In this chapter we will examine the homolysis (homolytic cleavage) of σ bonds to form highly reactive radical species. When a bond breaks homolytically, the two electrons of the breaking bond end up on different atoms. The resulting radical species possess a single unpaired electron on an atom lacking a full octet of electrons. This makes radicals very electron deficient and unstable. They are often formed in low concentrations and are rarely stable enough to isolate, though they can serve as intermediates in chemical processes, as we will see. Biological systems take advantage of the high reactivity and transient nature of radicals to mediate a host of transformations required for life. Molecular oxygen exists as a diradical and we will see how it acts as a powerful oxidant, attacking organic molecules to initiate radical reactions. These oxygen-mediated radical reactions cause cellular damage and we will take a close look at how the antioxidant vitamins E and C (Figure 8.1) prevent this damage by acting as radical scavengers.
The antioxidant vitamins E and C.
8.2 Formation, Stability, and Molecular Orbital View of Radicals
The homolysis of a covalent bond to form two radicals is illustrated using two single-headed (“fishhook”) arrows, each of which indicates the movement of a single electron (Figure 8.2). Note that fishhook arrows are reserved for keeping track of electron count in radical reactions and are not interchangeable with the standard arrows used to indicate the movement of pairs of electrons in acid/base or nucleophile/electrophile chemistry. The homolysis of molecular hydrogen (H2) yields two free atoms of hydrogen. The process requires an amount of heat (104 kcal/mol) that is equal to the amount of heat produced when two free atoms of hydrogen combine to form a covalent bond. This is referred to as the bond dissociation energy. We will use bond dissociation energies to help understand the strength of bonds and the relative reactivity of radicals.
Homolytic cleavage of a bond is illustrated using “fishhook” arrows, which indicate the movement of a single electron. The homolysis of molecular hydrogen (H2) yields two hydrogen atom radicals.
Homolysis of the C–H bond in methane yields a methyl radical in a process that requires 105 kcal/mol of energy (Figure 8.3). The homolysis of a C–H bond at a primary (101 kcal/mol), secondary (98.5 kcal/mol), or tertiary (96.5 kcal/mol) substituted carbon atom requires sequentially less energy. This trend in dissociation energies corresponds to the relative stability of the resulting primary, secondary, and tertiary radicals, with the tertiary radical being the most stable.
The formation of methyl radical and the relative stability of carbon radicals.