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kilomentor | 18 January, 2007 09:35
A chemical substrate is a reactant in a chemical transformation, which contains a large share of the atoms, which are intended to be retained in the target structure.
A chemical reagent is a reactant in a chemical transformation which is relatively inexpensive, usually commercially available or available in just a few steps from cheap commercial materials and which contains a large proportion of atoms which are not intended to be retained in the target structure.
A by-product is a chemical substance, which is formed during an attempted process step, which is not an intended or expected product. Most frequently a by-product when it is produced it consumes either substrate or reagents or both and is responsible for a significant or insignificant loss in the theoretical yield.
A co-product is a product of a chemical transformation which according to the stoichiometry must be formed at the same time as the product and which is related in the rate at which it forms to the rate of formation of the product.
A catalyst is a material which changes the rate of a reaction but which is not consumed by the reaction. True catalysts are needed in only amounts of 1-10 molar percent.
Assay yield is the calculated quantity of product material determined by analysis of the reaction mixture at the time when the reaction is deemed complete divided by the theoretical quantity of material that would be present after a quantitative conversion based on the limiting ingredient according to the assumed stoichiometry as a percentage. The significance of the assay yield is that it is a measure of the completeness of the reaction process without any confounding influence from the effectiveness of the isolation of the pure product.
Recovery yield is the quantity of the isolated product of adequate purity to take to the next step divided by the calculated quantity of product material determined by analysis of the reaction mixture at the time when the reaction is deemed complete, expressed as a percentage The significance of the recovery yield is that it is a pure measure of the efficiency of the isolation of the desired product from the reaction mixture.
Overall yield is the product of the assay yield represented as a fraction and the recovery yield represented as a fraction, converted to a percentage. This is the classical chemical yield that is described in the chemical literature.
Practical purity is the purity and the distribution of impurities, which is acceptable in an intermediate in order to obtain the required degree of purity in the target product. The significance of practical purity is that it a pragmatic objective. A low absolute purity of an intermediate is acceptable if the impurities are removed during the course of the subsequent steps or are removed in subsequent necessary purification operations. Another objective of the definition is to highlight that it is wasted effort to introduce purification operations for an intermediate, when the impurities, which are being removed would be removed in subsequent manipulations.
Process step is defined as all the unit operations, which are combined in order to go from one point where an intermediate can be accumulated, stored and analyzed to another such point in a batch process. A process step differs from a reaction step. A reaction step cosmists of all the operations required in practice to go from a set of isolated starting materials and reagents to another isolated pure substance, which is on the reaction path.
A phase shift in a reaction or isolation is the mass transfer of a chemical intermediate, co-product, by-product or reagent from a first phase where it was resident during a reaction of isolation to a second phase. Most frequently this is a transfer from on liquid medium to a second immiscible liquid medium as in an aqueous-organic extraction, but a phase shift could be a distillation in which the volatile materials are converted from liquid to gas or a crystallization or precipitation where a substance passes from a liquid solution into a solid. The importance according to Curran who popularized the concept, is that the more phase shifts the chemical intermediates pass through the more robust the separations in the process are, because at each phase shift there is the potential to leave impurities behind in the first phase.
Convergent synthesis describes a synthetic route in which large pieces of a chemical structure are assembled as individual targets and then the large pieces are couple together in the final operations of the synthesis. Convergent syntheses are mathematically more efficient that linear syntheses because the longest reaction path in a convergent synthesis is shorter than in a linear route and this is important, because the overall yield in a chemical synthesis is the product of the overall yields (each expressed as a decimal fraction). The more linear steps, the more fractions that must be multiplied and the smaller the product will be.
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