Chemistry Classification of Carbohydrates and Preparation of Glucose

Topics Covered :

● Introduction
● Carbohydrates
● Cassification of Carbohydrates
● Monosaccharides
● Glucose
● Preparation of Glucose

Introduction :

`=>` A living system grows, sustains and reproduces itself.

`=>` The most amazing thing about a living system is that it is composed of non-living atoms and molecules.

`=>` The pursuit of knowledge of what goes on chemically within a living system falls in the domain of biochemistry.

`=>` Living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc.

`=>` Proteins and carbohydrates are essential constituents of our food. These biomolecules interact with each other and constitute the molecular logic of life processes.

`=>` In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms.

Carbohydrates :

`=>` Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds.

● Some common examples are cane sugar, glucose, starch, etc.

● Most of them have a general formula, `color{red}(C_x(H_2O)_y)`, and were considered as hydrates of carbon from where the name carbohydrate was derived.

● For example, the molecular formula of glucose `color{red}((C_6H_(12)O_6))` fits into this general formula, `color{red}(C_6(H_2O)_6)`.

● But all the compounds which fit into this formula may not be classified as carbohydrates.

● Acetic acid `color{red}((CH_3COOH))` fits into this general formula, `color{red}(C_2(H_2O)_2)` but is not a carbohydrate.

● Similarly, rhamnose, `color{red}(C_6H_(12)O_5)` is a carbohydrate but does not fit in this definition.

`=>` A large number of their reactions have shown that they contain specific functional groups.

`=>` Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis.

`=>` Some of the carbohydrates, which are sweet in taste, are also called sugars.

● The most common sugar, used in our homes is named as sucrose whereas the sugar present in milk is known as lactose.

● Carbohydrates are also called saccharides (Greek : sakcharon means sugar).

Classification of Carbohydrates :

Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following three groups :

(i) `color{green}("Monosaccharides ")` : A carbohydrate that cannot be hydrolysed further to give simpler unit of polyhydroxy aldehyde or ketone is called a monosaccharide. About `20` monosaccharides are known to occur in nature. Some common examples are glucose, fructose, ribose, etc.

(ii) `color{green}("Oligosaccharides ")` : Carbohydrates that yield two to ten monosaccharide units, on hydrolysis, are called oligosaccharides.

● They are further classified as disaccharides, trisaccharides, tetrasaccharides, etc., depending upon the number of monosaccharides, they provide on hydrolysis.

● Amongst these the most common are disaccharides. The two monosaccharide units obtained on hydrolysis of a disaccharide may be same or different.

● For example, sucrose on hydrolysis gives one molecule each of glucose and fructose whereas maltose gives two molecules of glucose only.

(iii) `color{green}("Polysaccharides ")` : Carbohydrates which yield a large number of monosaccharide units on hydrolysis are called polysaccharides.

● Some common examples are starch, cellulose, glycogen, gums, etc.

● Polysaccharides are not sweet in taste, hence they are also called non-sugars.

`=>` The carbohydrates may also be classified as either reducing or non-reducing sugars.

● All those carbohydrates which reduce Fehling’s solution and Tollens’ reagent are referred to as reducing sugars.

● All monosaccharides whether aldose or ketose are reducing sugars.

● In disaccharides, if the reducing groups of monosaccharides i.e., aldehydic or ketonic groups are bonded, these are non-reducing sugars e.g. sucrose.

● On the other hand, sugars in which these functional groups are free, are called reducing sugars, for example, maltose and lactose.

Monosaccharides :

`=>` Monosaccharides are further classified on the basis of number of carbon atoms and the functional group present in them.

`=>` If a monosaccharide contains an aldehyde group, it is known as an aldose and if it contains a keto group, it is known as a ketose.

`=>` Number of carbon atoms constituting the monosaccharide is also introduced in the name as is evident from the examples given in Table 14.1

Glucose :

`=>` Glucose occurs freely in nature as well as in the combined form.

`=>` It is present in sweet fruits and honey.

`=>` Ripe grapes also contain glucose in large amounts.

Preparation of Glucose :

(i) `color{green}(["From sucrose (Cane sugar) "])` : If sucrose is boiled with dilute `color{red}(HCl)` or `color{red}(H_2SO_4)` in alcoholic solution, glucose and fructose are obtained in equal amounts.

`color{red}(undersettext(Sucrose) (C_(12) H_(22)O_(11)) +H_2O overset(H^+)→ undersettext(Glucose)(C_6H_(12) O_6) +undersettext(Fructose)(C_6H_(12) O_6))`

(ii) `color{green}("From starch ")` : Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute `color{red}(H_2SO_4)` at `393 K` under pressure.

`color{red}(undersettext(Starch or cellulose ) ((C_6H_(10)O_5)_n) +nH_2O underset(393K ; 2-3 atm) overset(H^+) → undersettext(Glucose)(n C_6H_(12) O_6))`