Hydrogenation, Fermentation, and Polymerization: Chemical Processes
Hydrogenation
Hydrogenation is a type of chemical reaction (redox) in which the visible end result is the addition of hydrogen (H2) to another compound. The usual targets of this reaction are unsaturated organic compounds such as alkenes, alkynes, ketones, nitriles, and amines. Most hydrogen is produced by direct addition of diatomic hydrogen under pressure and in the presence of a catalyst.
A typical example of hydrogenation is the addition of hydrogen to double bonds, converting alkenes to alkanes. Hydrogenation has important applications in the pharmaceutical, petrochemical, and food industries.
Fermentation
Fermentation is a catabolic process of incomplete oxidation, a completely anaerobic process, the end product being an organic compound. These end products are those that characterize the different types of fermentations. It was discovered by Louis Pasteur, who described it as “la vie sans l’air” (life without air). Fermentation is typically carried out by yeasts. Also, some metazoans and protists are able to do it. The fermentation process is anaerobic because it occurs in the absence of oxygen, which means that the final acceptor of electrons from NADH produced in glycolysis is not oxygen, but an organic compound that is reduced to re-oxidize NADH to NAD+. The organic compound that is reduced (acetaldehyde, pyruvate, etc.) is a derivative of the substrate that has been oxidized previously. In living organisms, fermentation is an anaerobic process and it does not involve the mitochondria and the respiratory chain. They are typical of microorganisms such as some bacteria and yeasts. Fermentation also occurs in most cells of animals (including humans), except in the neurons that die quickly if they cannot perform cellular respiration. Some cells, like red blood cells, lack mitochondria and are forced to ferment. The muscle tissue of animals performs lactic fermentation when oxygen delivery to muscle cells is not sufficient for aerobic metabolism and muscle contraction. From the energy standpoint, fermentations are not very profitable when compared with aerobic respiration, as from a glucose molecule can be obtained only 2 molecules of ATP, while in aerobic respiration 36 are produced. This is due to oxidation of NADH, instead of entering the respiratory chain, transfers electrons to organic compounds with low oxidizing power.
In the fermentation industry, fermentation can be oxidative, i.e., in the presence of oxygen, but aerobic oxidation is incomplete, as in the production of acetic acid from ethanol.
Fermentation can be:
- Natural: when environmental conditions allow the interaction of microorganisms and susceptible organic substrates.
- Artificial: when humans lead and control the contact conditions mentioned above.
Polymerization
Polymerization is a chemical process by which reactive monomers (low molecular weight compounds) are grouped together chemically, leading to a high molecular weight molecule called a polymer, which can be a linear chain or a three-dimensional macromolecule.
There are many types of polymerization and various systems to categorize them. The main categories are:
Addition Polymerization and Condensation
Polymerization is by addition if the monomer molecule becomes part of the polymer without loss of atoms, i.e., the chemical composition of the resulting string is equal to the sum of the chemical compositions of the monomers that form it. Condensation polymerization occurs if the monomer molecule loses atoms when it becomes part of the polymer. Usually, a small molecule, such as water or HCl gas, is lost. Condensation polymerization generates byproducts.