranil/isopropanol system as an example. 

Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

Song and Beak161 have used intramolecular and intermolecular hydrogen-deuterium kinetic isotope effects to investigate the mechanism of the tin tetrachloride catalysed ene-carbonyl enophile addition reaction between diethyloxomalonate and methylenecy-clohexane (equation 105). These ene reactions with carbonyl enophiles can occur by a concerted (equation 106) or a stepwise mechanism (equation 107), where the formation of the intermediate is either fast and reversible and the second step is slow k- > k-i), or where the formation of the intermediate (the k step) is rate-determining. 

Predict the products of the intermolecular hydrogen abstraction reactions of ketones and discuss the mechanisms of these reactions. 

Waals radii of two atoms (2.90 A). Nevertheless, each reaction site is far apart the hydrogen abstraction from the benzyl group proceeds and the product was obtained in optically active form. In this case, another mechanism besides the direct intramolecular hydrogen abstraction from the benzyl group may be involved, like intermolecular hydrogen abstraction or hydrogen shift. 

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

Kinetic and equilibrium studies of the sorption of methanol on various coals and on partially acety-lated samples of these coals have been used to elucidate a mechanism for this process. The data are interpreted in terms of partial acetylation blocking surface sites and perhaps interfering with intermolecular hydrogen bonding. It is proposed that the rate-determining step is a set of parallel, competing, second-order reactions involving transfer of methanol from the surface to the interior of the coal. All types of surface sites appear to participate, and the pressure-independent rate constant is considered to be the sum of the rate constants for each type of surface site. The dependence of the experimental rate constant on methanol pressure is a characteristic of the coal rank. 

On the other hand, if the removed hydrogen ion migrates from one oxide ion to the other ions, intermolecular hydrogen exchange of olefins may occur during the isomerization reaction. Accordingly if a dissociative mechanism of... 

Intra- and intermolecular hydrogen transfer processes are important in a wide variety of chemical processes, ranging from free radical reactions (which make up the foundation of radiation chemistry) and tautomeriza-tion in the ground and excited states (a fundamental photochemical process) to bulk and surface diffusion (critical for heterogeneous catalytic processes). The exchange reaction H2 + H has always been the preeminent model for testing basic concepts of chemical dynamics theory because it is amenable to carrying out exact three-dimensional fully quantum mechanical calculations. This reaction is now studied in low-temperature solids as well. 

Photochromic transformations of 1,4-naphthoquinone were discovered after the introduction of a methyl substituent in the peri position for suppression of its photochemical reactivity toward intermolecular hydrogen abstraction." The mechanism of photochromism is based on reversible photoenolization (Scheme... 

---