Organic Reactive Intermedites

( 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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