In the Kilomentor blogs particular terms are used. Their definitions are given here:
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 consumes either substrate or reagents or both and is
responsible for some loss from 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 interference from the effectiveness of the isolation of
the pure product.
Recovery yield is the ratio of quantity of isolated
product of adequate purity to take to the next step as a fraction of the
calculated quantity of product material determined by analysis of the reaction
mixture at the time when the reaction is deemed complete; such ratio 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 goal of
this measure 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.
A 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 consists
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.
Convergent synthesis describes a synthetic
route in which large pieces of a chemical structure are assembled as individual
targets and then these large pieces are couple together in the final operations
of the synthesis.
Convergent syntheses
are mathematically more efficient than linear syntheses because the longest
reaction path in a convergent synthesis is shorter than in a linear route. 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.
A phase switch 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 or isolation to a
second phase. Most frequently this is a
transfer from one liquid medium to a second immiscible liquid medium as in an
aqueous-organic extraction, but a phase switch could for example 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 switches the chemical intermediates pass through the more robust the separations in the
process are, because at each phase switch there is the potential to leave
impurities behind in the second phase
.
Some Phase Switches
Freeze drying
Adsorbtion of
aromatic-like materials on charcoal
Solubility/insolubility
in methanol (inorganic salts)
Silver nitrate
complex formation in solution or adsorbed on silica gel
Steam distillation
Crystallization
Short path
distillation-molecular distillation
Counter-current
extraction
Solubility
insolubility in ether, cyclohexane, carbon tetrachloride or toluene
Co-distillation with
a high boiling hydrocarbon
Addition of saturated
salt solution to a DMF solution-precipitating the product and salt
Dissociation
extraction
Derivatizing agents which give precipitated
or extractable solids
Derivatizing
polymeric materials
Sodium bisulfite
derivatives of aldehydes
Claisen alkali
Sulfur trioxide
Fuming sulphuric acid
Calcium chloride,
calcium bromide, lithium bromide complexes
Enamines of ketones
Girard P or Hydrazinobenzenesulfonic
acid
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