chemistry

Radical substitution

Radical substitution is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Radical substitution rather than just read about it. In short: In organic chemistry, a radical-substitution reaction is a substitution reaction involving free radicals as a reactive intermediate. The reaction always involves at least two steps, and possibly a third.

Key takeaways

Radical substitution belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Radical substitution to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Radical substitution from memory before moving on to harder problems.

Reference excerpt

In organic chemistry, a radical-substitution reaction is a substitution reaction involving free radicals as a reactive intermediate. The reaction always involves at least two steps, and possibly a third.

In the first step called initiation (2,3), a free radical is created by homolysis. Homolysis can be brought about by heat or ultraviolet light, but also by radical initiators such as organic peroxides or azo compounds. UV Light is used to create two free radicals from one diatomic species. The final step is called termination (6,7), in which the radical recombines with another radical species. If the reaction is not terminated, but instead the radical group(s) go on to react further, the steps where new radicals are formed and then react are collectively known as propagation (4,5). This is because a new radical is created, able to participate in secondary reactions.

Radical substitution reactions In free radical halogenation reactions, radical substitution takes place with halogen reagents and alkane substrates. Another important class of radical substitutions involve aryl radicals. One example is the hydroxylation of benzene by Fenton's reagent. Many oxidation and reduction reactions in organic chemistry have free radical intermediates, for example the oxidation of aldehydes to carboxylic acids with chromic acid. Coupling reactions can also be considered radical substitutions. Certain aromatic substitutions takes place by radical-nucleophilic aromatic substitution. Auto-oxidation is a process responsible for deterioration of paints and food, as well as production of certain lab hazards such as diethyl ether peroxide. More radical substitutions are listed below:

The Barton–McCombie deoxygenation involves substitution of a hydroxyl group for a proton. The Wohl–Ziegler reaction involves allylic bromination of alkenes. The Hunsdiecker reaction converts silver salts of carboxylic acids to alkyl halides. The Dowd–Beckwith reaction involves ring expansion of cyclic β-keto esters. The Barton reaction involves synthesis of nitrosoalcohols from nitrites. The Minisci reaction involves generation of an alkyl radical from a carboxylic acid and a silver salt, and subsequent substitution at an aromatic compound

References

Illustrations

Worked examples

Example 1 — a first encounter with Radical substitution

Start with the simplest possible case. Write down what Radical substitution claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Radical substitution before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Radical substitution ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Radical substitution

In research: Radical substitution appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Radical substitution in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Radical substitution is common in secondary-school and first-year university syllabi. It links to neighbouring topics Free radical reactions, Reaction mechanisms, so understanding it makes those chapters shorter.
In everyday life: Look for Radical substitution outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Radical substitution in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Radical substitution means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Radical substitution out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Radical substitution in simple terms?

Why does Radical substitution matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Radical substitution?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Radical substitution.