The study of free radicals is based mainly on organic chemistry and treated as special although it sometimes takes the role of unusual class of reactions despite the fact that in industry and nature, the transfer of one electron is found to be more common than the ones we get to see in inorganic chemistry where a pair of electrons are transferred.
The organic chemistry free radicals in solutions was fully developed by kharasch and then by Hey & Waters. The free radicals failed to attract wide spread interest among organic chemists and the correlation of solution phase work with extensive kinetic studies in gas phase. The main reason for this is that the simplest free radical reactions occur in gas phase and few organic chemists have apparatus to study such methods.
The meaning of radicals pertains to a series of explanation as the idea evolved around organic chemistry and then slowly percolated to entire chemical length and is not kept restricted to just one domain. The modern meaning of radical was due to a series of advances and reverses in understanding of chemical problems. The concept of radical was introduced by Lavoisier in his acid theory which were comprising of oxygen. The designation of element or group of elements which combined with oxygen in acid also considered as radical. The oxygen theory of acids gave away to the organic forms where it is frequently made use of to signify a group of elements retaining their identity through a series of reactions. The early 19th century paved way for many radical discovery. The radical proved a useful organising concept in both inorganic and organic chemistry. Kolbe obtained gases by electrolysis of solutions of fatty acids interpreted as free radicals.
The existence of free organic radicals was also with ideas that were developing about structure. Majority of structural formulae is written by allotting fixed valency to the elements. Kekule supported the doctrine of quadri valency carbon leading to rationalization of the formulae of many organic compounds.
Around 1900 Gombergs discovery of tri phenyl methyl brought the radical chemistry in focus. The idea was to prepare hexa phenyl ethane but the products of the reaction contained oxygen and was very much different than what he expected for hexa phenyl ethane. This rapidly oxidized in air and reacted with halogens which he promptly termed as tri phenyl methyl radical.
Ph3CCl + Ag Ph3C. + AgCl
Electron structure of free radicals: Let us look at the electronic structure of free radicals is analysed in terms of distributions of α and β electrons and also the spin properties. The analysis has been specified at theoretical level. The distribution of α and β in free radicals is about carbon centred free radicals, the pie π donor substituents remove the unpaired electrons from α carbon atom and the C substituent bond has a three electron bond character. While, the π acceptor substituents gets the unpaired electrons towards the bond Cα substituent which also specifies three electron bond character. A pair of substituent of opposite polarity delocalises the unpaired electron and bonds between Cα and the substituents have both three electron bond character.
The unpaired electron of π carbon radical is generally delocalized, and this effect is very important in all kinds of captodative species and in case of σ carbon radicals it is closer to Cα.
Stability of free radicals:
Although stability is considered in chemistry but there was no specific definition so far. This concept is used for characterising either an isolated species without referring to actual chemical process or explained from thermodynamic point of view. A system is said to be stable thermodynamically if the corresponding standard free enthalpy change is positive.
Here, ∆Ho and ∆So are enthalpy and entropy changes of the chemical process under consideration. When this condition is fulfilled the system is chemically inert and no further reaction is possible.
From kinetic point of view the stability of a compound may be defined by its lifetime under lab conditions. The rate constant and activation energy of a chosen reaction of the considered compound can also be used. This stability is related to concept of kinetic stabilisation.
The free radical chemistry has got tremendous importance in last decade and the radical intermediate are recognized and studied in depth for almost every field of chemical activity. This includes chemical activity in organic, physical, inorganic and biological systems. The experimental results and theories from all the related fields are collated to show how the free radical concept for intermediates brought in underlying order and unity for a difficult topic like free radicals. The free radical structure and reactivity is very much in line with carbanions and carbonium ions.
Organic chemistry represent an extensive body of facts and most of the ideas is meant for intermediate species that arise along the way for the starting compound till the final product is produced. The cations, anions and radicals are created during the intermediate stage for all kinds of chemical transformations. These radical units are formed due to the bond split which occur heterolytically or even homolytically.
R- + X+ R-X R+ + X-
Or, R-X R. + X.
Ions or radicals formed from a substrate further react, with other ions or radicals acting as reactants. These changes in chemical bonds are accompanied by one electron shift. This one electron shift concept is ideal for nucleophilic substitution.
The species is not a radical pair; it is a covalent molecule of product which results from SN2 reaction. The process of transfer of the R group to Nu- reactant proceeds in synchronicity with one electron. Shift and R-Z bond disruption. Two radicals particles formed in the course of reaction remain immediately close and hence unite rapidly. A one electron shift may or may not lead to the formation of radical particles. There are many reactions that consist not of one electron shift but instead one electron transfer. The initial products of this one electron transfer get the ion radical formation.
The ion radicals are different from their starting molecules as far the count of electrons is concerned that goes into the reaction and the bond split along with bond formation which is not observed. Reactions with participation of ion radicals bring in their own requirements and opportunities. These ion radicals have dual characters and contain unpaired electron and hence close to radicals. They also bear charge and naturally close to ions. The ion radicals are ready to react with radicals and quite like other radicals they dismutate and recombine. The ion radicals are able to react with particles of opposite charge and are prone to form ionic aggregates. Ion radicals are sensitive to medium effects.
|| Substitutive name
|| Accepted name
|| Hydroxy methyl
|| Chlorine dioxide
|| Hydrogen dioxide
|| Nitrogen monoxide
Formation of free radicals proceeds as a result of several complementary processes. In electric discharge the molecules release the radicals and ions by collisions with electrons and ions as well as high temperature of plasma. Due to high reactivity the primary free radicals cannot be detected but only stable radicals produced during reaction are seen. During the generation of free radicals from molecular precursors, these are formed in pairs. In each of these pairs radicals are found in close proximity and react reversibly to form original molecule. The escape of free radicals from initial pair occurs in gaseous phase. Free radicals do not appear in pairs but their spatial distribution is heterogeneous and the concentration of free radicals in system found to be lower than that at the electrode.
The formation of free radicals depends on the extent of deformation and high concentration of radicals is achieved much before the macroscopic rupture of the reagent. The alkyl radicals formed are prone to react with atmospheric oxygen and original structure along with polymer system properties are changed accordingly.
Most of radicals are related to oxygen centre free radicals and most of the common are in superoxide anion. Apart from these there are hydroxyl radicals, single oxygen radicals as well as hydrogen peroxide forms of anions. These are found when oxygen gets an extra electron and that gives a molecule with just one unpaired electron.
The hydroxyl radicals that are formed are also found to have short life but can be very catastrophic to body cellular mechanism. These are formed when oxygen and hydrogen peroxide undergo Harber Weiss reaction and the release of hydroxide radical from hydrogen peroxide. The single oxygen doesn’t follow Hund’s rule of filling electrons and that leaves one orbital empty resulting in empty energy level. When this oxygen is energetically excites one electron to empty orbital it results in unpaired electrons. These then transfer the energy to a new molecule and finally act as a catalyst to form the free radical. These can also interact with many other molecules and form newer free radicals.