20.5:
Radical Formation: Abstraction
When a radical abstracts a species from a compound, it leads to the formation of a new radical. If the species abstracted is a hydrogen atom, it is known as hydrogen abstraction.
However, this is different from the proton transfer process. A transfer of protons is ionic, where only the nucleus of the hydrogen atom undergoes relocation.
In the hydrogen abstraction method, the whole hydrogen atom—the proton and the electron—is moved across.
Similar to hydrogen abstraction is halogen abstraction. Here, instead of a hydrogen atom, a halogen atom is abstracted to produce a new radical.
20.5:
Radical Formation: Abstraction
The electron of an atom can be abstracted from a compound by a relatively unstable radical to generate a new radical of relatively greater stability. For example, an initiator which forms radicals by homolysis can abstract a suitable species like a hydrogen atom or a halogen atom from a compound to generate a new radical. This ability of radicals to propagate by abstraction is a crucial feature of radical chain reactions.
Even though homolysis produces radicals, it is different from radical abstraction. In homolysis, a spin-paired molecule produces two radicals; in abstraction, a radical reacts with a spin-paired molecule and produces one new radical and a new spin-paired molecule.
Most importantly, there is a fundamental difference between the mechanisms of hydrogen abstraction and proton transfer. Proton transfer is ionic in nature, with the movement of only the nucleus of the hydrogen atom. This yields an anion of the other reactant with the retention of its electron. However, the relocation of the hydrogen atom with its electron in hydrogen abstraction produces a radical species. Like hydrogen abstraction, halogen abstraction is possible, where a halogen atom is abstracted with one unpaired electron.