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A chemical reaction can be defined as the process of conversion of chemical compounds to new chemical compounds. It involves the formation of new chemical bond and also cleavage of some chemical bonds in starting compound. The starting compound of a chemical reaction is called as reactants and the newly formed compound is known as product. So we can say that a chemical reaction is conversion of reactant molecules to product molecules. A chemical reaction can be represented in terms of chemical equation. Chemical equation involves the chemical formulae of reactant and products which are separated by single headed or double headed arrow.

$Reactant \rightarrow Product$

The double headed arrow represents the reversible reaction whereas single headed arrow is for irreversible reaction. The physical state of both reactant and product must be written in parenthesis as sub-script. We should always write a balance chemical equation in which the number of atoms must be same on both sides of arrow. For example;

$BaCl_2(aq) + H_2SO_4(aq) \rightarrow BaSO_4(s) + 2 HCl(aq)$

Can we say that a chemical equation gives all the information for a chemical reaction? Do you get any idea about mechanism and type of reaction with chemical equation? Answer is NO! The mechanism of a chemical reaction represents the steps through which the reactant molecules convert to products.

In organic chemistry, most of the chemical reactions complete in more than one step to form the desired product. Such chemical reactions are called as multistep reactions. A multi-step reaction involves the conversion of reactant molecules to short-lived intermediates which are highly reactive and readily convert to other intermediates or products. Due to high reactivity and short life time it is difficult to isolate the reaction intermediates. The presence of reaction intermediate and their reactions with other reagents determine the structure of products.

Today we know many synthetic intermediates also which can be prepared, isolated and purified therefore they can be used as starting materials in a synthetic chemistry. On the contrary, reactive intermediate are short lived and help to assign the reaction mechanisms from the starting substrate to stable products.  They cannot isolate but can be detected with the help of spectroscopic methods or can be trapped chemically with other chemical compounds. Some common reaction intermediates are:
• Carbocation
• Carbanion
• Carbene
• Benzyne intermediate
Carbocations are mainly part of nucleophilic substitution reactions. They have positively charged carbon atom which is bonded to three other atoms. Carbocation does not have any nonbonding electrons. The simplest carbocation is methyl carbocation (CH3+) in which the positively charged carbon atom is bonded with three H atoms. The positively charge carbon atom is sp2 hybridized with a trigonal planar geometry and bond angles of 120Â°. The vacant un-hybridized p orbital - lies perpendicular to the plane of Câ€”H bonds. Due to positive inductive effect and positive charge on carbon atom, as the number of alkyl groups increases in carbocation, the stability of carbocation also increases. Stability and reactivity order for carbocations are listed below.

$(CH_3)_3C+ > (CH_3)_2CH+ > CH_3CH_2+ > CH_3+$

Some of the carbocations are stable due to delocalization of pi-electrons and positive charge such as benzylic carbocation, allylic carbocation etc.  Carbanions is another type of reaction intermediate in which carbon atom has negative charge due to an unshared pair of electron.  The central carbon atom of carbanion is sp3 hybridized with tetrahedron geometry. Due to presence of unshared electrons the stability order for carbanions is just opposite to carbocations.

$(CH_3)_3C+ < (CH_3)_2CH+ < CH_3CH_2+ < CH_3+$

Another type of reaction intermediates are free radicals. Free radicals can be defined as the reaction intermediate which have one or more unpaired electrons. The unpaired electrons contribute to net magnetic moment therefore radicals are paramagnetic.

These reaction intermediates can be detected by electron spin resonance or electron paramagnetic resonance. Simplest free radical is CH3â€¢ in which the unpaired electrons are present in center sp2 hybridized carbon atoms with other 3 H atoms arranged in trigonal planer geometry.

So an alkyl radical has planar trigonal geometry with sp2 bonding with the odd electron in a p orbital. Another possible geometry of free radical is pyramidal structure. This structure is due to sp3 hybridization in which the odd electron is in sp3 orbital.

Free radicals have unpaired electrons with sp2 hybridization and trigonal planer geometry. The presence of unpaired electrons makes the free radical paramagnetic in nature and more reactive compare to other reaction intermediates such as carbocation, carbanion etc. The stability and reactivity of free radicals determine by the presence of alkyl substituent on center carbon atom of free radical.  The stability of free radicals is

Tertiary > Secondary > Primary > Methyl free radical

It can be explained with the help of hyper-conjugation concept. The resonance stabilization of the radical due to delocalization of the odd electron is shown below.  For example the odd electron of ethyl radical is delocalized onto the 3 beta hydrogen atoms. This delocalization of electrons between unpaired electrons and sigma bond is called as hyper-conjugation.

The stabilization of allylic radicals and benzyl radicals is also due to resonance of unpaired electrons with vinyl and phenyl groups respectively.

Hence the stability of different free radicals can be summarized as;

Benzyl > Allyl> Tertiary > Secondary > Primary > Methyl > Vinyl free radical

One of the most common examples of different free radicals is tert-butyl, isopropyl, ethyl and methyl free radicals.

## How Are Free Radicals Formed?

During a chemical reaction some chemical bonds are broken down in the reactant molecules and some new bonds are formed that results the formation of new compounds (products). The cleavage of chemical bonds in reactant molecules results the formation of reaction intermediates; carbocation, carbanion, free radicals etc. The chemical bond fission can be two types; hetrolytic and homolytic fission.  The hetrolytic cleavage breaks the chemical bond in two unequal parts. One part of molecule gets both the bonding electrons whereas another part does not get any bonding electron.  It results the formation of one cation and one anion.

In case of hetrolytic fission, the more electronegative bonded atom will attract both bonding electrons and form anion whereas electropositive element will form cation.

On the contrary, in hemolytic bond fission, the bonding electrons distributed equally on both bonded atoms to form free radicals. So we can say that homolytic bond cleavage forms free radicals whereas hetrolytic bond cleavage forms carbocation and carbanion.

We know that free radicals are formed by hemolytic cleavage of chemical bond between bonded atoms. Homolytic cleavage distributes electrons equally and forms free radicals with unpaired electrons. Such type of bond cleavage requires some initiator to break the chemical bond equally. Usually light is used to cleaved the bond in hemolytic manner. The movement of single electron can be shown by curved arrows with a single barb.

This is also called as photolytic cleavage as photochemical energy is used to break the chemical bond. These reaction intermediates form spontaneously due to exposure to heat, light or something in the environment. Even in human body, free radical formation plays an important role in different biochemical reactions. The immune system of our body forms free radicals to neutralize viruses and bacteria. Many biomolecules are more susceptible to free radical attacks such as fats, DNA, RNA, cellular membranes, proteins, vitamins and carbohydrates.