Reactions describing molecularity

The general definition of a condensation reaction is a one that involves product formation by expulsion of water (or other small molecule) as a by-product. By this definition, activation and methylolation are also condensations. In more precise terms the chain-building process should be described as a condensation polymerization, however, in the jargon of the phenolics industry, the term condensation is usually reserved for the chain-building process. This terminology is not necessarily observed in the literature [88]. Many literature reports correctly refer to methylolation as a condensation reaction. The molecular weight development of the phenol alcohol adducts may also be classified as a step-polymerization. 

What does an atom experience in an encounter with another atom The nucleus, which contains most of the atom s mass, is confined to a tiny volume. Electrons, on the other hand, are spread out through space. Therefore, a collision between two atoms is a collision of their electron clouds. The electron clouds repel each other but are attracted by the nuclei. Chemists describe molecular structure, properties of materials, and chemical reactions in terms of how electrons respond to these electrical forces. 

The mechanism of this reaction describes what happens at the molecular level. Two NO2 molecules collide, forming a collision complex. In this collision complex, a bond may form between the two nitrogen atoms, producing an N2 O4 molecule. 

A mechanism is a description of the actual molecular events that occur during a chemical reaction. Each such event is an elementary reaction. Elementary reactions involve one, two, or occasionally three reactant molecules or atoms. In other words, elementary reactions can be unimolecular, bimolecular, or termolecular. A typical mechanism consists of a sequence of elementary reactions. Although an overall reaction describes the starting materials and final products, it usually is not elementary because it does not represent the individual steps by which the reaction occurs. 

The molecular view represents a set of initial conditions for the reaction described in Example. Each molecule represents a partial pressure of 1.0 bar. Determine the equilibrium conditions and redraw the picture to illustrate those conditions. 

The platinum metals are valuable by-products from the extraction of common metals such as copper and nickel. The anodic residue that results from copper refining is a particularly important source. The chemistry involved in their purification is too complicated to describe here, except to note that the final reduction step involves reaction of molecular hydrogen with metal halide complexes. 

C21-0022. Draw molecular pictures showing a t q)ical Lewis acid-base reaction and a typical Br< )nsted acid-base reaction. Describe in words the differences and similarities of these two reactions. 

The condensation reactions described above are unique in yet another sense. The conversion of an amine, a basic residue, to a neutral imide occurs with the simultaneous creation of a carboxylic acid nearby. In one synthetic event, an amine acts as the template and is converted into a structure that is the complement of an amine in size, shape and functionality. In this manner the triacid 15 shows high selectivity toward the parent triamine in binding experiments. Complementarity in binding is self-evident. Cyclodextrins for example, provide a hydrophobic inner surface complementary to structures such as benzenes, adamantanes and ferrocenes having appropriate shapes and sizes 12) (cf. 1). Complementary functionality has been harder to arrange in macrocycles the lone pairs of the oxygens of crown ethers and the 7t-surfaces of the cyclo-phanes are relatively inert13). Catalytically useful functionality such as carboxylic acids and their derivatives are available for the first time within these new molecular clefts. 

Observed monomer concentrations are presented by Figure 2 as a function of cure time and temperature (see Equation 20). At high monomer conversions, the data appear to approach an asymptote. As the extent of network development within the resin advances, the rate of reaction diminishes. Molecular diffusion of macromolecules, initially, and of monomeric molecules, ultimately, becomes severely restricted, resulting in diffusion-controlled reactions (20). The material ultimately becomes a glass. Monomer concentration dynamics are no longer exponential decays. The rate constants become time dependent. For the cure at 60°C, monomer concentration can be described by an exponential function. 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

Molecularity must be integral, but order need not be there is no necessary connection between molecularity and order, except for an elementary reaction the numbers describing molecularity, order, and stoichiometry of an elementary reaction are all the... 

Adsorption kinetics involve a time-dependent process that describes the rate of adsorption of chemical contaminants on the solid phase. The standard chemical meaning of kinetics usually covers the study of the rate of reactions and molecular processes when transport is not a limiting factor however, this definition is not... 

In the reactions described in Section III,C, a three-atom fragment in the heterocycle is replaced by the same atoms of the reagent and with the same sequence of atoms. This type of reaction has been referred to as an inter-molecular transfragment reaction (67JA6911) and therefore can be clearly distinguished from molecular rearrangements, such as the Dimroth reactions (68MI1 see also Chapter IV) and photochemical transposition reactions. 

Application of the coupling reactions described in the previous section has given many organometallic complexes of ligands containing diyne fragments, which may not be directly coordinated to a metal center. The following surveys this area briefly and describes some recent applications of these systems in the construction of novel molecular architectures. 

With one exception, naphthalen-l,4-imines with a double bond between C-2 and C-3 are not known to dissociate thermally by either possible retro-Diels-Alder pathway (the reverse of reactions described in Section III, A, 1 and 2), and the enthalpy requirements for the formation of a benzyne or an acylic acetylene are doubtless unfavorable. However, the mass spectra of compounds 93-99 reveal one important fragmentation of the molecular ions to be loss of dimethyl acetylene-dicarboxylate, and another fragmentation pathway involves the formation of nitrilium ions MeC=NR and PhC=NR from 93-95 and 96-99, respectively. ... 

The course of Cyt P450 catalysis is in principle well understood. The most important function of the heme group consists of converting molecular oxygen into an especially reactive atomic form, which is responsible for all of the reactions described above. 

Albeit a number of conclusions can be gained from parity considerations, chemistry is described in terms of real space variables and particle ideology. Reaction coordinate, molecular species, molecular structure and properties are to be related to the present approach. This cannot be made rigorously because quantum mechanics is about quantum states and not objects in real space. This confusion has been fatal to a correct understanding of molecular phenomena in spite of the effort made by Primas [15]. 

Hydrolysis or complexation The following very schematic reactions describe the most common representation of a solution process, starting from a stable stock solution of some metal complexes to which reagents are added. The first step consists in the substitution or addition of molecular groups in the coordination sphere of a cation M. Most common reactions are (9) ... 

The generalization of the reaction described by Kammerer and Bohmer, using p-cresylformaldehyde polymers with higher molecular weight as templates, may lead to poly(amino acids) with higher molecular weight. 

The reaction described above can also be carried out at higher concentration whereby the probability of intramolecular reaction (cyclization) vanishes. So called chain extension processes result from the stoichiometric reaction of a "living" bifunctional precursors with an efficient bifunctional electrophilic deactivator. This polycondensation reaction induces a very large increase of the molecular weight, but is also results in an enhanced polydispersity. - Fractionation is necessary if well defined substances are required. However the average distance between successive hinges along the chain fluctuates only very little. 

Daylight has extended SMILES rules to accommodate general descriptions of molecular patterns and chemical reactions (13). These SMILES extensions are called SMARTS and SMIRKS. SMARTS is a language for describing molecular patterns while SMIRKS defines rules for chemical reaction transformations. 

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