INTRODUCTIONFREE RADICALS:-FREE RADICALS:- Reaction which involved intermediates formed by the homolytic fission of the covalent bonds

These intermediates having an unpaired electron & are know as free radicals

These are species having one or more unpaired electrons (odd no.)

Free radical is an atom or group of atom which an unpaired electron in its valance shell

TYPEA OF FREE RADICAL:-

I) stable free radicals or free radicals of long life II) Short lived free radicals or transient free radicals

most of the common free radical are extremely reactive species and could not be isolated by traditional method. Such radicals are know as short lived free radicals in contras to stable radicals which are not very reactive and have been found to exist in equilibrium with the normal compound

I) stable free radicals or free radicals of long life :-Gomberg in 1900 discovered the first free radicals i.e. Triarylmethyl, Actually, he was trying to prepare Hexphenylethane by condensing Triphenylmethyl chloride with zinc dust in Benzene as solvent in the absence of air

The product obtained by him was colourless, crystalline solid. This compound showed the following unexpected

characteristici) it is very reactive towards many substance such as oxygen, halogens, hydrogen, nitric oxide etc, forming

addition productsii) it developed yellow color in organic solvent which

disappeared in airiii) as the solution was paramagnetic, it indicated the

presence of odd electrons in the productiv) it reacted with iodine Triphenyliodomathane, nitric oxide Triphenylnitrosomethane

Oxygen Triphenyl methyl peroxidev) the molecular weight of the product was 243 while the

molecular weight of Hexaphenylethane was 486

Reaction of Triarylmethyl free radicals:-1) Addition reactiona) Addition of hydrogen:- Triphenylmethyl radicals gets reduced with platinum / hydrogen producing Triphenylmethane

b) Addition of Oxygen :- Triphenylmethyl radicals on absorbing atmospheric oxygen form colourless oxide very rapidly

c) Addition of halogen:- Triphenylmethyl radicals reacts rapidly with halogen at room temp.

d) Addition of Nitric oxide & Nitrogen peroxide:- On passing nitric oxide into Triphenylmethyl solution the bluish green nitroaotriphenyl methane formed which gets further polymerized to a colourless product of unknown structure

e) Addition of metals :- When solution of Triphenylmethyl radicals are treated with metals such as sodium, potassium & magnesium, they form the organomerallic derivatives

f) Addition of Olefinic compound:- Two molecules of triphenylmethyl radicals have been found to add on various unsaturated compounds on their double bonds in the following way

g) Addition of p-benzoquinone :- Triphenylmethyl radicals adds to p-benzoquinone to produce ditriphenylmethyl ether of hydrocarbon

2) disproportionation :- Triphenylmethyl radical is disproportionate in which one molecule of the radical gets reduced at the expense of the phenyl group of another molecule which thus gets oxidized

This reaction is enhanced by light

3) Irreversible dimerisation :- In the presence of mineral acids, triphenylmethyl radicals undergo irreversible dimerisation.

II) Short lived or transient free radicals :- They are short lived free radicals and some example under this category are methyl, ethyl, acetyl, benzoate, phenyl, etc.

1) Thermal decomposition :- compound having weak bonds produce free radicals on heatinga) Paneth’s method :- It was the first method for producing shot lived free radicals. # vapors of tetra methyl lead + H/N2

# Quartz tube pressure 1-2 mm by using high speed vacuum pump

# Tube strongly heated (600-8000)at point X # Lead mirror deposited at the point X formed from tetramethyl lead# Free radicals carried by H2 gas# The mirror of metallic lead is deposited at another point Y to detect the methyl radicals # Methyl radical with h2/n2 gas combine with metallic lead at Y forming tetramethyl lead# Thus the lead mirror at Y disappears

# If the distance between X&Y greater than 32cm- the mirror does not disappear- b’coz methyl radical have shot life- and combine readily in absence of lead mirror to for Ethane

half life period for Methyl radical is found as 0.006 second

b) Thermal or pyrolytic decomposition of other compound :- Thermal method of producing free radicals is applicable to compounds having weak covalent bonds. Organic peroxides (R-O-OH) or per acids. Which have low O-O bond energy (30-440 Kcal/mole) undergo decomposition readily at quite low temp. into free radicals

Similarly, thermal decomposition of aliphatic Azo compound

2) Photochemical decomposition or photolysis :- Some molecules undergo homolysis by absorbing radiation in UV/Visible range. It is know as photolysis

It is also possible that a free radicals may produce a new free radical by further cleavage or by reacting with another free radical

3) Redox reaction by inorganic ions :- The free radicals are prepared in solution by redox reaction involving one electron transfer. The diazonium sails on heating with cuprous salts get reduced to aryl radicals. It is a transient intermediate in Sand-Meyer reaction.

Similarly, Fenton’s reagent (mixture of H2O2 & ferrous salt) the H2O2 gets oxidized into hydroxyl radicals. They generate new free radical by abstracting hydrogen atoms

Radicals are also produced by the reduction of inorganic ions

Electrolytic method is used to generate free radical In Kolbe electrolytic synthesis of hydrocarbon, the carboxylate ions get oxidized at the anode giving free radicals. Then they dimerise to form hydrocarbons.

