Free-radical addition

Tiny bits of stuff can move.

These bits are very active. They want to join with others. This happens because they have a lonely part.

Heat or light can start this. The bits join in a chain. One bit makes a new bit. This keeps going fast.

Sometimes two bits meet. They join to stop the chain. This is how it ends.

It is like a fast game. The bits move and change. It makes new things.

In chemistry, some tiny bits are very active. We call these free radicals. They are active because they have one lonely electron.

These radicals work in a set of steps. First, a radical is made. This is called initiation. Next, the radical joins with others. This step is called chain propagation. It works like a chain reaction. One radical makes a new radical. This keeps the reaction going. Finally, two radicals meet and join. This is called termination. It stops the chain.

One way this works is called free-radical addition. In this way, a radical attaches to a molecule. This can happen with things like alkenes. A common example uses hydrogen bromide. Heat or light can start the process. The bromine atoms add to the alkene. This makes a new part of the molecule. This specific way of adding is called the peroxide effect.

Some reactions can also make rings. This is called radical cyclization. It can even make a ketone. This is a type of chemical compound. These steps show how small bits change the world.

Chemistry involves many tiny, busy movements. One important way is called free-radical addition. This happens when free radicals join a molecule. A free radical is a very active piece of matter. It has one lonely electron in its outer shell. This unpaired electron makes the radical want to react. It looks for other things to bond with quickly.

These reactions can change many different types of substances. They work with molecules called alkenes or aromatic rings. Some of these molecules even have heteroatoms inside them.

This process works like a fast-moving chain. First, there is a step called radical initiation. This is when a radical is born from something else. Next comes chain propagation. In this step, a radical hits a normal molecule. This creates a brand new radical species. This new radical then attacks another part of the molecule. This keeps the chain moving forward. Finally, there is chain termination. This happens when two radicals meet and join together. When they join, they become a stable, non-radical piece.

Scientists have studied these chains for a long time. A man named Morris Kharasch studied a special result. He called it the peroxide effect. This effect describes how certain additions happen in a specific way. In these reactions, the radical attaches to the easiest spot. This spot is usually the least crowded carbon atom. The radical then stays on the more crowded carbon. This specific pattern is known as anti-Markovnikov addition. It is a very useful trick for making new molecules.

One famous example uses a chemical called hydrogen bromide. When it reacts, it turns into monatomic bromine. These bromine atoms add to an alkene at the best site. This creates a bromoalkyl radical. That radical then grabs a hydrogen atom from another HBr molecule. This step regenerates the bromine to keep the chain going. Hydrogen bromide is very selective and does not make many messy byproducts. Other acids like HF, HCl, or HI do not work this way. Their radical formation is too difficult for this process.

Free-radical addition is a tool for building things. It can even help make ring shapes through radical cyclization. This can turn certain molecules into a group called ketones. Some reactions use sulfur halides to make new sulfur compounds. Other reactions use silicon, germanium, or phosphorus. These processes help us understand how atoms find their places. They show how energy from heat or light can start a big change. Even small, lonely electrons can change how matter is built.

Free-radical addition is a specific type of reaction in organic chemistry. It involves molecules called free radicals. A free radical is a highly reactive species. This reactivity comes from having an unpaired electron in its valence shell. Because electrons usually prefer to be in pairs, a radical is very eager to react. These reactions can occur with many different unsaturated substrates. These include olefinic molecules, which have double bonds, or aromatic rings. Some of these substrates also contain heteroatoms, which are atoms other than carbon and hydrogen.

The process follows a specific radical chain mechanism. This mechanism is divided into three distinct stages. First is radical initiation. This is the step where a radical is created from a non-radical precursor. This often happens because of heat or light. These conditions cause weak bonds to undergo homolysis. Homolysis is when a bond breaks and each part takes one electron. Next is chain propagation. This is a repeating cycle where a radical reacts with a non-radical. This reaction produces a new radical species, keeping the chain moving. Finally, there is chain termination. This occurs when two radicals react with each other. They form a stable, non-radical species, which ends the chain.

In a free-radical addition, the chain propagation stage includes two specific steps. In the first step, an adding radical attaches to a multiply-bonded precursor. This results in a new radical with a lower bond order. In the second step, this newly formed radical abstracts a substituent from the adding reagent. This step is crucial because it regenerates the original adding radical. This regeneration allows the chain to continue. The direction of this addition is often very specific. The adding radical usually attacks the alkene at the most sterically accessible carbon. This is typically the least substituted carbon. The radical then stabilizes on the more substituted carbon. This specific result is called anti-Markovnikov addition. Morris Kharasch famously referred to this phenomenon as the "peroxide effect."

A classic example of this process uses hydrogen bromide. In this reaction, hydrogen bromide radicalizes into monatomic bromine. These bromine atoms add to an alkene at the most accessible site. This creates a bromoalkyl radical. The radical then sits on the more substituted carbon. This radical then abstracts a hydrogen atom from another HBr molecule. This action regenerates the monatomic bromine to continue the reaction. Hydrogen bromide is a very useful and selective reagent. It does not produce many detectable quantities of polymeric byproducts. Other hydrohalic acids do not work this way. For example, forming radicals from HF, HCl, or HI is extremely endothermic. This makes those reactions chemically disfavored.

Free-radical addition can also involve different types of chemical groups. Some reactions involve halogenated compounds with stable radicals. For instance, sulfur halides can add radically to create sulfoxides or sulfides. Other reactions involve unsubstituted compounds that can dissociate from hydrogen. These reactions can sometimes lead to polymerized byproducts. A common example is the thiol-ene reaction. In this case, thiols, disulfides, or hydrogen sulfide add across a double bond. If the substrate polymerizes easily, these chemicals might catalyze polymerization instead. In thermal silane additions, a process called telomerization usually proceeds to about 6 units.

Some reactions can even change the shape of a molecule through radical cyclization. This can happen between the two propagation steps. One example is self-terminating oxidative radical cyclization. In this process, inorganic radicals oxidize alkynes to create ketones. This reaction is not catalytic. It requires the oxidized radical source in stoichiometric amounts. This means the amount of reagent used is proportional to the amount of product made. In one version, a nitrate radical adds to an alkyne. This generates a very reactive vinyl nitrate ester radical. This radical then abstracts an intramolecular hydrogen atom five atoms away. This leads to a 5-exo-trig ring-closure. The resulting radical then fragments into a ketone and nitrogen dioxide.

Finally, these reactions can connect to many other areas of chemistry. Radical reactions with aromatic rings, or arenes, are often studied. However, bonds between arenes and their substituents are famously strong. This makes generating an aryl radical difficult. Because of this, these reactions are often viewed as nucleophilic aromatic substitution. One known example is the Meerwein arylation. Understanding these radical movements helps scientists build complex molecules. It shows how small changes in energy or structure can drive major chemical shifts.