- They are polyhydroxy-aldehydes or ketones or substances which give these substances on hydrolysis and contain at least one chiral atom.
- They have general formula of C x (H2O)
- Rhamnose, deoxyribose, rhamnohexose do not obey this formula but are carbohydrates.
Types of carbohydrates
- These are simplest carbohydrate which can’t be hydrolyzed further into smaller compounds.
- They are called as aldose or ketose depending upon whether they have aldehyde or ketone group.
- Depending upon the number of carbon atoms present they are called as triose, tetrose etc.
- All monosaccharide’s are sweet smelling crystalline, water soluble and are also capable of diffusing through cell membranes.
For example: Glucose is aldohexose while fructose is a ketohexose. Both of them have 6 carbon atoms. The simplest monosaccharide is a triose (n=3).
Example: Glyceraldehyde and Dihydroxyacetone. They have one or more asymmetric carbon and are optically active.
Their structures are:
All naturally occurying monosaccharides belong to D—series that is OH group at their penultimate C-atom.
Open chain structures:
- D-glucose and D-mannose, differ only in configuration at C-2 and are known as epimers.
- Similarly D-glucose and D-galactose, differ in configuration around C-4 atom and are also known as epimers.
- Thus a pair of diastereomers, differing only in configuration around C-2 or any other chiral carbon except C-1 is called epimers.
Close chain structure
- All the pentose’s and hexoses exist in cyclic hemiacetal structure.
- In free state, they have generally six membered cyclic structure known as pyranose form and in combined state, some of them have 5 membered cyclic structure called as furanose .
Due to cyclic hemiacetal or hemiketal structure all the pentoses and hexoses exist in two stereoisomeric forms
- Alpha form
- Beta form
- Both alpha and beta form are Anomers.
- Their structure is given below :
These carbohydrates on hydrolysis give 2 to 9 molecules of monosaccharides.
They are further of few types:
- Disaccharides (C12H22o11): On hydrolysis, they give 2 molecules of monosaccrides which are held together by Glycosidic linkage
Example: sucrose etc
- Trisaccharides (C18H32o16): On hydrolysis, they form three molecules of monosaccharides.
- Tetra-saccharides: (C24H42o21): Such as stachyose which gives four monosaccrides on hydrolysis.
These are the carbohydrates which on hydrolysis, yield more than nine monosaccharides molecules.
Example: Starch etc
- Glucose exist in two forms : i.e. alpha –D glucose with specific rotation of 112degree and beta D-glucose with specific rotation of +19 degree.
- However, when either of these two forms is dissolved in water and allowed to stand, it gets converted into same equilibrium mixture of both the alpha and beta forms with a small amount of open chain form having specific rotation of 52.7 degree.
- As a result of this, equilibrium the specific rotation of freshly prepared solution of alpha glucose decreases from +112 degree to 52.7 degree while that for beta glucose increases from +19 to 52.7 degrees.
- The phenomenon of change of change in specific rotation of optically active compounds with time to an equilibrium value is known as Mutarotation.
- The alpha D (+) glucose and beta (+) glucose, differ in configuration at C-1 carbon and the compounds differing in configuration at C-1 are called Anomers.
Fructose: It is represented by six membered ring as shown:
Beta –D+(fructose) furanose structure
Fructose is assigned furanose structure.
Lactose: It is made up of molecule and a molecule of galactose. The units are linked together.
Maltose: It has the structure as shown below
- On treatment, with acid or with enzyme maltose gets hydrolysed to 2 molecules.
- That is alpha D-glucose.
- Since one of the glucose units exist in hemiacetal form it is a reducing sugar.
Sucrose: It has a structure shown below
- On hydrolysis, with dilute mineral acid or enzyme invertase sucrose gives glucose and fructose in equal amounts.
- Sucrose and glucose are dextrorotatory while fructose is laevorotatory and has higher value of specific rotation.
- Thus the process is accompanied by inversion of optical activity. The mixture formed is invert sugar. That is sucrose àglucose + fructose
(water and invertase )
Sweetness of sugars
- All the monosaccharide and disaccharides are sweet in taste and hence also known as sugars.
- Sucrose is given sweetness value of 100. The sweetness of other sugars is compared with the value of sucrose.
- The sweetness of fructose -173, invert sugar 130, sucrose 100, glucose 74,glactose 32,maltose 32 and that of lactose is 16.
- All the monosaccharide and disaccharides are reducing agents due to hemiacetal and hemiketal forms which easily change in to aldehydic form in the alkaline medium.
- Although fructose doesn’t contain any aldehydic group yet it gives Tollen’s reagent test and Fehling’s solution test because under the basic conditions of reagent the fructose gets converted into the mixture of glucose and mannose both of which contains aldehydic group.
- This is called Lobry De Bruyn Van Eikensten rearrangement.
- The alpha and beta glucose reacts with one molecule of ethanol to form the corresponding methyl glucosides.
- When glucose is treated with methanol in presence of HCl the hemiacetal form changes to acetal form.
Starch: It serves as a storage polysaccharide in plants .It consist of two components of alpha glucose.
- It is a linear polymer of glucose and is soluble in water.
- Its percentage in starch is about 10-20 %.
- These are linked together by alpha linkage involving C-1 of glucose unit to C-4 of the other.
- It is branched chain polymer of alpha glucose and is insoluble in water.
- There are about 1000 units of glucose.
(c ) Cellulose
- It is found in all plants
- It constitutes 50% of total organic matter in the living beings.
- Cotton is pure cellulose.
- Cellulose is linear polymer of beta D-glucose.
- The chains are arranged to form bundles and linked together by hydrogen bonds between glucose molecules of adjacent organic solvents.
- When it is treated with concentrated sulphuric acid in cold it slowly passes into solution.
- This solution when diluted with water gives starch like substance amyloid which is known as parchment paper.
- On boiling with water it is hydrolyzed into D-glucose.
- Cellulose gives many useful products when treated with different chemicals like rayon, gum, cotton etc.
- Cellulose is directly used in making cloth and paper.
- In glycogen there are about 25 glucose units. Its structure is similar to amylopectin and is a condensation polymer of alpha glucose.
- Glycogen in short term food storage in animals.
Tests for carbohydrates
- For this Molisch test is performed.
- In it Molisch reagent is used which is 10%alcoholic solution of alpha naphthol and it is added to aqueous solution of carbohydrate followed by concentrated sulphuric along the sides of tube.
- As a result a violet ring is formed at the junction of two layers.