2023-10-25 22:00:00
Considered from the angle of the physical, physicochemical or chemical properties common to a part of the molecule, an atom or a group of atoms, the study of reactivity encompasses all the methods of identification and determination of functions and, through them, of the molecule itself. But knowledge of reactivity is also and above all the study of the properties of functional groups, the influence of other functions and the size of the molecule.
The information provided by this study is of vital importance. It is, in fact, obvious that a poorly reactive function within the molecule will be less sensitive to the external environment such as air, humidity, excipients or solvents. Conversely, knowledge of the reactivity specific to each function or attributable to its interactions allows us to know the weak points of the molecule. For example, camphor is stable and we know the low reactivity of its ketone function to obtain a dinitrophenylhydrazone likely to be the cause of its instability.
The study of the reactivity of organic molecules is essential in the pharmaceutical, cosmetic or agri-food fields where we must predict the evolution of the quality of products under the influence of external factors likely to transform them such as temperature, humidity, light, oxygen, pH, in order to define the storage and transport conditions, the expiration date. Multiple reactions can be implemented such as racemization, isomerization, cyclization, hydrolysis, redox, esterification, photolysis, biochemical or enzymatic reactions.
It should also be noted that the study of the reactivity of the molecule sometimes makes it possible to predict the metabolism of a drug: the successive degradations that an organic molecule undergoes in vitro under the influence of various reagents or in conditions close to biological conditions. (pH, ionic force) allow a first working hypothesis for predicting the degradation of the drug in the body.
There is no universal study protocol; therefore, responsiveness studies must be fit for purpose. For example, reactivity studies to check the stability of pharmacoactive substances or drugs in an unfavorable climatic environment will be carried out under specific temperature and humidity conditions. However, three factors are still of particular importance: light, temperature and air. Light appears to be a primordial factor, the rise in temperature only accelerates the phenomenon already induced, the air, through the presence of oxygen, causes oxidation. The study of reactivity includes the study of the reaction mechanism but we cannot be satisfied with a simple demonstration of degradation products; these must be isolated and identified, and their toxicity studied.
In terms of analytical technique, it is necessary to surround oneself with a body of evidence chosen according to the reaction to be elucidated and not according to the performance of the devices. The analyst must thus be able to choose the means appropriate to the study of the reactivity of organic species, adapt them to the goal pursued and apply them in particular areas of chemical activity.
After a brief reminder of chemical kinetics, we will consider, through examples, the influence of each of the main factors involved in the degradation of organic molecules.
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