Glucose | Preparation, Properties, Uses & Structure |

Glucose.
Glucose.

Glucose is the most common monosaccharide. It is known as Dextrose because it occurs in nature principally as the optically active dextrorotatory isomer. Glucose is found in most sweet fruits, especially grapes (20-30%), and honey. It is an essential constituent of human blood. The blood normally contain 65 to 110 mg (0.06 to 0.1%) of glucose per 100 ml. in diabetic persons the level may be much higher. In the combined form glucose occurs in abundance in cane sugar and polysaccharides such as starch and cellulose.

Preparation of glucose.

Glucose is produced commercially by the hydrolysis of starch with dilute hydrochloric acid at high temperature under pressure.
Preparation of glucose.
An aqueous suspension of starch obtained from corn is acidified with hydrochloric acid, It is then heated with high-pressure steam in an autoclave. When the hydrolysis is complete, the liquid is neutralised with sodium carbonate to a pH of 4-5. The resulting solution is concentrated under reduced pressure to get the crystals of glucose.


Uses of glucose.

Glucose is used.
  1. As a sweetening agent in syrups and confectionery.
  2. As food for infants.
  3. As a reducing agent in silvering of mirrors and to convert indigo blue to indigo white in vat dyeing.
  4. As a raw material for wine and alcohol manufacture.

Structure of glucose.

The structure of glucose may be discussed under the following heads.
  1. Open Chain formula.
  2. Configuration.
  3. Cyclic structure.
  4. Haworth representation.

Open-chain Formula.

The open-chain formula of glucose is constructed from the following facts:
(1) Molecular Formula.
Elemental analysis and molecular weight determination have established that glucose has the molecular formula C6H12O6.
(2) Presence of 6-carbon unbranched chain.
The complete reduction of glucose with concentrated hydrogen iodide and red phosphorus gives n-hexane. This proves that glucose molecule is made of an unbranched six-carbon chain.
(3) Presence of 5 OH group.
Glucose reacts with acetic anhydride to form pentaacetyl derivative. This shows the presence of five hydroxy groups. Since glucose is a stable compound, no two OH groups are attached to the same carbon. In other words, the five OH group are on different carbons.
(4) Presence of C=O group.
Glucose reacts with hydroxylamine to form an oxime. It suggests the presence of a carbonyl group.
(5) Presence of terminal CHO function.
On mild oxidation with bromine water, glucose is converted to glucose acid which when reduced with excess of Hl yields n-hexanoic acid.
glucose Presence of terminal CHO function.
This show that glucose contains a six carbon straight chain with CHO at one end, which has been oxidised to COOH.
(6) Construction of open-chain formula.
Knowing that glucose has a straight 6-carbon chain with a terminal CHO, the five OH groups can be placed one each on the remaining five carbons. Supplying hydrogen atoms to these carbons to satisfy their tetracovalency, the open-chain structure of glucose can be written as :
glucose Construction of open-chain formula.

Configuration of D-Glucose.

The configuration of D-glucose was proved by Emil fischer by arguments similar to the ones stated below.
(1) Construction of four possible D-pentoses.


Taking the configuration of D-glyceraldehyde as the standard, two possible D-aldotetroses (A and B) may be constructed by adding a CHOH just below CHO, placing OH to the right and then to the left.
glucose Construction of four possible D-pentoses.
Similarly, each of the two D-tetroses (A and B) gives two D-aldopentoses. Thus four possible D-aldopentoses are :
Configuration of D-Glucose.
(2) D-Arabinose has configuration II or IV.
Oxidation of D-arabinose with nitric acid oxidises the terminal CHO and CH2OH groups yielding two optically active (asymmetric) dicarboxylic acids. The forms II and IV can form two optically active (asymmetric) diacids, while I and III can give meso acids only that have a plane of symmetry. Therefore, D-arabinose is either II or IV.
(3) Configuration II confirmed for D-arabinose.
D-arabinose by Killiani -Fischer synthesis yields two epimeric aldohexoses, D-glucose and D-mannose. These on oxidation with nitric acid form two optically active (asymmetric) dicarboxylic acids. This is theoretically possible only if D-arabinose has the configuration II and not IV.
glucose Configuration II confirmed for D-arabinose.
Proceeding similarly, you will find that if D arabinose had configuration IV, of the two dicarboxylic acids derived from it, one would be meso and one asymmetric (optically active). Hence D-arabinose has the configuration II.
(4) Ruff degradation of D-glucose and D-mannose produces D- arabinose in each case.
In ruff degradation the CHOH below CHOH is destroyed. Therefore, the configuration of the two aldohexoses, D-glucose and D-mannose, can be derived by adding a new CHOH below CHO in form II of D-arabinose.
Ruff degradation of D-glucose and D-mannose produces D- arabinose in each case.
Hence D-glucose has configuration V or VI.
(5) D-glucose and L-glucose yield the same dicarboxylic acid.
This means that these two sugars differ only in respect of the position of the terminal groups (CHO and CH2OH). Therefore, the exchange of the terminal groups in D-glucose should be able to give a different aldohexose (L-glucose). Let us now examine configuration formulas V and VI (One of which is D-glucose) from this angle.
D-glucose and L-glucose yield the same dicarboxylic acid.
If VII is rotated through 180° in the plane of the paper, it gives an aldohexose VIII, different from V. A similar procedure with formula VI does not give rise to a different sugar.
From the above arguments it is evident that D-glucose has the configuration as show by the from V.

