Classification of Carbohydrates and Glucose
- Carbohydrates are called saccharides.
- Most of the carbohydrates have the general formula Cx(H2O)y.
Classification of Monosaccharides
Monosaccharides are classified based on the number of carbon atoms and the functional group present in them.
Different types of monosaccharides are listed in the given table
Preparation of glucose
By boiling sucrose with dilute HCl or H2SO4 in alcoholic solution
By boiling starch with dilute H2SO4, at 393 K, under pressure
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
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.
Emil Fischer determined the configuration of four chiral carbon atoms (C-2, C-3, C-4, C-5) in glucose.
The correct configuration of glucose is given as
Glucose is correctly named as D(+)-glucose.
The designation (+) implies dextrorotatory nature of glucose and hence, it is also called as dextrose.
The designation D indicates the configuration of glucose.
D/L configuration system
Glyceraldehyde has one chiral carbon atom (C-2) and exists as two enantiomers.
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.
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.
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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