Free Radical Addition

Free radical addition is an organic reaction where a radical adds to a double bond, creating a new carbon radical intermediate. In Organic Chemistry, it often explains why some alkene additions give anti-Markovnikov products.

What is Free Radical Addition?

Free radical addition is an alkene reaction that goes through radical intermediates instead of carbocations. A radical, which has an unpaired electron, adds to a carbon-carbon double bond and creates a new carbon-centered radical. That second radical then reacts in the next step, so the product forms by a chain process rather than a one-step ionic mechanism.

In Organic Chemistry, you usually see this idea when an alkene reacts under radical conditions, especially with a radical initiator or with oxygen present. The reaction often starts when a weak bond breaks homolytically, making two radicals. From there, one radical adds to the double bond, and the resulting carbon radical is stable enough to continue the chain.

The big reason this term shows up with Markovnikov's rule is that radical addition can give the opposite regiochemistry. Instead of placing the new bond based on carbocation stability, the reaction follows the stability of the radical intermediate. That means the pathway that leaves the more substituted carbon radical usually wins, which can lead to anti-Markovnikov placement.

A simple way to picture it is this: the first radical step chooses the carbon that gives the more stable radical after addition. So if adding one fragment to an unsymmetrical alkene creates a secondary radical instead of a primary one, that pathway is favored. The mechanism is about radical stability, not the same electrophilic logic you use for acid-catalyzed alkene additions.

This is also why free radical addition matters in polymer chemistry. Once a radical starts adding to one alkene, the new radical can keep adding more alkene units, making a chain. Oxygen can sometimes help generate or sustain radical species through peroxy radicals, which keeps the propagation cycle going.

Why Free Radical Addition matters in Organic Chemistry

Free radical addition gives you the logic behind a whole class of alkene reactions that do not follow the usual carbocation pattern. In Organic Chemistry, that means you need to know not just the product, but why the product forms where it does. If you can track radical stability, you can predict regiochemistry instead of memorizing outcomes.

This term also connects mechanism to synthesis. Many reactions that install a new group onto an alkene use radical conditions when ionic conditions would give the wrong product or a messy mixture. That comes up in homework problems where you compare two possible mechanisms and decide whether the reaction is ionic or radical.

It also builds the habit of reading reaction conditions carefully. Light, peroxides, and oxygen can point you toward a radical pathway, while acids and carbocations point you toward electrophilic addition. Once you see that difference, questions about alkene addition become much easier to sort out.

Free radical addition is one of the clearest examples of how structure controls reactivity. The more stable intermediate is usually the one that forms, so the product distribution reflects radical stability, not just the starting alkene.

How Free Radical Addition connects across the course

Free Radical

A free radical is the reactive species that starts and carries the chain in free radical addition. The unpaired electron is what makes the mechanism so different from carbocation chemistry. If you can identify where the radical is formed and where it moves next, you can usually predict the product pattern.

Electrophilic Addition

Electrophilic addition is the usual alkene-addition pathway taught alongside Markovnikov's rule. It goes through a carbocation intermediate, so its product orientation depends on carbocation stability. Free radical addition often shows up as the exception, because it follows radical stability instead.

Markovnikov's Rule

Markovnikov's rule predicts where a hydrogen and other group add in many alkene reactions. Free radical addition can give the opposite orientation, which is why the rule and radical mechanisms are often discussed together. The key difference is the intermediate, carbocation versus radical.

Hyperconjugation

Hyperconjugation helps explain why more substituted radicals are more stable. That stability matters in free radical addition because it influences which carbon radical forms after the first addition step. This is the same kind of stability thinking you use for carbocations, but applied to radicals.

Is Free Radical Addition on the Organic Chemistry exam?

A problem set question usually gives you an alkene plus radical conditions and asks for the major product. You use the mechanism to decide which carbon gets the new group, then check whether the radical intermediate formed after the first addition is more substituted and therefore more stable. If the prompt mentions peroxide, oxygen, or radical initiators, that is a strong clue that you should think radical, not electrophilic.

In a mechanism question, you may need to show homolytic bond cleavage, radical addition to the double bond, and chain propagation. If the product looks anti-Markovnikov, that is often the point of the question. The fastest way to answer correctly is to identify the intermediate first, then build the product from the most stable radical pathway.

Free Radical Addition vs Electrophilic Addition

These reactions can both add groups across a carbon-carbon double bond, so they are easy to mix up. Electrophilic addition goes through a carbocation, while free radical addition goes through a radical. That difference changes the product orientation and the reaction conditions you look for.

Key things to remember about Free Radical Addition

• Free radical addition is an alkene reaction that proceeds through radical intermediates, not carbocations.

• The product orientation depends on radical stability, so it can differ from Markovnikov's rule.

• Radical initiators, oxygen, or other radical-friendly conditions are clues that the mechanism is radical.

• The reaction often runs as a chain process, with one radical creating the next radical intermediate.

• This mechanism shows up often in polymer formation and in reactions where anti-Markovnikov placement is expected.

Frequently asked questions about Free Radical Addition

What is free radical addition in Organic Chemistry?

Free radical addition is an alkene reaction where a radical adds across a double bond to make a new carbon radical intermediate. That intermediate then reacts again to give the final product. In Organic Chemistry, it is a major example of a chain mechanism and often produces anti-Markovnikov products.

How is free radical addition different from Markovnikov addition?

Markovnikov addition usually refers to electrophilic addition through a carbocation intermediate. Free radical addition follows a radical pathway, so the major product can be placed differently on the alkene. The deciding factor is radical stability, not carbocation stability.

Why does oxygen matter in free radical addition?

Oxygen can help generate or maintain radical species, often through peroxy radicals. That can keep the chain reaction moving. If oxygen is mentioned in a reaction setup, it is a clue that radical chemistry may be involved.

How do I know the major product in a free radical addition problem?

Look for the conditions that signal a radical mechanism, then ask which first-added intermediate is the most stable radical. The favored product is usually the one formed through the more stable carbon radical, not the one you would pick using carbocation logic.