Biomolecules

Classification of Carbohydrates and Glucose

• Carbohydrates are called saccharides.

• Most of the carbohydrates have the general formula C_x(H₂O)_y.

• Classification

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

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

• 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

• Structure

• 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

• $\text{Specific rotation of glucose}={\left[\mathrm{\alpha }\right]}_{\mathrm{D}}^{20}=+52.7^{\circ }$

• 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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