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Radicals - Introduction

Stability of Radicals

Due to the unpaired electrons, free radicals do not have an electron octet. Therefore, they are usually instable and highly reactive. As a result of their high reactivity, radicals merely show a low selectivity. Thus, radicals are a threat to biological cells.

Stability sequence

Dissociation energies (kJ/mol) of C-H bonds in several hydrocarbons.

Much like carbocations, radicals usually appear only during intermediate stages. However, in comparison to carbocations, carbon-centered radicals normally exhibit a longer half-life period. The stability of radicals - as in case of carbocations - depends on their structure. Higherly substituted radicals are, in comparison, more stable than lower substituted ones. The stability of radicals can be determined by the dissociation energies of the C-H bonds that must be homolytically cleaved in order to obtain the radicals.

Compare the results of measuring the stability of radicals in the sequence below. The differences in radical stability are significantly lower than these in carbocations. This is only one explanation for the lower tendency of radicals, in comparison to carbocations, to rearrange.

benzylic > allylic > tertiary > secondary > primary > methyl.

Stabilization by hyperconjugation

The sequence of radical stability may be explained by the differing amounts of hyperconjugation. The more alkyl substituents a radical carbon atom possesses, the more stabilized it becomes from hyperconjugation.

The interaction of the double-occupied C-H σ bonding orbital with the single-occupied, non-bonding p orbital of the radical carbon atom is comparable to the stabilization by hyperconjugation in carbenium ions. However, they differ greatly in one important factor. The stabilization of carbenium ions, for example, is the result of the overlapping of a double-occupied C-H bonding orbital with an unoccupied, non-bonding 2p orbital. In radicals, on the other hand, this stabilization is obtained by the overlapping of a C-H bonding orbital with a single-occupied, non-bonding 2p orbital.

The stability of radicals can also be increased by aromatic substituents at the radical carbon atom. The central radical carbon atom of the triphenylmethyl radical, for instance, carries three phenyl groups. Therefore, the radical is highly resonance-stabilized. The triphenylmethyl radical is, in fact, so stable that it is at equilibrium with a dimer in a solution at room temperature even if the radical consumes only two percent of the equilibrium mixture. However, the large steric hindrance most likely prevents the formation of hexaphenylethane.

Triphenylmethyl radical

Exercise: Stability of radicals 1

Exercise: Stability of radicals 2

Exercise: Stability of radicals 3

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