What Are Quinones? A Complete Guide for Chemistry Students
Quinones are a category of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by converting an even number of –CH= groups into –C(=O)– groups with the required double bond rearrangement, resulting in "a complete conjugated cyclic dione structure." The 1,4-benzoquinone or cyclohexadienedione, commonly referred to as "quinone," is the class's archetypal member (thus the name of the class). 1,2-benzoquinone (ortho-quinone), 1,4-naphthoquinone, and 9,10-anthraquinone are other significant examples.
In theories about the relationship between chemical constitution and colour, the quinone structure is crucial. Quinones, such as benzoquinones, naphthoquinones, anthraquinones, and polycyclic quinones, can be found as biological pigments (biochromes). Quinones are present in bacteria, fungi, and higher plant types, but only a few species contain them. Sea urchins, aphids, lac insects, and certain scale insects are examples of animals that get their quinone compounds from the plants they consume. Some chemical measures of acidity or alkalinity, as well as different dyes used to colour certain types of fabrics, are naphthoquinones.
This article will study quinone structure and benzoquinone structure in detail.
Properties of Quinone
Quinones are oxidized aromatic derivatives that are commonly produced from reactive aromatic compounds with electron-donating substituents including phenols and catechols, which increase the nucleophilicity of the ring and lead to high redox potential needed to break aromaticity. (Quinones are aromatic but not conjugated.) Quinones are Michael acceptors that have been stabilized by conjugation. The reduction can rearomatize the compound or split the conjugation depending on the quinone and the reduction site. Conjugate addition almost always results in the conjugation being broken.
Given below is the structure of benzoquinone structure:
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Quinones are usually made from aromatic amines, polyhydric phenols, and polynuclear hydrocarbons that have been oxidized. The reduction of quinones to the corresponding dihydroxy form is their most significant characteristic reaction. P-benzoquinone (C6H6O2) is reversibly reduced to hydroquinone (C6H6O2) in acidic solution; the reaction is characterized by a measurable electrical potential of a magnitude that depends on the hydrogen ion concentration of the reaction medium as well as the concentrations of benzoquinone and hydroquinone. To assess hydrogen ion concentrations in unknown solutions, the so-called quinhydrone electrode is used, which contains equal quantities of p-benzoquinone and hydroquinone. Hydroquinone is primarily used as a photographic developer.
P-benzoquinone is a bright yellow solid with a strong odour that is slightly soluble in water and melts at about 115° C. Hydroquinone is a crystalline white solid that is water and alcohol soluble.
Benzoquinone Aromatic
The chemical compound 1,4-benzoquinone, also known as para-quinone, has the formula C6H4O2. It forms bright-yellow crystals with an unpleasant odour that resembles chlorine, bleach, and hot plastic or formaldehyde when it is pure. The oxidized derivative of 1,4-hydroquinone is this six-membered ring compound. The molecule serves as a ketone, capable of forming oximes, an oxidant, capable of forming the dihydroxy derivative, and an alkene, capable of undergoing addition reactions, especially those typical of, -unsaturated ketones. Both strong mineral acids and alkali react with 1,4-benzoquinone, causing condensation and decomposition of the compound.
Quinone Uses
Quinone is primarily used as a precursor to hydroquinone, which is used as a reducing agent and antioxidant in photography and rubber manufacturing. Benzoquinonium is a benzoquinone-based skeletal muscle relaxant and ganglion blocking agent.
Ubiquinone-1 is a 1,4-benzoquinone that is found in nature and is involved in respiration.
Ubiquinones, as their name suggests, are found in all living things and are components of the respiratory system. Blattellaquinone is a cockroach sex pheromone. They are thought to exist in all breathing species. Some act as electron acceptors in electron transport chains, such as those involved in photosynthesis (plastoquinone, phylloquinone) and aerobic respiration (phylloquinone, phylloquinone) (ubiquinone). Animals use phylloquinone, also known as Vitamin K1, to carboxylate specific proteins involved in blood coagulation, bone development, and other processes.
