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دراسة علمية نظرية بوسطةGassan Qais AliDepartment of Chemistry, College of Science, Al-Mustansiriya University, Baghdad, Iraq.
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( I)(II)
(C)
Ex:
6.
In the simplest cases an atom in the compound has only seven electrons in its valence shell and the unpaired electron is localised on either a carbon atom or on a heteroatom. Note the characteristic 'yl' termination of the systematic names for free radicals and the representation of the unpaired electron by a dot at the appropriate atom:
Preparation of Free Radicals:1. Homolytic cleavage of weak single covalent bonds.
Note the use of 'fish-hook' single-headed curved arrows, i.e. to indicate the movement of a single electron.
Thermal cleavage:
N-N, O-O, S-S, Cl-Cl, Br-Br, C-N, N-Cl, O-Cl, O-Br
Photolytic cleavage compounds with low-energy electronic absorptions only:
2. Redox reactions of non-radical precursors:
Reduction.
Oxidation:
Stability of Free Radicals:
allylic benzylic > 3 alkyl > 2 alkyl > 1 alkyl
1.Substitution effect - radical centres are electron-deficient, hence stabilised by attached electron releasing alkyl groups and electron with drawing
2.Resonance effect:
The combination of electronic and steric effects can result in very stable - and, in suitable circumstances, even isolable - free radicals:
Detection of Free Radicals:
1. Electron Spin Resonance (ESR) Spectroscopy.
The degeneracy of spin of an unpaired electron is lifted in a strong magnetic field. E corresponds to microwave radiation. Hyperfine splitting due to electron-proton spin coupling aids structural interpretation.
2. Matrix Isolation:
Free radicals generated and trapped in a radiation-transparent solid argon matrix at very low temperature may be studied spectroscopically:
Reaction of Free Radicals:
Despite what we might expect, simple free radicals do not normally undergo rearrangement:
The transition states for both rearrangements are very similar:
Consider the orbitals involved in the transition state: H 1s and 2 C 2p.
Combination of the H 1s and 2 C 2p atomic orbitals gives three molecular orbitals:
(A) is bonding for the hydrogen atom and both carbon atoms.
(B) is bonding for the two C atoms but antibonding for the C2-H interaction.
(C) is antibonding for the two C atoms. In addition there is no net interaction between the hydrogen atom and the two carbon atoms.
(A) is the lowest energy orbital while (B) and (C) are approximately equivalent in energy.
-Unsaturated free radicals will undergo rearrangement with migration of the unsaturated group:
4. Dimerization:
Leads to non-radical products - termination steps in radical chain-reactions.
5. Disproportionation - one radical is oxidised by another:
Hydrogen abstraction from one n-propyl radical by the other results in the radical accepting hydrogen being reduced to an alkane. The radical losing hydrogen is simultaneously oxidised to an alkene.This leads to non-radical products - i.e. is a termination step in radical chain-reactions.6. Fragmentation:
4. Dimerization:
Leads to non-radical products - termination steps in radical chain-reactions.
FREE RADICALS IN ORGANIC SYNTHESIS
Free Radical Substitution of Hydrogen by Other Atoms.
(a) Photochemical halogenation of saturated hydrocarbons
(Fig. 4.1a) (Fig.4.1b) (Fig.4.1c)
Reaction of Carbenes:
Addition Reaction
Insertion Reaction
Exp:
Exp:
Dimerization reaction
Rearrangement Reaction
Abstraction Reaction
Rearrangement:
Benzyne
Ex:
3.
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