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The reaction creates a ring of carbon atoms closed by one bridging oxygen atom. The resulting molecule has a hemiacetal or hemiketal group, depending on whether the linear form was an aldose or a ketose. The reaction is easily reversed, yielding the original open-chain form. In these cyclic forms, the ring usually has five or six atoms.
In an alkaline solution, a reducing sugar forms some aldehyde or ketone, which allows it to act as a reducing agent, for example in Benedict's reagent. In such a reaction, the sugar becomes a carboxylic acid. All monosaccharides are reducing sugars, along with some disaccharides, some oligosaccharides, and some polysaccharides.
If reaction is between the C-4 hydroxyl and the aldehyde, a furanose is formed instead. [1] The pyranose form is thermodynamically more stable than the furanose form, which can be seen by the distribution of these two cyclic forms in solution. [2] Formation of pyranose hemiacetal and representations of β-D-glucopyranose
In solution, reducing monosaccharides exist in equilibrium between their acyclic and cyclic forms with less than 1% in the acyclic form. The open chain form can close to give the pyranose and furanose with both the α- and β-anomers present for each. The equilibrium population of conformers depends on their relative energies which can be ...
The reaction between C-1 and C-5 yields a six-membered heterocyclic system called a pyranose, which is a monosaccharide sugar (hence "-ose") containing a derivatised pyran skeleton. The (much rarer) reaction between C-1 and C-4 yields a five-membered furanose ring, named after the cyclic ether furan.
The reaction turns the =O group into a hydroxyl, and the hydroxyl into an ether bridge (−O−) between the two carbon atoms, thus creating a ring with one oxygen atom and four or five carbons. If the cycle has five carbon atoms (six atoms in total), the closed form is called a pyranose, after the cyclic ether tetrahydropyran, that has the ...
The β-D-glucopyranosyl group which is obtained by the removal of the hemiacetal hydroxyl group from β-D-glucopyranoseIn organic chemistry, a glycosyl group is a univalent free radical or substituent structure obtained by removing the hydroxyl (−OH) group from the hemiacetal (−CH(OH)O−) group found in the cyclic form of a monosaccharide and, by extension, of a lower oligosaccharide.
In chemistry, a Haworth projection is a common way of writing a structural formula to represent the cyclic structure of monosaccharides with a simple three-dimensional perspective. Haworth projection approximate the shapes of the actual molecules better for furanoses —which are in reality nearly planar—than for pyranoses that exist in ...
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