What is Pyran?
Pyran is a heterocyclic series of chemical compounds with five carbon atoms and one oxygen atom in a ring structure. Only one of two probable simple pyran compounds has been identified; it was synthesised in 1962 and proved to be extremely unstable. Tetrahydropyran, which is formed by hydrogenating the dihydro molecule, is one of the stable members of this family. The glucose unit found in sucrose, starch, cellulose, and glycogen is a good example of pyranose sugars, including the tetrahydropyran ring.
The pyrones have a carbonyl group (>C=O) that replaces the pyrans' methylene group (>CH2). Toad venoms and squill poisonous principles are pyrones, which belong to the steroid family.
The pyran ring exists as a positively charged ion in the pyrylium salts, which are coloured compounds. Flavylium salts, which are closely related, are pigments found in roses, blue cornflowers, and other flowers.
Pyran Structure
The only difference between the two isomers of pyran, 2H-pyran and 4H-pyran, is the placement of the double bonds. Pyrano Flavonoids are biologically significant compounds with pyrans in the ring. Pyran-based molecules are used as antimicrobials, antivirals, mutagenic chemicals, antiproliferative, sex pheromones, anticancer chemicals, cancer treatments, and central nervous system activity because they are physiologically active molecules with varied pharmacological actions. Some pyran derivatives are commonly used in cosmetics and pigments, as well as possible biodegradable agrochemicals.
2H-pyran
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4H pyran
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Tetrahydropyran
Tetrahydropyrans, commonly known as oxanes, are chemical compounds made up of a saturated six-membered ring with five carbon atoms and one oxygen atom in each ring. The core of pyranose carbohydrates like glucose is the tetrahydropyran ring structure. The 2-tetrahydropyranyl group is widely employed in chemical synthesis as a protective group for the protection of alcohols as tetrahydropyranyl ether. This shields the alcohol from a wide range of reaction circumstances. The deprotection is straightforward, comprising acidic hydrolysis and the subsequent production of 5-hydroxypentanal as a byproduct. Tetrahydropyran-containing compounds have been regarded as a valuable addition to the growing list of chemicals used in the construction of liquid crystals for LCD applications.
Tetrahydropyran Structure
Tetrahydropyranyl ethers are extensively utilised in organic synthesis, even though tetrahydropyran is a rare molecule. The 2-tetrahydropyranyl (THP) group, in particular, is a common protective group for alcohols. 2-tetrahydropyranyl ethers are formed when alcohols combine with 3,4-dihydropyran. These ethers can withstand a wide range of reactions. After that, acid-catalyzed hydrolysis can be used to recover the alcohol. The parent alcohol and 5-hydroxypentanal are both formatted during this hydrolysis. Diastereomers are formed by THP ethers produced from chiral alcohols. Another disadvantage is that the ethers have complicated NMR spectra, which makes analysis difficult.
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Pyranose
Pyranose is a name that refers to saccharides that have a six-membered ring with five carbon atoms and one oxygen atom in their chemical structure. There could be additional carbons outside the ring. The name comes from its resemblance to the oxygen heterocyclic pyran, except that the pyranose ring lacks double bonds. A pyranoside is a pyranose in which the anomeric OH at C(l) has been transformed to an OR group.
Formation of Pyranose Structure
The hydroxyl group on a sugar's carbon 5 (C-5) reacts with the aldehyde at carbon 1 to generate the pyranose ring. This results in the formation of an intramolecular hemiacetal. When the C-4 hydroxyl reacts with the aldehyde, a furanose is produced instead. The distribution of these two cyclic forms in solution shows that the pyranose form is thermodynamically more stable than the furanose form.
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Conformations of Pyranose
This leads to 38 distinct basic pyranose conformations: 2 chairs, 6 boats, 6 skew-boats, 12 half-chairs, and 12 envelopes.
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These conformers can interconvert; but, because each form has a significantly different relative energy, there may be a large barrier to interconversion. Quantum mechanics may be used to compute the energy of these conformations, and an example of probable glucopyranose interconversions is presented.
The pyranose ring has conformations that are superficially comparable to those of the cyclohexane ring. However, the ring oxygen is mentioned in the pyranose nomenclature, and the presence of hydroxyls on the ring has a distinct effect on its conformational preference. The pyranose ring also has its own structural and stereochemical properties.
Nomenclature of Pyranose
To name pyranose conformations, the conformer must first be identified. The name comes from the common conformers, which are identical to those found in cyclohexane. Chair (C), boat (B), skew (S), half-chair (H), and envelope (E) are all common conformations (E). After that, the ring atoms are numbered, with the anomeric, or hemiacetal, carbon always being 1. In the acyclic form, oxygen atoms are labelled O and are referred to by the carbon atom to which they are bonded.
When looking at the top face of the ring, the atoms should be numbered clockwise.
The reference plane should be chosen in the chair and skew conformations. The reference plane in the chair conformation is designed so that the lowest-numbered atom (typically C-1) is exo-planar. The plane in the skew conformation has three neighbouring atoms and one with the lowest possible number of coplanar atoms.
Atoms above the plane are written as a superscript before the conformer label.
