Molecular oxygen is the greenest of all possible oxidizing agents, but its specific electronic configuration eliminates the possibility of its direct reaction with organic substances. Single-electron transfer between triplet O2 and singlet organics is prohibited according to the Wigner spin selection rule. Moreover, attaching the first electron is disadvantageous even from a thermodynamic point of view. The authors will not consider the high-temperature processes, however, and will focus on the mechanisms for generating active oxygen species under normal temperature conditions. All methods of “soft” activation of oxygen can be conditionally divided into three main groups: photo-excitation; direct or sensitized initiation of a radical-chain oxidative process; and catalysis by transition metal ions with the inclusion of oxygen in the coordination sphere. New perspectives on molecular oxygen activation pathways and their application in some oxidative reactions including degradation of organic pollutants, organic synthesis, and environmentally significant reactions are discussed.
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Oxygen is the most common element on Earth. Its content in the earth's crust exceeds 46% and is in absolute terms ~ 2.8×1019 tons. Slightly less, namely ~ 1.15×1018 tons of oxygen is contained in ocean water. However, only a negligible part of it e.g., at the level of 0.004% of the total content is represented in the atmosphere in free form, as gaseous molecular oxygen. Hence, there are approximately 1.18×1015 tons of oxygen, which is relatively small compared to its content in water or sand but nevertheless more than enough, to maintain life on Earth1,2. Note that one kilogram of biomass (from the simplest phytoplankton as representatives of the genus homo sapiens) accounts for about five hundred kilograms of pure oxygen, which for a medium-sized person would be enough (based on a norm of 700 g per 70 kg of weight) for 137 years or two lives - this is without “recharging” and refueling due to the activities of oxygen-extracting plants.
It is intuitive to draw attention to the widespread misconception that forests (and primarily in the Amazon or Siberia) are the “lungs” of the planet, supplying oxygen to the rest of life on Earth. In fact, that's not the case. Indeed, a third of all trees in the world are concentrated on more than 6 million hectares of Siberian taiga, although this ecosystem has only a minor impact on the level of carbon dioxide in the atmosphere. It is noteworthy that Taiga is fully lit by the sun for no more than one month a year because it takes any tree about 50 years to grow here! Amazon forests - the largest rainforest array on the planet - do not have such a problem. This huge green massif stretched across the territories of eight countries, and it surpasses all the rainforests taken together in its area. According to experts, the juicy greens of the Amazon basin produce a fifth of the Earth's oxygen. At first glance, this seems a lot, however, it is necessary to assess based on the scientific data.
In fact, oxygen is produced not only by those plants that grow in the forest. All plant organisms, including the inhabitants of water bodies, and residents of the steppes, and deserts constantly produce oxygen. Plants, unlike animals, fungi, and other living organisms, can synthesize organic substances, using the energy of light for this. This process is called photosynthesis. As a result of photosynthesis, oxygen is released. It is a byproduct of photosynthesis. Oxygen is released very, very much, in fact, 99% of the oxygen that is present in the Earth's atmosphere is of plant origin. And only 1% comes from the mantle, the underlying layer of the Earth.
On average, approximately 1 hectare of forests annually releases 4 tons of oxygen and consumes 5 tons of carbon dioxide. A person exhales up to 1 kilogram of carbon dioxide per day, 365 kg per year. Consequently, 1 hectare of forest absorbs carbon dioxide, which is exhaled by 13 people. However, forest plants not only release oxygen but also consume it, and it's not just and also not so much about daily biorhythms. Yes, indeed, if during the day a plant releases oxygen under the influence of photosynthesis, converting carbon dioxide into it, then at night the same process (in fact, not exactly this, but its dark chemical component) goes in the opposite direction: oxygen is consumed, and carbon dioxide is released.
In an extremely simplified form, these gross processes can be represented as: