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Free radicals

Their formation is initiated mostly by ultraviolet light.
For example, Description: 39798.png
The alkyl group bearing an odd unpaired electron is called alkyl radical. The radicals are deficient in one electron to complete its octet and are paramagnetic in nature. A free radical has a carbon in the sp2 hybridized state, thus exhibit planar (flat) structure. If a free radical with three different atoms or group of atoms is attacked by another radical, it leads to the generation of a stereocenter, existing as enantiomeirc pair.
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Free radicals can be attacked either by another free radical or by a neutral molecule whose bond (whether π or σ) will be broken homolytically. Free radicals have very less or no tendency to rearrange to give a more stable radical.
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Whenever an intermediate rearranges, it goes through a transition state. The transition states for the rearrangement of carbocation, radical, and carbanion are shown in the adjacent figure.These transition states involve two, three, and four electrons, respectively, and electrons in excess of two can be accommodated only in an antibonding molecular orbital of much higher energy. However, 1,2-aryl and halogen shifts are known in radicals, which involve stabilized, bridged transition state. The driving force for the rearrangement is to attain stability.
We are aware that the stability order of free radicals is 3° > 2° > 1° alkyl, if electron releasing groups are attached.
For example,
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Carbocation is the carbon containing intermediate bearing a positive charge and having a sextet of electrons in the outermost shell. The carbocation can act as an electrophile. Let us first see the carbocation formation from different substrates.
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The most characteristic feature of carbocations is that they undergo rearrangements readily (if possible). The driving force for the rearrangement is to attain stability. The stability order of carbocations is 3° > 2° > 1° alkyl, if electron-donating groups are attached. The rearrangement may or may not involve change in carbon skeleton.
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The migrating aptitude of various groups is
  1. H > 3° > 2° > 1° alkyl > CH3
  2. Ph > CH3
  3. p-anisyl > p-tolyl > phenyl > p-chlorophenyl > p-nitro phenyl
Carbocation has a trigonal planar structure, as the carbon atom of carbocation is also sp2 hybridized. A carbocation can act as an electrophilic reagent and leads to the formation of racemic mixture if the three groups attached to the carbocationic carbon are different and incoming nucleophile is different than the three groups already present.
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Carbanions are anions of carbon, generated by the removal of one of the groups attached to a carbon atom without removing the bonding electrons.
For example, nitromethane reacts with alkali to yield an anion, which is stabilized by resonance.
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In general, compounds containing methylene groups flanked by nitro, cyano, or carbonyl groups show acidic character as the hydrogens can be removed as protons leaving a resonance stabilized anion. Alkyl carbanions can only be formed under extreme conditions, since the attached groups are not capable of dispersing the negative charge.
Carbanion assumes a pyramidal configuration, as carbon atom here is sp3 hybridized. The unshared electron pair would be accommodated in an sp3 hybridized orbital. The unshared electron pair would be accommodated in an sp3 orbital. The carbanions readily undergo pyramidal inversion also called umbrella inversion.
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Carbanions are capable of acting as nucleophiles and will attack the substrate on a site of lower electron density.
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