Radical | Free Radical | Chemistry | thetutee

Radical

In chemistry, a radical is a molecule with at least one unpaired electron, sometimes known as a free radical. Most molecules have an even number of electrons, and the covalent chemical bonds that hold the atoms in a molecule are usually made up of pairs of electrons shared by the atoms joined by the bond. Most radicals are thought to have generated via the cleavage of conventional electron-pair bonds, with each cleavage producing two distinct entities, each containing a single unpaired electron from the broken link (in addition to all the rest of the normal, paired electrons of the atoms).

Stable Radical

Free radicals may be electrically neutral while having unpaired electrons. Free radicals are frequently very reactive due to their odd electrons. They react with intact molecules, abstracting parts of the molecules to complete their own electron pairs and generating new free radicals in the process, or they combine with one another or with single atoms that also carry free electrons to produce ordinary molecules with all of their electrons paired. Because of its single unpaired electron, any simple free radical can join with another radical or atom possessing a single unpaired electron in all of these processes. Diradicals with unpaired electrons on each of two atoms (providing an overall even number of electrons) can be generated under specific conditions, and these diradicals have a combining power of two.

Some free radicals are stabilised by their distinctive structures; given the correct conditions, they may remain for long periods of time. Most free radicals, particularly simple ones like methyl (CH3) and ethyl (C2H5), are only capable of a brief independent existence.Moses Gomberg developed the first somewhat stable free radical, triphenylmethyl, in 1900.

 The core carbon in this molecule is trivalent because it has three substituents rather than four, and its unshared electron is indicated by a dot. Triphenylmethyl free radicals are only stable in specific organic solvents; in the presence of air, water, or strong acids, they are promptly destroyed via irreversible processes. Free radicals are produced by the breakdown of the nitrogen–nitrogen bond in aromatic hydrazines with the general structure R2NNR2, or the core nitrogen–nitrogen link in aromatic tetrazanes with the general structure R2NRNNRNRNR2. As a result,  1,1-diphenyl-2-picrylhydrazyl exists as a stable violet solid. Similar free radicals, such as the 2,4,6-tri-tert-butylphenoxy radical, are also known, but with the odd electron on oxygen.

When a chemical like benzophenone is treated with metallic sodium to produce the coloured compound (C6H5)2CO-, another sort of stable radical ion is formed: a metal ketyl. Sodium interacts with complex aromatic hydrocarbons, such as naphthalene, to produce brightly coloured radical ions. Organic free radicals with the group > NO form a last class of generally stable organic free radicals. Diphenylnitrogen oxide (C6H5)2NO, for example, is produced by oxidising diphenylhydroxylamine (C6H5)2NOH.

The presence of stable free radicals appears to be dependent on certain structural properties. The semiquinone radical ion demonstrates one particularly important criterion. The higher oxygen atom has a negative charge, whereas the bottom one contains an odd electron, as shown. Molecular structure, however, is arbitrary, and the same molecule would be represented if the charge and the odd electron were swapped. 

When this occurs, the actual average distribution of electrons inside the molecule is assumed to be intermediate between the two configurations just described. Delocalization, or resonance, is a phenomenon that, according to quantum physics, greatly enhances the stability of a material and, in this case, the likelihood of its existence. The stability of the other free radicals outlined before is explained by similar reasoning.

Unstable Free Radical

Simple free radicals like methyl (CH3) exist and perform important roles as transitory intermediates in a variety of chemical processes. Friedrich A. Paneth and W. Hofeditz originally established the existence of the methyl radical in 1929 with the following experiment. At low pressure, tetramethyllead (Pb(CH3)4) vapours were combined with gaseous hydrogen (H2) and passed through a silica tube. The tetramethyllead dissolved and a mirror of metallic lead was formed on the tube's interior surface when a piece of the tube was heated to roughly 800° C. The decomposition's gaseous emissions were discovered to be capable of triggering the disappearance of a second lead mirror put at a cooler location in the tube.

They not only interacted with lead and other metals, but they also vanished quickly and spontaneously, mostly by dimerization to ethane (H3CCH3). Subsequent research has substantially improved techniques for creating reactive free radicals in the gas phase.

 Several unstable species, including ethyl (C2H5), propyl (C3H7), and hydroxyl (OH), have been discovered to be acquired through a variety of techniques, including (1) photochemical decomposition of a variety of organic and inorganic materials, (2) reaction between sodium vapour and an alkyl halide, and (3) discharge of electricity through a gas at low pressure. Atoms that result from the breakdown of a diatomic molecule (for example, the chlorine atom, Cl, from the breakdown of the chlorine molecule, Cl2) can also be generated and exhibit the features of this type of short-lived radical.

The reactions that the many known unstable free radicals undergo are most usually used to establish their existence. Ethyl radicals, which are produced from the tetraethyllead, Pb(C2H5)4, melt zinc and antimony mirrors in this way. The zinc and antimony ethyl derivatives Zn(C2H5)2 and Sb(C2H5)3, respectively, have been isolated and chemically characterised. Stable radicals have also been detected spectroscopically in a few cases. The key method of flash photolysis is applied here, which involves using a single, powerful flash of light to create a high concentration of free radicals for a brief period of time.

Free radicals are thought to be momentary intermediates in many high-temperature reactions (such as combustion and hydrocarbon thermal cracking), many photochemical methods, and a number of other important organic chemistry reactions, despite the fact that their concentrations are generally too low for direct detection. Chloromethane, CH3Cl, and hydrogen chloride, HCl, are formed when methane, CH4, interacts with chlorine, Cl2. Light speeds up the reaction tremendously.

Chlorine atoms are created in (1) and destroyed in (4), whereas the isolated products come from (2) and (3). (3). Because the chlorine atoms destroyed in (2) are regenerated in (3), a single atom of chlorine can produce a large number of chloromethane molecules. Chain reactions are processes in which an intermediate is continuously regenerated, and their study is an important part of chemical kinetics. 

Similar chains involving transitory free radicals are involved in the halogenation of many other chemical compounds, in many polymerization reactions used in the production of plastics and synthetic rubber, and in the interaction of molecular oxygen, O2, with a wide variety of organic molecules.