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Before we get to know about glucose, we have to know about carbohydrates. Because glucose is an important carbohydrate and more importantly it’s a monosaccharide. Carbohydrates originally come from plants and form a huge group of organic compounds. Some common illustrations of carbohydrates are sugarcane sugar, glucose, starch, etc. Generally, they possess a common formula, Cx (H2O) y.
Glucose is one of the most important monosaccharides. It is the building block of almost every oligosaccharide and polysaccharide.Carbohydrates are of 3 types which are monosaccharides, oligosaccharides and polysaccharides. So, let’s see what monosaccharide means.
What are Monosaccharides ?
A carbohydrate that can not be hydrolysed beyond to offer a smaller yet simpler unit of polyhydroxy aldehyde or ketone is known as a monosaccharide. Around 20 monosaccharides are understood to be present in nature. Some common illustrations are glucose, fructose, ribose, etc.
Glucose
Glucose occurs voluntarily in nature as well as in the associated form. Glucose is an aldohexose and is also comprehended as dextrose. It’s the monomer of numerous of the larger carbohydrates, videlicet bounce, cellulose. It’s presumably the most plenteous organic compound on the globe. And is a monosaccharide because it cant be divided into 2 carbohydrates further.
Preparation of glucose
1. From sucrose (Cane sugar):
When we heat sucrose with dilute HCl or H2SO4 in an alcoholic solution, the obtained solution contains glucose and fructose in equal proportions.
C12H22O11 (sucrose) + H2O + (H+) → C6H12O6 (glucose) + C6H12O6 (fructose) + (H+)
2. From starch:
Commercially glucose is attained by hydrolysis of starch by steaming it with diluted H2SO4 at 393 K under a lot of pressure.
(C6H12O5)n (starch) + nH2O + (H+) →nC6H12O6 (glucose)
Structure of Glucose
Glucose is an aldohexose and is also comprehended as dextrose. It’s the monomer of numerous of the larger carbohydrates, like starch, cellulose. It’s presumably the most plenteous organic compound on the globe. When scientists got the evidence they decided on these structural changes:
1. Its molecular formula was found by scientists to be C6H12O6.
2. When heated for a long time with HI glucose gives n-hexane which means the six-carbon are aligned into a straight chain.
3. Glucose when reacted with hydroxylamine gives oxime and when added a molecule of hydrogen cyanide gives us cyanohydrin. These reactions conclude that there is a carbonyl group (>C = O) present in glucose.
4. Glucose will get oxidised by a mild oxidising reagent so we react it with bromine water which gives six-carbon carboxylic acid known as gluconic acid. This indicates that the carbonyl part exists as an aldehyde functional group.
5. When glucose is acetylated that means reacting with acetic anhydride it gives Penta acetate. That means glucose has 5 –OH groups. Because we know it exists as a sturdy compound, five – OH groups should be fixed to distinct carbon particles.
6. On oxidation with nitric acid, glucose as easy as gluconic acid both give a dicarboxylic acid, saccharic acid. This gives us a hint of the presence of a primary alcoholic (–OH) group in glucose.
Cyclic Structure of Glucose
The structure of glucose clarified maximum of its characteristics and properties but the following chemical reaction and information couldn’t be cleared by this structure.
1. Despite possessing the aldehyde group, glucose doesn’t give positive Schiff’s test and it doesn’t form the hydrogen sulphite as a side product with NaHSO3.
2. The pentaacetate of glucose has no reaction with hydroxylamine. So we can conclude that —CHO group is absent.
3. Glucose found in nature has 2 forms one is α and the second one is β. The α- shape of glucose is attained by crystallisation from a concentrated mixture of glucose while the β- shape is obtained by crystallisation from boiling and saturated hydrated solution.
This kind of behaviour wasn’t shown by glucose when performed experiments. It was thought that if the —OH group could add to the —CHO group then it would lead to the formation of cyclic hemiacetal structure. It was then found that glucose forms a six-membered ring in which — OH at CARBON-5 is engaged in ring conformation. Hence we can conclude from this that the CHO group is not present so the existence of glucose in two forms as shown below.
These cyclic forms have an open-chain structure and also exist in equilibrium.
The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at C1, called anomeric carbon Cyclic Structure of Glucose (the aldehyde carbon before cyclisation). These α-form and β-form are anomers, anomers are compounds with differences in configuration of only one carbon atom. The six-membered cyclic form of glucose is known as pyranose structure (α – or β –), in metaphor with pyran. Pyran is a cyclic organic compound with one oxygen bond and five-carbon molecules in the ring.
Conclusion
Glucose is one of the most important monosaccharides. It is the building block of almost every oligosaccharide and polysaccharide. A carbohydrate that can not be hydrolysed beyond to offer a smaller yet simpler unit of polyhydroxy aldehyde or ketone is known as a monosaccharide. Preparation of glucose from sucrose (Cane sugar): If sucrose is boiled with dilute HCl or H2SO4 in an alcoholic solution, glucose and fructose are obtained in equal amounts. Preparation of glucose from starch: Commercially glucose is attained by hydrolysis of starch by steaming it with dilute H2SO4 at 393 K under a lot of pressure.
Glucose found in nature has 2 forms one is α and the second one is β. The α- shape of glucose is attained by crystallisation from a concentrated mixture of glucose while the β- shape is obtained by crystallisation from boiling and saturated hydrated solution. The two cyclic hemiacetal forms of glucose differ only in the configuration of the hydroxyl group at C1, called anomeric carbon Cyclic Structure of Glucose.
