Reactivity and stability of organic molecules

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 methods for identifying and measuring functions and, through them, 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 bulk of the molecule.

The information provided by this study is of the utmost importance. It's obvious that a function with low reactivity 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 enables us to identify the molecule's weak points. For example, camphor is stable, and we know the low reactivity of its ketone function to produce a dinitrophenylhydrazone, which could be the cause of its instability.

The study of the reactivity of organic molecules is essential in the pharmaceutical, cosmetics and agri-food sectors, where we need to predict the evolution of product quality under the influence of external factors likely to transform them, such as temperature, humidity, light, oxygen and pH, in order to define storage and transport conditions and expiration dates. A wide range of reactions can be implemented, such as racemization, isomerization, cyclization, hydrolysis, oxidation-reduction, esterification, photolysis, biochemical or enzymatic reactions.

It should also be noted that the study of a molecule's reactivity can sometimes be used to predict a drug's metabolism: 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 strength) provide a first working hypothesis for predicting drug degradation in the body.

There is no universal study protocol, so reactivity studies must be adapted to the objectives. For example, reactivity studies to verify the stability of pharmaco-active substances or drugs in adverse climatic conditions will be carried out under specific temperature and humidity conditions. However, three factors are of particular importance: light, temperature and air. Light appears to be the most important factor, while higher temperatures merely accelerate the phenomena already induced, and the presence of oxygen in the air causes oxidation. The study of reactivity includes a study of the reaction mechanism, but it is not enough simply to identify degradation products; these must be isolated and identified, and their toxicity studied.

When it comes to analytical techniques, it's essential...