Biomolecules
Classification of Carbohydrates and Glucose
- Carbohydrates are called saccharides.
- Most of the carbohydrates have the general formula Cx(H2O)y.
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Classification
Classification of Monosaccharides
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Monosaccharides are classified based on the number of carbon atoms and the functional group present in them.
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Different types of monosaccharides are listed in the given table
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Carbon atoms
General term
Aldehyde
Ketone
3
Triose
Aldotriose
Ketotriose
4
Tetrose
Aldotetrose
Ketotetrose
5
Pentose
Aldopentose
Ketopentose
6
Hexose
Aldohexose
Ketohexose
7
Heptose
Aldoheptose
Ketoheptose
Glucose
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Preparation of glucose
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By boiling sucrose with dilute HCl or H2SO4 in alcoholic solution
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By boiling starch with dilute H2SO4, at 393 K, under pressure
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Structure
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Glucose has been assigned the above structure based on the following evidences:
(i) Molecular formula − C6H12O6
(ii) Suggestion of straight chain
(iii) Confirmation of carbonyl (> C = O) group
(iv) Confirmation of the presence of carbonyl group as aldehydic group
(v) Confirmation of the presence of five −OH groups
(vi) Indication of the presence of a primary alcohol
Optical isomerism in glucose
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Glucose contains four chiral carbon atoms and each one of them can have two distinct spatial arrangement of groups around them. Therefore, glucose has 2⁴ = 16 optical isomers.
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Emil Fischer determined the configuration of four chiral carbon atoms (C-2, C-3, C-4, C-5) in glucose.
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The correct configuration of glucose is given as
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Glucose is correctly named as D(+)-glucose.
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The designation (+) implies dextrorotatory nature of glucose and hence, it is also called as dextrose.
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The designation D indicates the configuration of glucose.
D/L configuration system
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Glyceraldehyde has one chiral carbon atom (C-2) and exists as two enantiomers.
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Conventionally, (+)-glyceraldehyde has OH group attached to C-2 on right side and this configuration is called D while(−)-glyceraldehyde has OH group attached to C-2 on left side and this configuration is called L.
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So, all the compounds which can be chemically correlated to (+)-glyceraldehyde are known to have D configuration while those which can be chemically correlated to (−)-glyceraldehyde are known to have L configuration. This is relative configuration system of chiral compounds.
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The D/L configuration of a monosaccharide is assigned on the basis of the configuration of the lowest chiral carbon in its Fischer projection formula.
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