The oxidation of linseed oil, which causes it to heat up and produce volatile organic compounds and larger molecules, is based on a mechanism that involves the initiation and propagation of radical chains by a highly reactive form of oxygen called singlet oxygen (1�g). This review explains how this mechanism works and how it affects the properties and applications of linseed oil. It also examines the role of metal complexes that contain cobalt, iron or manganese atoms in speeding up the oxidative drying of linseed oil, which makes it harden and form a protective film. It provides some numerical values for the rate constants of the reactions that form peroxides, which are intermediates in the oxidation process.
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Linseed oil is a natural product obtained from the seeds of the flax plant (Linum usitatissimum). It has been used for centuries as a drying oil for paints, varnishes and wood finishes. It is also used as a nutritional supplement, a laxative and a component of linoleum. Linseed oil contains a high proportion of unsaturated fatty acids, mainly linolenic acid (18:3) and linoleic acid (18:2), which make it susceptible to oxidation by atmospheric oxygen.
Oxidation of linseed oil is a complex process that involves free radical reactions. Free radicals are atoms or molecules that have unpaired electrons and are very reactive. They can be formed by various sources, such as heat, light or chemical agents. One of the most important sources of free radicals in the oxidation of linseed oil is singlet oxygen (1�g), which is an excited state of molecular oxygen (3Σg) that has a different electronic configuration and higher energy. Singlet oxygen can be generated by photochemical reactions or by metal complexes.
Singlet oxygen can initiate the oxidation of linseed oil by abstracting a hydrogen atom from one of the unsaturated fatty acids, forming a hydroperoxide radical (ROO�). This radical can then react with another molecule of linseed oil, forming a hydroperoxide (ROOH) and a carbon-centered radical (R�). The carbon-centered radical can then react with molecular oxygen, forming another hydroperoxide radical and continuing the chain reaction. This process leads to the formation of various volatile organic compounds, such as aldehydes, ketones and alcohols, which contribute to the odor and flavor of linseed oil. It also leads to the formation of higher molecular weight compounds, such as polymers and cross-linked networks, which increase the viscosity and hardness of linseed oil.
The oxidation of linseed oil can be accelerated by the presence of metal complexes, which act as catalysts. Catalysts are substances that increase the rate of a chemical reaction without being consumed or changed in the process. Metal complexes are compounds that consist of a metal atom or ion surrounded by other atoms or molecules, called ligands, that are attached to it by coordinate bonds. Metal complexes can have different shapes, charges and properties depending on the type and number of ligands.
Some of the most common metal complexes used in the oxidative drying of linseed oil are cobalt, iron and manganese complexes. These metal complexes can generate singlet oxygen by transferring electrons to molecular oxygen in a process called autoxidation. They can also decompose hydroperoxides into radicals, which can propagate the chain reaction. The metal complexes can also coordinate with the unsaturated fatty acids in linseed oil, forming metal-organic complexes that can undergo further oxidation and polymerization reactions.
The use of metal complexes in the oxidative drying of linseed oil has some advantages and disadvantages. On one hand, they can improve the drying time, the film formation and the durability of linseed oil-based products. On the other hand, they can also cause some undesirable effects, such as discoloration, yellowing and cracking of the film. The optimal amount and type of metal complex depends on various factors, such as the composition of linseed oil, the environmental conditions and the desired properties of the final product. 8cf37b1e13