Covalent bond fission – Homolytic and heterolytic

Organic reactions usually involve making and breaking of covalent bonds. The fission of bonds can take place in two ways.

Bond breaking is also known as bond fission.

1. Homolytic fission
2. Heterolytic fission
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Heterolytic fission results in the formation of two different chemical species in the sense that one is a cation and the other an anion.

Homolytic fission results in two electrically uncharged radicals.

Radicals have an unpaired electron.
Radicals are particles that have an unpaired electron. They may be single atoms (e.g. chlorine radical, Cl.) or groups of covalently bonded atoms (e.g. methyl radical,.CH3).

Some radicals, called biradicals, have two unpaired electrons, for example, .O. (1s22s22p4) and .O2..

Because of the unpaired electron, radicals can be very reactive. However, there are some that are relatively stable and behave somewhat like ordinary molecules. An example is nitrogen monoxide.

Heterolytic fission versus Homolytic fission...

The hydrogen chloride molecule (H-Cl) is polar owing to the greater electronegativity of the chlorine atom. Heterolytic fission is more common where a chemical bond is already polar. Hydrogen chloride is highly soluble in water and becomes fully ionised; it is a strong acid. Solvents with polar molecules favour heterolytic fission.

Homolyic fission is favoured by non-polar solvents, or by gaseous conditions, and the presence of visible or ultraviolet light.

http://www.avogadro.co.uk/light/fission/bondfission.htm
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Reaction Intermediates

The species produced during cleavage of bonds are called reaction intermediates. The important ones are:

1. Free radical: A free radical is an atom or group of atoms having an unpaired electron. Thee are produced during the homolytic fission of a covalent bond. These are very reactive. The free radicals are classified as primary, secondary or tertiary depending upon whether one, two or three carbon atoms are attached to the carbon atom carrying the odd electron.

The order of stability of alkyl free radicals is

cH3<1 br="">
2. carbocation: It is a group of atoms which contain positively charged carbon having only six electrons. It is obtained by heterolytic fission of covalent bond involving carbon atoms.

Relative stabilty: The methyl group has +I inductive effect. So the alkyl group (as a specific example methyl group)attached to +vely charged carbon (carbocation) tends to releaase electrons towards carbon. As a result, it decreases +charge on the carbocation. Due to this inductive effect, the positive charge on the carbocation gets reduced and dispersed (Dispersion is distribution of charge over other atoms in the molecule). The dispersal of charge results into stability. Therefore, more the number of alkyl groups, the greater will be the dispersal of charge and therefore, more stable will be the carbocation.

So the order of stabilty is

CH3^+<1 br="">
3. Carbanion: It is a species containing a carbon atom carrying a negative charge. They are generated during heterolytic fission of covalent bonds containing carbon, when an atom linked to carbon goes without the bonding electrons.

Carbanions are very reactive species.

They are classified as primary, secondary and tertiary.

The order of stability is reverse of that of carbocations and free radicals.

CH3‾ >1°>2°>3°

4. carbene: The carbenes are reactive neutral species in which the carbon atom has six electrons in the valence shell out of which two are shared. The simplest carbene is methylene (:CH2). It is formed wbehg diazomethan is decomposed by the action of light.

CH2N2 --> :CH2 + N2

Types of attacing reagents

1. Free radicals
2. Electrophiles
3. Nucleophiles


Typesof organic reactions

1. substitution reactions
2. Addition reactions
3. Elimination reactions
--i) α-Elimination
--ii) β-Elimination
--iii)γ-Elimination
4. Rearrangement reactions
5. Condensation reactions
6. Isomerism reactions