20.3
Radical Formation: Overview
A bond can be broken either by heterolytic bond cleavage to formions or homolytic bond cleavage to yieldradicals. A fishhook arrow is used to represen…
Radicals can be made from spin-paired molecules or other radicals.
There are two possible routes from spin-paired compounds. First, homolysis, where one spin-paired molecule can form two radicals. Secondly, electron transfer or reduction.
Alternatively, there are three ways to create radicals from other radicals. Here, in general, one spin-paired molecule and one radical can interact to yield either a radical or a spin-paired molecule and a radical.
First is the substitution or abstraction method that involves the interaction of a radical with a spin-paired molecule to generate the corresponding spin-paired molecule and a radical.
Second is the addition method where a radical adds to an alkene to produce a new radical.
Last is the elimination method, which is the reverse of the addition method, and yields a new spin-paired molecule and a radical.
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Q1: What are the two main ways to form radicals from spin-paired molecules?
Radicals form from spin-paired molecules through homolysis or electron transfer. Homolysis breaks a bond to create two radicals, represented by a fishhook arrow showing single electron motion. Electron transfer, also called reduction, adds an electron to a spin-paired molecule to generate a radical.
Q2: How does radical substitution or abstraction work?
In radical substitution or abstraction, a radical interacts with a spin-paired molecule, typically abstracting a hydrogen or halogen atom. This interaction produces a new spin-paired molecule and a different radical. The process allows radicals to transfer reactivity between molecules while maintaining the total number of radicals in the system.
Q3: What happens when a radical adds to an alkene?
When a radical adds to an alkene's pi bond, it produces a carbon-centered radical. This addition method generates a new radical from the starting radical and unsaturated compound. The resulting carbon-centered radical can then participate in subsequent radical reactions or chain propagation steps.
Q4: How does radical elimination differ from radical addition?
Radical elimination is the reverse of radical addition. While addition forms a carbon-centered radical from a radical and an alkene, elimination breaks a radical compound to yield a new radical and an unsaturated compound with a pi bond. Both processes interconvert radicals and unsaturated molecules.
Q5: What is the difference between homolytic and heterolytic bond cleavage?
Homolytic bond cleavage breaks a bond to form two radicals, each receiving one electron from the broken bond. Heterolytic bond cleavage, by contrast, forms ions by unequally distributing electrons. A fishhook arrow represents homolytic cleavage, distinguishing it from heterolytic processes that generate charged species.
Q6: Can radicals be generated from other radicals, and if so, how?
Yes, radicals form from other radicals through three mechanisms: substitution or abstraction, addition, and elimination. In each case, one radical and one spin-paired molecule interact to produce either a new radical or spin-paired molecule and a radical. These pathways enable complex radical chain reactions.
Q7: What does electron transfer accomplish in radical formation?
Electron transfer, or reduction, adds an electron to a spin-paired molecule to create a radical. Unlike homolysis, which generates two radicals from one molecule, electron transfer produces a single radical by introducing an additional electron. This method provides an alternative route to radical formation from neutral compounds.