Radical (chemistry)

In chemistry, a radical—often called a free radical—is an atom, molecule, or ion that possesses at least one unpaired valence electron.

Most electrons in stable molecules exist in pairs. Because radicals lack these pairs, they are typically highly chemically reactive. They often seek to find a partner by interacting with other substances. Many organic radicals have very short lifetimes due to this reactivity. However, some radicals are stable enough to be isolated and studied in a lab.

Radicals form through several distinct chemical pathways. One primary method is homolysis, which occurs when a covalent bond breaks in half. In this process, each fragment of the broken bond retains one of the shared electrons.

The energy required for this is called the homolytic bond dissociation energy. For example, splitting an H2 molecule requires +435 kJ/mol, while splitting Cl2 requires +243 kJ/mol. Strong bonds may require high-energy photons or flames to induce homolysis. Another method is reduction, which involves a one-electron transfer. This can create radical anions, which are stable if the charge can be delocalized.

Radicals can also be created from other radicals through three main types of reactions. The first is abstraction, where a radical pulls an atom, such as hydrogen, from another molecule.

The second is addition, where a radical joins to a spin-paired substrate. This is the fundamental mechanism behind radical polymerization, a process used to create many modern plastics.

The third is elimination. This is essentially the reverse of addition, where an unstable radical breaks down into a new radical and a spin-paired molecule.

Understanding how long a radical lasts is a complex topic in modern chemistry. In 1976, researchers Griller and Ingold published an influential review that changed how chemists classify these particles. They distinguished between "stabilized" radicals and "persistent" radicals. A radical is considered stabilized if the bond to a hydrogen atom is weaker than in a standard alkane. Persistence, however, is usually a steric effect, meaning the physical shape of the molecule protects the radical center.

A famous example of a persistent organic radical is TEMPO. Its methyl groups act like a shield around the reactive core, allowing it to behave like a normal compound.

At a deeper level, radical structure is defined by the Singly-Occupied Molecular Orbital, or SOMO.

In molecular orbital theory, the SOMO is the highest-energy orbital that contains only one electron. Radicals are often thermodynamically unstable because the positions of the nuclei cannot easily minimize the energy of this single electron. To find stability, radicals often use delocalization. This means the unpaired electron is spread out over a large part of the molecule. This can happen through interaction with electron-donating groups or electron-withdrawing groups. This dual interaction is sometimes called the "capto-dative effect."

Radicals play vital roles across many scientific fields. In biochemistry, many natural products are created by enzymes that generate radicals. In living organisms, radicals like superoxide and nitric oxide act as messengers in redox signaling. They help regulate essential processes, such as controlling vascular tone and blood pressure.

Radicals are also critical in combustion, atmospheric chemistry, and plasma chemistry. Even the air around us contains important radical species. For instance, dioxygen (O2) is a stable diradical, meaning it contains two unpaired electrons. This gives oxygen its paramagnetic character, allowing it to be attracted to magnets.

Beyond biology, radicals are essential to industrial technology. Radical addition is the backbone of the polymerization process used to manufacture plastics like PMMA.

Additionally, many radicals act as important hydrogen-atom donors. Vitamin E, or alpha-tocopherol, is a well-known example. While the tocopherol radical itself is too unstable to be isolated, the molecule is a highly effective antioxidant because it can donate hydrogen to neutralize other reactive species.

By studying these reactive intermediates, scientists can better understand everything from human metabolism to the creation of new materials.