Atoms below the plane are written as a subscript after the conformer label.
Did You Know?
Furan is a five-membered aromatic ring with four carbon atoms and one oxygen atom that makes up a heterocyclic organic compound. Furans are a type of chemical compound that contains such rings.
Furan is a colourless, highly volatile, flammable liquid with a boiling point near room temperature. It is somewhat soluble in water and soluble in typical organic solvents such as alcohol, ether, and acetone. The odour is described as "strong, ethereal, chloroform-like." It is harmful to humans and may cause cancer. Furan is a starting point for a variety of speciality compounds.
Furan is aromatic because one of the lone pairs of electrons on the oxygen atom is delocalized into the ring, forming a 4n + 2 aromatic system similar to benzene (see Hückel's rule). The molecule is flat and lacks discrete double bonds due to its aromaticity.
FAQs on Pyran
1. What exactly is pyran in chemistry?
Pyran is a six-membered heterocyclic, non-aromatic compound. Its chemical structure consists of a ring with five carbon atoms and one oxygen atom. The chemical formula for its simplest isomers is C₅H₆O. Pyran primarily exists in two isomeric forms, 2H-pyran and 4H-pyran, which differ in the location of their double bonds.
2. How do the structures of 2H-pyran and 4H-pyran differ?
The difference between 2H-pyran and 4H-pyran lies in the position of the saturated CH₂ group (an sp³-hybridised carbon). In 2H-pyran, this saturated carbon is at position 2 of the ring. In 4H-pyran, it is at position 4. This single structural difference changes the placement of the double bonds within the ring and breaks the potential for continuous electron delocalisation.
3. What is the fundamental difference between a pyran ring and a furan ring?
The main differences between pyran and furan are based on their structure and chemical properties:
- Ring Size: Pyran has a six-membered ring (five carbons, one oxygen), while furan has a smaller five-membered ring (four carbons, one oxygen).
- Aromaticity: Furan is an aromatic compound that follows Hückel's rule. In contrast, pyran is non-aromatic because its structure contains a saturated carbon that interrupts conjugation.
- Chemical Formula: The simplest form of pyran is C₅H₆O, whereas for furan it is C₄H₄O.
4. Why is pyran not considered an aromatic compound?
Pyran fails to meet the criteria for aromaticity, specifically Hückel's rule. Its structure contains a saturated, sp³-hybridised carbon atom (a CH₂ group) which breaks the continuous loop of p-orbitals required for cyclic delocalisation. Furthermore, it only has 4 π-electrons from its two double bonds, not the (4n+2) π-electrons needed for a system to be aromatic.
5. How is a pyranose ring formed in monosaccharides like glucose as per the CBSE syllabus?
As per the CBSE Class 12 syllabus on Biomolecules, a pyranose ring in glucose is formed through an intramolecular reaction. The hydroxyl group (–OH) on carbon 5 (C-5) of the open-chain glucose structure acts as a nucleophile and attacks the electrophilic carbonyl carbon of the aldehyde group (C-1). This reaction forms a stable, six-membered cyclic hemiacetal, which is known as the pyranose structure.
6. What is tetrahydropyran (THP) and what is its main role in organic synthesis?
Tetrahydropyran (THP) is the fully saturated derivative of pyran, meaning it has no double bonds in its ring. Its primary role in organic chemistry is as a protecting group for alcohols. An alcohol can react with a derivative of THP to form a THP ether, which is stable under many reaction conditions. This 'protects' the alcohol group from reacting, and it can be easily removed later by simple acid hydrolysis.
7. Why does the pyranose ring adopt chair and boat conformations like cyclohexane?
A pyranose ring adopts these conformations to achieve maximum stability. A flat, six-membered ring would have significant angle strain (bond angles forced to be 120° instead of the ideal tetrahedral 109.5°) and torsional strain from eclipsing C-H bonds. To relieve these strains, the ring puckers into three-dimensional shapes. The most stable of these is the chair conformation, which minimises both types of strain, just like in a cyclohexane ring.
8. Can pyran be classified as an ether?
Yes, from a functional group perspective, pyran can be classified as a cyclic ether. An ether is characterised by an oxygen atom connected to two alkyl or aryl groups (R-O-R'). In pyran, the ring's oxygen atom is single-bonded to two carbon atoms, fitting this C-O-C structural definition. However, it is more specifically identified as a heterocyclic compound due to its ring structure containing a non-carbon atom.
9. What are some important applications of compounds that contain a pyran ring?
Compounds containing a pyran ring structure have diverse and significant applications:
- Carbohydrate Chemistry: The pyranose form is the fundamental structure for most carbohydrates, including glucose, which is a building block for essential polysaccharides like starch and cellulose.
- Natural Pigments: Cationic pyran rings, known as pyrylium salts, are responsible for the vibrant colours in many flowers, such as roses and blue cornflowers.
- Pharmaceuticals: Many pyran-based molecules are biologically active and are investigated for use as antimicrobial, antiviral, and even anticancer agents.
- Organic Synthesis: The derivative tetrahydropyran (THP) is a widely used protecting group for alcohols during complex chemical syntheses.