Cyclic Structure of D-Glucose.

(1) Open-Chain Structure not wholly true.
Fischer realised that the open-chain pentahydroxy aldehyde structure of glucose did not wholly explain its chemical behaviour. Unlike simple aldehydes, glucose did not form the crystalline bisulphite compound and failed to give the shiff's test. Furthermore, the penta-acetate and pentamethyl-ether derivatives of glucose are not oxidised by Tollens' regent or Fehling's solution, indicating the absence of CHO group.
(2) Cyclic structure suggested to explain mutarotation.
The French chemist Tarnet established the existence of two crystalline forms of glucose, α-glucose and β-glucose. α-Glucose had specific rotation +112° , while β-glucose + 19°. The optical rotation of each of these forms changed gradually with time till finally a constant value of +53°  was reached. To explain this phenomenon of mutarotation, it was visualised that the α and β glucose were in reality the cyclic hemiacetal forms of glucose which were interconvertible via the open-chain form. The constant value of + 19° represented the state of equilibrium between α-D-glucose and β-D-glucose.
Cyclic Structure of D-Glucose.
The concentrations of α-D glucose, β-D-glucose, and the open-chain glucose at equilibrium are 36%, 64%, and less than 0.01% respectively. This explains why D-glucose can react both as an aldehyde and a cyclic hemiacetal in which CHO is absent.
(3) Glyeoside formation confirms cyclic structure.
Glucose when treated with methanol in the presence of dry HCl, gives two isomeric acetals or glycosides,. These crystalline glycosides, methyl-α-D-glucose and methyl-β-D glucoside, have been actually isolated. These are optically active but do not give any reactions of free CHO group. Evidently, the two glycosides are the methyl derivatives of α- and β-D-glucose, formed as a result of the reaction between the hemiacetal OH of these forms and methanol.
Glyeoside formation confirms cyclic structure.
thus the cyclic structure of D-glucose stands confirmed but whether it has a 5-membered or a 6-membered ring is still to be proved.
(4) Determination of ring size.
So far we have represented structure of cyclic hemiacetals or anomers of D-glucose as having a ring of six members, five carbons and one oxygen. This has been proved to be correct and a five-membered ring has been ruled out.
Hirst (1926) prepared tetra-O-methyl-D-glucose. by treating methyl-D-glucoside with dimethyl sulphate and subsequent acid hydrolysis of the pentamethyl derivative formed. The oxidation of tetra-O-methyl-D-glucose with nitric acid yielded trimethoxyglutaric acid.
Determination of ring size.
Obviously, the two carboxylic carbons (1,5) of the trimethoxyglutaric acid are the one's originally involved in ring formation. Hence , there must have existed an oxide ring between C-1 and C-5. Tracing back the reaction sequence, it stands proved that D-glucose has a six-membered ring. The presence of a 6-membered ring in D-glucose has also been confirmed by x-ray analysis.

The Hawoeth Representation.

So far we have used Fischer projection formulas for representing the cyclic forms of D-glucose. Haworth thought that these structures were awkward. He introduced the hexagonal representations resembling the heterocycle pyran which contain five carbon and one oxygen in the ring. Thus, he claimed the names α-D-glucopyranose and β-D-glucopyranose for the hexagonal structures of α-D-glucose and β-D-glucose.
The Hawoeth Representation.
It may be noted that in hawaoth formula, all the OH groups on the right in Fischer formula are directed below the plane of the ring, while those on the left go above the plane. The terminal CH2OH projects above the plane of the ring.

Physical properties of glucose.

Glucose is a white crystalline solid, mp 146°C. When crystallised from cold water, it forms glucose monohydrate (C6H12O6.H2O), mp 86°C. it is extremely soluble in water, only sparingly so in ethanol, and insoluble in ether. It is about three-fourths as sweet as cane-sugar (sucrose). It is optically active, and the ordinary naturally occurring form is (+)-glucose.


Chemical properties of glucose.

We have seen that D-glucose is an equilibrium mixture of a straight chain form and a cyclic hemiacetal form.
Glucose | Preparation, Properties, Uses & Structure | Glucose | Preparation, Properties, Uses & Structure | Reviewed by Genuine Chemistry on May 11, 2019 Rating: 5

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