Glucose,Preparation of Glucose,Physical properties of glucose

Glucose

Glucose is most common monosaccharide. It is known as Dextrose because it occurs in

nature principally as optically 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 conatains 65 to 110 mg (0.06 to 0.1%) of glucose per 100 ml. In diabetic persons the level may be much higher. In combined form glucose occurs in abundance in cane sugar and polysaccharides such as starch and cellulose.

Preparation of Glucose

1. From sucrose (Cane sugar)

When sucrose in boiled with dilute HCl or H2SO4 in alcoholic solution, glucose and fructose are obtained in equal amounts.

2. From Starch

Glucose is produced commercially by the hydrolysis of starch by boiling it with dilute H2SO4 at high temperature under pressure.

in this process, an aqueous solution of starch obtained from corn is acidified with dilute H2SO4.

It is then heated with high pressure steam in an autoclave. When the hydrolysis is complete, the liquid is neutralized with sodium carbonate to pH of 4-5. The resulting solution is concentrated under reduced pressure to get the crystals of glucose.

Physical properties of glucose

Some important physical properties of glucose are mentioned as under:

1. It is colourless sweet crystalline compound having m.p.419 K.

2. It is readily soluble in water, sparingly soluble in alcohol and insoluble in ether.

3. It forms a monohydrate having m.p. 391 K.

4. It is optically active and its solution is dextrorotatory. The specific rotation of fresh

solution is + 1120 C.

5. It is about three fourth as sweet as sugarcane i.e., sucrose.

Chemical properties of glucose

Chemical properties of glucose can be studied under the following headings:

(A)Reactions of aldehydic group

1. Oxidation.

(a) Glucose gets oxidized to gluconic acid with mild oxiding agents like

bromine water

Only-CHO group is affected.

(b) A strong oxidizing agent like nitric acid oxidizes both the terminal groups viz. –CH2OH and –CHO groups and saccharic acid or glucaric acid is obtained.

(d) Glucose gets oxidized to gluconic acid with ammonical silver nitrate ( Tollen’s reagent)

and alkaline copper sulphate ( Fehling solution). Tollen’s reagent is reduced to metallic silver (silver mirror) and Fehling solution to cuprous oxide which is a red precipitate.

(i) With Tollen’s reagent

(ii) With Fehling solution

2. Reduction 

(a) glucose is reduced to sorbitol or Glucitol on treatment with sodium

amalgam and water.

(b) On reduction with conc. HI and red P at 373 K glucose gives a mixture of n-hexane and 2-

idohexane

3. Reaction with HCN

Like aldehydes, glucose reacts with HCN forming cyanohydrins.

4. Reaction with hydroxylamine

Glucose forms glucose oxime.

(B) Reactions of hydroxyl groups

1. Reaction with acetic anhydride or acetyl chloride.

Glucose forms penta acetate with

acetic anhydride of acetyl chloride.

2. Reaction with methyl alcohol

Glucose reacts with methy alcohol in the presence of dry

HCl gas to form methyl glucoside.

3. Reaction with metallic hydroxides

Glucose reacts with calcium hydroxide to form

calcium glucosate which is water soluble.

(C)Miscellaneous reactions

1. Action of acids

On warming with conc.HCl, glucose forms 5-hydroxy methyl furfural,

which on further reaction gives laevulinic acid.

2. Fermentation.

Glucose undergoes fermentation into ethyl alcohol in the presence of the

enzyme zymase.

3. Reaction with Alkalies

When warmed with strong sodium hydroxide solution, glucose

forms a brown resinous product. In dilute alkali solution, D-glucose rearranges to give a

mixture of D- glucose, D-mannose and d-fructose.

The above equilibrium is established via the enediol starting from any of these three hexoses.

That is why D-Fructose, although it has a ketonic C=O group, reduces Fehling’s solution or

Tollen’s reagent. The rearrangement reaction of a monosaccharides in weakly alkaline solutions

to give a mixture of isomeric sugars, is named as Lobry de Bruyn Van Ekestein rearrangement.

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