Similarly, free radicals are also formed during the electrolysis of ketones to pinacols

DETECTION OF FREE RADICALS :- The free radical have been detected by the following methods a) Mirror method :- when free radicals are carried in a current of inert gas over a metallic Mirror of Lead , bismuth etc. Then metal coating disappears. This occurs because free radicals attacks on the metal coating. The highly reactive alkyl free radicals in gaseous phase have been detected by this method.

b) Polymerisation reaction method :- The free radical initiated the polymerisation reaction & can there for be detected. A mixture of equal amount of styrene & methyl methacrylate get polymerised into a copolymer by a free radical

c) Colour reaction method :- The free radicals can be readily detected by the following colour reaction. A Nitrogen radical called 1,1-diphenyl-2-picryl hydrazyl is extremely stable & even forms Violet coloured crystals.

In, solution it react with other free radical to form colourless compounds. Therefore, it can also be used as a trap for free radicals

d) Magnetic moment method :- The free radicals have a net magnetic moment due to the unpaired electron. Hence, they are paramagnetic. Therefore, they can be detected by their values of magnetic susceptibility. This method is not reliable because only the FR of large concentration can be detected by this method.

e) ESR method :- It is the most powerful tool for the study of FR . It is based on the fact that when FR kept in magnetic field, it’s odd electron spin gets oriented preferentially in the more stable position. However, with the absorption of light energy, the spin can turn over to the unstable orientation .Transition between stable & less stable level produces a line in the ESR spectrum.

The number of lines in the ESR spectrum is always n+1 where n is the number of coupling protons,e.g.- Methyl radical shows 3+1 or 4 lines

on the other hand, the 1,3,5-cycloheptatriene radical shows 7+1 or 8 resonance linesThis method is very sensitive & can be applied to detect FR even at the conc. Of 10-7 M.

STABILITY OF FREE RADICALS:- Generally, free radical are very reactive the alkyl radical are very unstable while many other FR such as triarylmethyl radical are relatively more stable

Stability of alkyl free radical :- Like the carbonium ions, the simple alkyl FR have following order of stability

In general, smaller the amount of energy required for bond-breaking the more stable is the radical.

Bond dissociation energy of some compound :-

Hyperconjugation (resonance)

canonical structure of the primary ethyl free radical

canonical structure of the secondary isopropyl free radical

canonical structure of the tertiary butyl free radical

Other factor that increase stability of tertiary radical may be Steric factor

In (I) allylic & (II) benzylic radical, the p-orbital having the unpaired electron overlaps with TT molecular orbital & the unpaired electron gets delocalized

Stability of Triarylmethyl radical:- Resonance effect:-

Hyperconjugation stability e. g. p-alkyl substituent

Electron withdrawing substituents

Steric effectThree phenyl group in a triphenylmethyl radical occupy large volume, it is difficult for two radicals to come closer for a bond formation

CONFIGURATION OF FREE RADICALS :- It was found by Brown, Kharasch & chao that free radical chlorination of (+)-1-chloro-2-methylbutane results in the formation racemic 1,2-dichloro-2-methylbutane.

Racemisation of the (+)- isomer during FR chlorination has been explained on the basis of the following two probable configurations of FRa) Planar configuration :-b) Pyrimidal configuration:-

a) Planar configuration :- Here, the FR is considered to be a planar species in which the carbon atom having the odd electron is sp2 hybridised & the odd electron is present in the p orbital But the intermediate (FR) is planar. Therefore, attachment of chlorine atom to either face is equally probable. Hence, both the optical isomers of the product will be obtained i.e. racemisation will occur

b) Pyrimidal configuration :- Here, the FR is assumed to have a shape which resembles a shallow pyramid & the orbital having unpaired electron is more or less a hybrid between sp2 & sp3 orbital , the formation of racemic product in such cases has been explained on the basis of assumption that the inversion of such a pyramid is faster than the transfer reaction with chlorine molecule there by resulting in the formation of racemic product.

From the evidence, it follows that in radicals which are stabilised by the unpaired electron , the three bonds of the carbon are coplanar. On other hand, if the unpaired electron lies at a bridge head of bicyclic system, an sp2 configuration is completely ruled out by the geometric requirements & a pyrimidal configuration becomes more or less mandatory e.g. :- Apocamphoyl peroxide which readily undergoes dissociation in cabrontetrachloride to form the apocamphyl radical.

REACTION OF SHORT LIVED FREE RADICALS :- The FR are very reactive due to the presence of the unpaired electron. The F R are chain reaction involving three steps:-

a) Initiation :- In which free radicals is generated. b) Propagation :- In which these radicals react to give new free radicals. c) Termination :- In which radicals combine to give stable molecules. 1) Polymerisation2) Halogenation3) Autoxidation4) Aromatic substitution5) Rearrangement of radical

1) Polymerisation :-initiation

propagation

termination

when we need to control mol. Wt. of polymer chain terminator are added to reaction mixture

2) Halogenation :- a) Chlorination:-initiation

propagation

termination

b) Brominationc) Iodinationd) Fluorination

3) Autoxidation :- slow oxidation of organic substances by atmospheric oxygen is termed as autoxidation

4) Aromatic substitution :-

5) Rearrangement of radical:-

ADDITION TO CARBON-CARBON MULTIPLE BONDS :- Addition of hydrogen bromide to olefins

This type of reaction are called Non-stereospecific & some time it may be stereoselective

REFERANCE:-1)2)3)4)