Quinones are of pharmacological interest in a number of ways. They are a form of cytotoxin that is used in the fight against cancer. Daunorubicin, for example, is an antileukemic drug. Some of them have anti-tumour properties. They reflect some herbal medicine claims.
Quinone derivatives are used in many natural and synthetic colouring substances (dyes and pigments). They are only second to azo dyes in terms of value as dyestuffs, with a focus on blue colours. The first natural dye to be synthesized from coal tar was alizarin which was extracted from the madder plant.
In organic chemistry, benzoquinone is used as an oxidizing agent. Chloranil and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone are two strongly oxidizing quinones (also known as DDQ).
1,4-Benzoquinone is a toxic metabolite present in human blood that can be used to detect benzene intake or mixtures of benzene and benzene compounds, such as gasoline. The compound can disrupt cellular respiration, and extreme exposure has been linked to kidney damage in animals. It's excreted both in its natural state and as different forms of its own metabolite, hydroquinone.
Did You Know?
1,4-Benzoquinone can stain dark brown skin, cause erythema (redness and rashes on the skin), and cause localized tissue necrosis. It irritates the eyes and respiratory system in particular. Because of its ability to sublime at common temperatures, it poses a greater risk of airborne exposure than a room-temperature solid. The IARC found insufficient evidence to comment on the compound's carcinogenicity, but it did notice that it can easily move into the bloodstream, that it inhibits protease enzymes involved in cellular apoptosis, and that it showed activity in depressing bone marrow output in mice.
FAQs on Quinone: Definition, Properties, and Uses
1. What are quinones and what is their basic chemical structure?
A quinone is a class of organic compounds derived from an aromatic ring, like benzene, where two –CH= groups are replaced by two carbonyl (–C=O) groups. The simplest example is 1,4-benzoquinone, which has the chemical formula C₆H₄O₂. They are a type of fully conjugated cyclic dione.
2. What is the main difference between quinone and quinine?
Although their names sound similar, they are very different compounds. Quinone refers to a class of aromatic diones used in dyes and chemical synthesis. In contrast, quinine is a complex natural alkaloid compound derived from cinchona tree bark, famous for its use in treating malaria.
3. What are some important uses of quinones in industry and nature?
Quinones have several important applications across different fields. Key uses include:
- Dyes and Pigments: Many natural and synthetic dyes, like Alizarin, are based on a quinone structure.
- Biological Roles: They are crucial for processes like photosynthesis and cellular respiration, where they function as electron carriers.
- Chemical Synthesis: They are often used as oxidising agents in various organic reactions.
- Medicine: Some important drugs, including certain anticancer agents and Vitamin K, are structurally quinones.
4. Are benzoquinone and quinone the same thing?
Not exactly. The term "quinone" refers to the entire class of compounds. Benzoquinone is the simplest member of this class, specifically the one derived from a benzene ring. So, while benzoquinone is a quinone, the general term also includes more complex structures like naphthoquinones (from naphthalene) and anthraquinones (from anthracene).
5. How are quinones involved in biological processes like photosynthesis?
In biology, quinones act as vital electron and proton carriers. During photosynthesis, a specific quinone called plastoquinone is essential. It accepts electrons from Photosystem II and transports them to the next component in the electron transport chain, which is a critical step in converting light energy into chemical energy.
6. Why do phenols often turn dark when exposed to air?
Phenols are highly susceptible to oxidation. When exposed to air (oxygen) and light, they slowly oxidise to form benzoquinone. Benzoquinone itself is yellow, but it is highly reactive and can polymerise to form darker, complex products. This is why a sample of phenol often develops a dark pink or brownish colour over time.
7. Is Vitamin K considered a type of quinone?
Yes, Vitamin K is a fat-soluble vitamin whose chemical structure is based on 2-methyl-1,4-naphthoquinone. This quinone structure is essential for its biological function in blood clotting, as it allows the molecule to participate in the electron transfer reactions needed to activate clotting-factor proteins in the liver.
