Keeping Track of Reactions

Chapters 7-10 have introduced three basic kinds of organic reactions nucleophilic substitution, P elimination, and addition. In the process, many specific reagents have been discussed and the stereochemistry that results from many different mechanisms has been examined. How can we keep track of all the reactions  

To make the process easier, remember that most organic molecules undergo only one or two different kinds of reactions. For example  

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One may wonder why it is important to distinguish between and keep track of these two energies and Dq, when it seems that one would do. Actually, both are important. The bond energy Dg dominates theoretical comparisons and the dissociation energy Dq, which is the ground state of the real molecule, is used in practical applications like calculating thermodynamic properties and reaction kinetics. 

The circle m a hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the aromatic sextet —the six delocalized TT electrons of the three double bonds Robinson s symbol is a convenient time saving shorthand device but Kekule type formulas are better for counting and keeping track of electrons especially m chemical reactions... 

The number of reactions in this chapter is quite large and keeping track of all of them can be difficult—or it can be manageable The key to making it manageable is the same as always structure determines properties... 

It is easier to keep track of what species are in solution if we write down the reactions that control the solution s composition. These reactions are the dissolution of a soluble salt... 

As a consequence of these reactivity differences, it s usually possible to convert a more reactive acid derivative into a less reactive one. Acid chlorides, foi instance, can be directly converted into anhydrides, thioesters, esters, and amides, but amides can t be directly converted into esters, thioesters, anhydrides, or acid chlorides. Remembering the reactivity order is therefore a way tc keep track of a large number of reactions (Figure 21.2). Another consequence, a noted previously, is that only acyl phosphates, thioesters, esters, and amides are... 

Redox reactions are more complicated than precipitation or proton transfer reactions because the electrons transferred in redox chemishy do not appear in the balanced chemical equation. Instead, they are hidden among the starting materials and products. However, we can keep track of electrons by writing two half-reactions that describe the oxidation and the reduction separately. A half-reaction is a balanced chemical equation that includes electrons and describes either the oxidation or reduction but not both. Thus, a half-reaction describes half of a redox reaction. Here are the half-reactions for the redox reaction of magnesium and hydronium ions ... 

To summarize, the equation for a nuclear reaction is balanced when the total charge and total mass number of the products equals the total charge and total mass number of the reactants. This conservation requirement is one reason why the symbol for any nuclide includes its charge number (Z) as a subscript and its mass number as a superscript. These features provide a convenient way to keep track of charge and mass balances. Notice that in the equation for neutron decay, the sum of the subscripts for reactants equals the sum of the subscripts for products. Likewise, the sum of the superscripts for reactants equals the sum of the superscripts for products. We demonstrate how to balance equations for other reactions as they are introduced. 

Besides the resuspension of particles, the perfect sink model also neglects the effect of deposited particles on incoming particles. To overcome these limitations, recent models [72, 97-99] assume that particles accumulate within a thin adsorption layer adjacent to the collector surface, and replace the perfect sink conditions with the boundary condition that particles cannot penetrate the collector. General continuity equations are formulated both for the mobile phase and for the immobilized particles in which the immobilization reaction term is decomposed in an accumulation and a removal term, respectively. Through such equations, one can keep track of the particles which arrive at the primary minimum distance and account for their normal and tangential motion. These equations were solved both approximately, and by numerical integration of the governing non-stationary transport equations. 

In the flow-through model, any mineral mass present at the end of a reaction step is sequestered from the equilibrium system to avoid back-reaction. At the end of each step, the model eliminates the mineral mass (including any sorbed species) from the equilibrium system, keeping track of the total amount removed. To do so, it applies Equation 13.11 for each mineral component and sets each nk to a vanishingly small number. It is best to avoid setting nk to exactly zero in order to maintain the mineral entries Ak in the basis. The model then updates the system composition according to Equations 13.5-13.7 and takes another reaction step. 

A stoichiometric table for keeping track of the amounts or flow rates of all species during reaction may be constructed in various ways, but here we illustrate, by means of an example, the use of , the extent of reaction variable. We divide the species into components and noncomponents, as determined by a stoichiometric analysis (Section 5.2.1), and assume experimental information is available for the noncomponents (at least). 

DR. MEYER In electron transfer one can describe the problems in terms of well defined theoretical models. In the type of reactions which you are treating, you have found this nice empirical way to put things together but there is no fundamental basis for it. It is just a way to keep track of experimental facts. 

The traditional approach is to keep track of the amounts of the various chemical species in the system. At each point in time, the hydrogen ion concentration is calculated by solving a set of simultaneous nonlinear algebraic equations that result from the chemical equilibrium relationships for each dissociation reaction. 

The term on the left side of the equation is the accumulation term, which accounts for the change in the total amount of species iheld in phase /c within a differential control volume. This term is assumed to be zero for all of the sandwich models discussed in this section because they are at steady state. The first term on the right side of the equation keeps track of the material that enters or leaves the control volume by mass transport. The remaining three terms account for material that is gained or lost due to chemical reactions. The first summation includes all electron-transfer reactions that occur at the interface between phase k and the electronically conducting phase (denoted as phase 1). The second summation accounts for all other interfacial reactions that do not include electron transfer, and the final term accounts for homogeneous reactions in phase k. 

Equations used to calculate percent yield or dilution ratios A list of disposable equipment (e.g., rubber gloves, Bunsen burners) Step-by-step instructions of the procedure Warnings to other scientists about unusual hazards Quantitative statements of reaction times and temperatures Descriptions of the physical appearances of synthesis products IR or NMR data confirming product purity Statistical packages used (including the name of the software) Reports of other software used to keep track of data (e.g., Excel)... 

Oxidation states A concept that provides a way to keep track of electrons in oxidation-reduction reactions according to certain rules. 

The overall strategy for this calculation is to replace the partial pressures that appear in K by the molar concentrations, and thereby generate Kc. We need to keep track of the units so we write activities as Pj/bar and molar concentrations as Q]/(mol-L 1), as explained in the earlier side-notes. We consider a specific case the relation between K and Kc for the ammonia equilibrium, reaction C. 

Data storage in REACCS is hierarchical related data are stored separately but also are grouped under a single descriptive category. For example, in the Theilheimer database, the treename is the complete hierarchical name of a piece of data and is composed of three components entity, parent datatypes or category of data, and field datatype. All REACCS databases include VARIATION. VARIATION is usually the highest parent datatype in the reaction hierarchy. VARIATION can be used to store more than one complete set of reaction data with a reaction. To keep track of the data associated with different variations or multiple reactants and products in the same reaction, line numbers are appended to some of the datatypes in a treename. 

How can you tell when a redox reaction is taking place The answer is that we assign to each atom in a substance a value called an oxidation number (or oxidation state), which indicates whether the atom is neutral, electron-rich, or electron-poor. By comparing the oxidation number of an atom before and after reaction, we can tell whether the atom has gained or lost electrons. Note that oxidation numbers don t necessarily imply ionic charges. They are simply a convenient device to help keep track of electrons in redox reactions. 

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