chemistry

Organic peroxides

Organic peroxides 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 Organic peroxides rather than just read about it. In short: Organic peroxides are organic compounds containing the peroxide functional group (R−O−O−R′). If the R′ is hydrogen, the compounds are called organic hydroperoxides.

Key takeaways

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

Reference excerpt

Organic peroxides are organic compounds containing the peroxide functional group (R−O−O−R′). If the R′ is hydrogen, the compounds are called organic hydroperoxides. The O−O bond of peroxides easily breaks, producing free radicals of the form RO• (the dot represents an unpaired electron). Thus, organic peroxides are useful in organic chemistry as initiators for some types of polymerization, such as the acrylic, unsaturated polyester, and vinyl ester resins used in glass-reinforced plastics. MEKP and benzoyl peroxide are commonly used for this purpose. However, the same property also means that organic peroxides can explosively combust. Organic peroxides, like their inorganic counterparts, are often powerful bleaching agents.

Types of organic peroxides Organic peroxides are classified (i) by the presence or absence of a hydroxyl (−OH) terminus and (ii) by the presence of alkyl vs acyl substituents.

Diphenyl peroxide has never been observed, and quantum chemical calculations predict that it does not exist, because it immediately undergoes a nearly barrierless reaction akin to the benzidine rearrangement.

Properties The O−O bond length in peroxides is about 1.45 Å, and the R−O−O angles (R = H, C) are about 110° (water-like). Characteristically, the C−O−O−R (R = H, C) dihedral angles are about 120°. The O−O bond is relatively weak, with a bond dissociation energy of 45–50 kcal/mol (190–210 kJ/mol), less than half the strengths of C−C, C−H, and C−O bonds.

Biology

Peroxides play important roles in biology. Hundreds of peroxides and hydroperoxides are known, being derived from fatty acids, steroids, and terpenes. The prostaglandins are biosynthesized by initial formation of a bicyclic peroxide ("endoperoxide") derived from arachidonic acid. Many aspects of biodegradation or aging are attributed to the formation and decay of peroxides formed from oxygen in air. Countering these effects, an array of biological and artificial antioxidants destroy peroxides. In fireflies, oxidation of luciferins, which is catalyzed by luciferases, yields a peroxy compound 1,2-dioxetane. The dioxetane is unstable and decays spontaneously to carbon dioxide and excited ketones, which release excess energy by emitting light (bioluminescence).

Industrial uses

In polymer chemistry Many peroxides are used as a radical initiators, e.g., to enable polymerization of acrylates. Industrial resins based on acrylic and/or methacrylic acid esters are invariably produced by radical polymerization with organic peroxides at elevated temperatures. The polymerization rate is adjusted by suitable choice of temperature and type of peroxide. Methyl ethyl ketone peroxide, benzoyl peroxide and to a smaller degree acetone peroxide are used as initiators for radical polymerization of some thermosets, e.g. unsaturated polyester and vinyl ester resins, often encountered when making fiberglass or carbon fiber composites (CFRP). Examples include boats, RV units, bath tubs, pools, sporting equipment, wind turbine blades, and a variety of industrial applications. Benzoyl peroxide, peroxyesters/peroxyketals, and alkylperoxy monocarbonates are used in production of polystyrene, expanded polystyrene, and High Impact Polystyrene, and benzoyl peroxide is utilized for many acrylate based adhesive applications. Thermoplastic production techniques for many industrial polymerization applications include processes which are carried out in bulk, solution, or suspension type batches. Relevant polymers include: polyvinyl chloride (PVC), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polymethyl methacrylate (PMMA), Polystyrene, and Polycarbonates.

Other uses For flour, it is desirable to oxidize glutenin and gliadin, since it converts free thiol groups to disulfide bonds. Then, when flour is mixed with water, the glutenin and gliadin combine to form gluten. More disulfide bonds increases the strength of the gluten and also its other mechanical properties. The degree of oxidization controls the mechanical property of the gluten. Before industrialization, it was common to let the flour slowly oxidize by air. After industrialization, this is too slow, and benzoyl peroxide and hydrogen peroxide are used to treat flour as bleaching and "maturing" agents. Bleaching flour also turns it white, which may be aesthetically pleasing. For example, cake flour is often bleached with chlorine to make it fully white. Benzoyl peroxide is an effective topical medication for treating most forms of acne.

Preparation

From hydrogen peroxide Dialkyl peroxides, e.g., dicumyl peroxide, are synthesized by addition of hydrogen peroxide to alkenes or by O-alkylation of hydroperoxides. Diacyl peroxides are typically prepared by treating hydrogen peroxide with acid chlorides or acid anhydrides in the presence of base:

H2O2 + 2 RCOCl → (RCO2)2 + 2 HCl H2O2 + (RCO)2O → (RCO2)2 + H2O The reaction competes with hydrolysis of the acylating agent but the hydroperoxide anion is a superior nucleophile relative to hydroxide. Unsymmetrical diacyl peroxides can be produced by treating acyl chlorides with the peroxy acid. Peresters, an example being tert-Butyl peroxybenzoate, are produced by treating acid anhydrides or acid chlorides with hydroperoxides.

From O2

Cyclic peroxides can be obtained by cycloaddition of singlet oxygen (generated by UV radiation) to dienes. An important example is rubrene. Six-membered cyclic peroxides are called endo peroxides. The four-membered dioxetanes can be obtained by 2+2 cycloaddition of oxygen to alkenes. The hazards associated with storage of ethers in air is attributed to the formation of hydroperoxides via the direct albeit slow reaction of triplet oxygen with C-H bonds.

Reactions

Homolysis Organic peroxides are widely used to initiate polymerization of olefins, e.g. the formation of polyethylene. A key step is homolysis:

ROOR ⇌ 2 RO. The tendency to homolyze is also exploited to modify polymers by grafting or visbreaking, or cross-link polymers to create a thermoset. When used for these purposes, the peroxide is highly diluted, so the heat generated by the exothermic decomposition is safely absorbed by the surrounding medium (e.g. polymer compound or emulsion).

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Organic peroxides

Start with the simplest possible case. Write down what Organic peroxides 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 Organic peroxides 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 Organic peroxides 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 Organic peroxides

In research: Organic peroxides 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 Organic peroxides 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: Organic peroxides is common in secondary-school and first-year university syllabi. It links to neighbouring topics Food additives, Functional groups, Organic compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Organic peroxides 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 Organic peroxides in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Organic peroxides 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 Organic peroxides out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Organic peroxides in simple terms?

Organic peroxides are organic compounds containing the peroxide functional group (R−O−O−R′). If the R′ is hydrogen, the compounds are called organic hydroperoxides.

Why does Organic peroxides 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 Organic peroxides?

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 Organic peroxides.