Carbohydrates Notes
Carbohydrates
Carbohydrate are composed of mainly carbon, hydrogen and oxygen. Carbohydrates are technically hydrates of carbon in which hydrogen and oxygen are in 2:1 ratio. The empirical formula of carbohydrate is Cn(H2O)m, n and m may or may not be same. All carbohydrates does not follow this emperical formula like Rhamnose (C6H12O5). Many molecules which follow this emperical formula but they are not carbohydrate(HCHO & CH3COOH etc.).
Structurally, carbohydrates are polyfunctional compounds containing two types of functional group i.e. hydroxyl group and carbonyl group. They are polyhydroxy aldehydes or ketones or compounds which are converted to these on hydrolysis.
Classification of Carbohydrates
Carbohydrates are classified into two main classes, sugars and polysaccharides. Sugars are sweet crystalline substances that are soluble in water. These are further classified on the basis of their behavior on hydrolysis. The simplest form of carbohydrates is the monosaccharide. 'Mono' means 'one' and 'saccharide' means 'sugar'.
Monosaccharides
Monosaccharides are polyhydroxy aldehyde or ketone that
cannot be hydrolyzed further to give simpler sugar. They may again be classified on the basis of the nature of carbonyl group.
a. Polyhydroxy aldehydes are called aldoses. e.g. Glucose
b. Polyhydroxy ketones are called ketoses. e.g. Fructose
The aldoses and ketoses are further divided on the basis of the number of carbons present in their molecules, as trioses, tetroses, pentoses, hexoses etc. They are referred to as aldotrioses, aldotetroses, aldopentoses, aldohexoses, ketohexoses etc.
Oligosaccharides
Carbohydrates that produce two to ten monosaccharide units during the hydrolysis are called oligosaccharides. They can be further classified based on the number of monosaccharide units formed on hydrolysis.
If two monosaccharide units are obtained on hydrolysis it is called disaccharide. Similarly, for three, four, five... monosaccharide units, it is called tri, tetra, penta... saccharide respectively.
Polysaccharides
Carbohydrates that produce a large number of monosaccharide units on hydrolysis. These monosaccharide units are joined together by oxide bridges. These linkages are called glycosidic linkages. The common and widely distributed polysaccharides are not sweet in taste, so they are called non-sugars. Some common examples are starch, cellulose, glycogen, etc.
D and L Notations of Carbohydrates
The notations D and L are used to describe the configurations of carbohydrates and amino acids. Glyceraldehyde has been taken as arbitrary standard for the D and L notation because, it has an asymmetric carbon and can exist as a pair of enantiomers.
In a Fischer projection, the carbonyl group is always placed on the top position for monosaccharide. In this structure,
if the '–OH' group attached to the first asymmetric center from the bottom is on the right side, then, the compound is a D-sugar. If the '–OH' group is on the left side, then, the compound is a L-sugar. Almost all sugars found in nature are D-sugar.
They do not indicate whether the compound rotates polarized light to the right or to the left. For example, D-glyceraldehyde is dextrorotatory, whereas D-lactic acid is levorotatory.
Mutarotation
Normally D-(+)-glucose has a melting point of 146°C. When D-(+)-glucose is crystallized by evaporating an aqueous solution above 98°C, a second form of D-(+)-glucose with a melting point of 150°C can be obtained. When the optical rotations of these two forms are measured, they are found to be significantly different, but when an aqueous solution of either form is allowed to stand, its rotation changes. The specific rotation of one form decreases while the other form increases, until both solutions show the same value of rotation.
For example, a solution of α-D-(+)-glucose (mp 146°C) specific rotation gradually decreases from an initial value of +112.2° to +52.7°, while The β-D-(+)- glucose (mp 150°C) specific rotation gradually increases from an initial value of + 18.7° to + 52.7°. These forms of glucose reach equilibrium with the specific rotation of +52.7. This change or mutation in the specific rotation toward equilibrium is called mutarotation.
Osazone Formation
Aldose and ketose react with one equivalent of phenylhydrazine to produce phenylhydrazones.
When three equivalent of phenylhydrazine reacts with Aldose and ketose it forms bis-hydrazone known as an osazone. C-1 and C-2 both reacts with phenylhydrazine and forms osazone along with aniline, ammonia and water.
D-Glucose and D-fructose form the same osazone as their structure are same except at C-1 and C-2 position.
H-C=N-NH-Ph
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C=N-NH-Ph
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Structure of glucose
1. On the basis of elemental analysis and molecular weight determination the molecular formula of glucose is C6H12O6.
2. The reduction of glucose with red phosphorus and HI gives n-hexane confirm the six carbon atoms in glucose are in a straight chain.
3. It forms penta acetate on treatment with acetic anhydride which indicates the presence of five hydroxyl groups in the molecule. Since glucose is a stable molecule so, all the five hydroxyl groups are on seperate carbon atoms.
4. Glucose reacts with hydroxyl amine to form an oxime and with hydrogen cyanide to form cyanohydrins. It indicates the presence of a carbonyl group.
5. The mild oxidation of glucose with bromine water or sodium hypobromide yields a monocarboxylic acid (gluconic acid) containing same number of carbon atoms as in glucose. This confirms the carbonyl group must be an aldehyde group.
6. The catalytic reduction of glucose gives a hexahydric alcohol (sorbitol) which gives hexaacetate on treatment with acetic anhydride. The sixth hydroxyl group must be obtained by the reduction of aldehyde group, thus further confirming the presence of an aldehyde group and five hydroxyl groups in glucose.
7. Oxidation of gluconic acid with nitric acid yields a dicarboxylic acid (glucaric acid) with the same number of carbon atoms as in glucose. Thus besides aldehyde group, glucose must contain a primary alcoholic group also, which generates the second carboxylic group on oxidation.
On the basis of above reactions, Fisher assigned an open chain structure of glucose shown below-
Structure Determination Open Chain of D-Fructose
