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cyclic structure of monosaccharides

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This document discusses monosaccharides, which are simple sugars that cannot be further hydrolyzed. It notes that while monosaccharides are often represented as linear molecules, many adopt cyclic structures through intramolecular reactions between carbonyl and hydroxyl groups. It specifically examines why the aldehyde group in monosaccharides reacts with the hydroxyl group on the fifth carbon rather than the second. The document also discusses the alpha and beta anomeric forms that monosaccharides can take on when forming cyclic structures, and how glucose exists in both the alpha and beta forms in solution.

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PRESENTED BY SYEDA JAWERIA REHMAN BIO CHEMISTRY
Introduction Derived from a latin word  Also called simple sugars Sweet in taste Can not hydrolyse Examples o glucose o fructose o glactose
Cyclic Structures of Monosaccharides we have represented monosaccharides as linear molecules many of them also adopt cyclic structures conversion occurs because of the ability of aldehydes and ketones to react with alcohols
cyclic structure of monosaccharides
In some cases, OH and carbonyl groups on the same molecule are able to react with one another in an intramolecular reaction  monosaccharides larger than tetroses exist mainly as cyclic compounds
QUESTION?  why the aldehyde reacts with the OH group on the fifth carbon atom rather than the OH group on the second carbon atom next to it?
ANSWER  Recall that cyclic alkanes containing five or six carbon atoms in the ring are the most stable.  The same is true for monosaccharides that form cyclic structures: rings consisting of five or six carbon atoms are the most stable.
cyclic structure of monosaccharides
Stereoisomers  When a straight-chain monosaccharide forms a cyclic structure, the carbonyl oxygen atom may be pushed either up or down, giving rise to two stereoisomers known as anomers • OH group on the first carbon atom projected downward alpha • OH group on the first carbon atom pointed upward,Beta
cyclic structure of monosaccharides
 comparison for glucose α form melts at 146°C β form melts at 150°C specific rotation of +112°, specific rotation of +18.7°.
Mutaration  possible to obtain a sample of crystalline glucose in which all the molecules have the α structure or all have the β structure  dissolved in water  they open to form the carbonyl group  then reclose to form either the α or the β anomer  interconversion between anomeric forms  At equilibrium, the mixture consists of about 36% α- D-glucose, 64% β-D-glucose, and less than 0.02% of the open-chain aldehyde form
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cyclic structure of monosaccharides

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cyclic structure of monosaccharides

  • 1. PRESENTED BY SYEDA JAWERIA REHMAN BIO CHEMISTRY
  • 2. Introduction Derived from a latin word  Also called simple sugars Sweet in taste Can not hydrolyse Examples o glucose o fructose o glactose
  • 3. Cyclic Structures of Monosaccharides we have represented monosaccharides as linear molecules many of them also adopt cyclic structures conversion occurs because of the ability of aldehydes and ketones to react with alcohols
  • 5. In some cases, OH and carbonyl groups on the same molecule are able to react with one another in an intramolecular reaction  monosaccharides larger than tetroses exist mainly as cyclic compounds
  • 6. QUESTION?  why the aldehyde reacts with the OH group on the fifth carbon atom rather than the OH group on the second carbon atom next to it?
  • 7. ANSWER  Recall that cyclic alkanes containing five or six carbon atoms in the ring are the most stable.  The same is true for monosaccharides that form cyclic structures: rings consisting of five or six carbon atoms are the most stable.
  • 9. Stereoisomers  When a straight-chain monosaccharide forms a cyclic structure, the carbonyl oxygen atom may be pushed either up or down, giving rise to two stereoisomers known as anomers • OH group on the first carbon atom projected downward alpha • OH group on the first carbon atom pointed upward,Beta
  • 11.  comparison for glucose α form melts at 146°C β form melts at 150°C specific rotation of +112°, specific rotation of +18.7°.
  • 12. Mutaration  possible to obtain a sample of crystalline glucose in which all the molecules have the α structure or all have the β structure  dissolved in water  they open to form the carbonyl group  then reclose to form either the α or the β anomer  interconversion between anomeric forms  At equilibrium, the mixture consists of about 36% α- D-glucose, 64% β-D-glucose, and less than 0.02% of the open-chain aldehyde form