Products reaction path

Given that the objective is to manufacture a certain product, there are often a number of alternative reaction paths to that product. Reaction paths which use the cheapest raw materials and produce the smallest quantities of byproducts are to be preferred. Reaction paths which produce significant quantities of unwanted byproducts should especially be avoided, since they create significant environmental problems. 

If not available in published references, the heat of reaction can be calculated from the difference in heats of formation of the reactants and products, as described in any college chemistry text. The accuracy of the calculation obviously depends on knowledge of the reaction products, reaction path and the accuracy of the heat of formation data. Any changes of state (heats of solution, vaporization, etc.) must also be taken into account. Not taking account of these changes of state can have a marked effect on the calculation and may lead to a false sense of security (HSE 2000) or an overestimation of the hazard. 

The close-coupling approach works readily and simply if the reaction is purely melastic . The method can also be made to work very simply for a single product arrangement (as in collinear reactions), by using a twisted coordinate system, most conveniently reaction path coordinates [37, 38 and 39] as shown in figure B3.4.3. 

Figure B3.4.3. A schematic figure showing, for the DH2 collinear system, a reaction-path coordmate Q coimecting continuously the reactants and the single products asymptote. Also shown are the cuts denoting the coordinate perpendicular to Q.

Some fraction of such events will lead to the system remaining on the Odd surface until, further along the reaction path, the Odd surface again intersects the Even surface on the product side at which time quenching to produce ground-state products can occur. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

The issue of regioselectivity arises with arylhydrazones of unsymmetrical ketones which can form two different enehydrazine intermediates. Under the conditions used most commonly for Fischer cyclizations, e g. ethanolic HCI, the major product is usually the one arising from the more highly substituted enehydrazine. Thus methyl ketones usually give 2-methy indoles and cycliz-ation occurs in a branched chain in preference to a straight chain. This regioselectivity is attributed to the greater stability of the more substituted enhydrazine and its dominance of the reaction path. 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Generally, the sulfonation of naphthalene leads to a mixture of products. Naphthalene sulfonation at less than ca 100°C is kineticaHy controlled and produces predominandy 1-naphthalenesulfonic acid (4). Sulfonation of naphthalene at above ca 150°C provides thermodynamic control of the reaction and 2-naphthalenesulfonic acid as the main product. Reaction conditions for the sulfonation of naphthalene to yield desired products are given in Figure 1 alternative paths are possible. A Hst of naphthalenesulfonic acids and some of their properties is given in Table 1. 

A secondary reaction path produces by-product chlorate and has a proposed net reaction of ... 

Techniques for the synthesis of reaction paths on the basis of enumeration of all possible reactions are fairly weU developed. The SYNCHEM2 system conducts a backward search of possible reaction paths from a desired product (76). Because the procedure is limited by the rapid growth of... 

In the investigation of a ring enlargement, discrimination was possible between the reaction paths via a 1,3-dipole (142) and a quaternary acyl product (143), precursors of different triazolidones. Exclusive formation of (144) from (141) and phenyl isocyanate decided in favor of the dipolar intermediate (142) (81JOC320). 

There is a great deal of flexibility in the choice of laser radiation in the production of thin Aims by photochemical decomposition, and many routes for achieving the same objective can be explored. In most reactions of indusuial interest the reaction path is via tire formation of free radicals as intermediates, and the complete details of the reaction patlrs are not adequately defined. However, it may be anticipated that the success of the photochemical production of new materials in tlrin fllms and in fine powder form will lead to considerably greater effort in the elucidation of these kinetics. 

Figure 7 The discretized reaction path is represented by a number of intermediate configurations of the system connecting the fixed reactant (1, coil) and product (12, helix) states.

Enzymes increase the rate of chemical reactions by decreasing the activation energy of the reactions. This is achieved primarily by the enzyme preferentially binding to the transition state of the substrate. Catalytic groups of the enzyme are required to achieve a specific reaction path for the conversion of substrate to product. 

Because these various quantities are characteristics of the reactants and products but are independent of the reaction path, they cannot provide insight into mechanisms. Information about AG, AH, and AS does, however, indicate the feasibility of any specific reaction. The enthalpy change of a given reaction can be estimated from tabulated thermochemical data or from bond-energy data such as those in Table 1.3 (p. 14) The exan le below illustrates the use of bond-energy data for estimating the enthalpy of a reaction. 

This description provides information, via conventional structures, about the constitution of reactants, products, and the intermediate. Transition state structures are more provisional and may attempt to show the electronic distribution and flow in this region of the reaction path. The curved arrow symbolism is often used, as shown in structure 1 for the first elementary reaction. 

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