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Gluconic Acid is an organic compound with the molecular formula C6H12O and an Average Molar Mass of 196.16 g/mol. Its condensed structural formula is given by HOCH4COOH. It is one of the 16 stereoisomers of 2,3,4,5,6-pentahydroxy hexanoic acid. Gluconic Acid is formed due to the oxidation reaction of Glucose with mild oxidising agents. It is readily soluble in water. But, it is insoluble in ether or ethanol. In an aqueous solution, Gluconic Acid exists in equilibrium i.e., a balanced state with the cyclic ester glucono delta-lactone.
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Key Terms: Gluconic acid, glucose, oxidation, IUPAC name, structural formula, Fischer projection
What is Gluconic Acid?
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Gluconic Acid is a white, odorless crystalline powder. It is a monocarboxylic acid with 6 carbon atoms. It is found in honey, plants, and wine. As a food additive, it is known as an acidity regulator. Gluconic Acid is also obtained from a fungal source known as Aspergillus niger.
Gluconic Acid is produced by the oxidation of the first carbon of glucose with antiseptic and chelating properties. In alkaline solution, this agent exhibits strong chelating activities towards anions i.e., calcium, aluminium, iron, and other heavy metals.
At neutral pH, in an aqueous solution, Gluconic Acid forms Gluconate ions. The salts of gluconic acid are known as "gluconates". Oxidation of glucose leads to the formation of various species of Gluconic acid, gluconate salts, and gluconate esters. Some drugs are injected in the form of gluconates.
Properties of Gluconic Acid
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Molecular Formula: C6H12O
Average Molecular Mass: 196.16 g/mol.
Other Names: Dextronic acid, Glycogenic acid, D-Gluconic acid.
Boiling Point: 130–132 °C.
Melting Point: Below 12°C
Acidity(pKa): 3.86
Structural Formula of Gluconic Acid
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As we can see, the chemical structure of gluconic acid consists of six carbon chains that are linked to five hydroxyl groups which are positioned in the same way as in the open chained form of glucose, ending with the carboxylic acid group.
Structural Formula of Gluconic Acid
Formation of Gluconic Acid by Glucose
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Glucose is a simple sugar with six carbon atoms. When glucose is treated with a mild oxidizing agent like bromine water (Br2), it gets oxidised to Gluconic Acid.
CHO(CHOH)4CH2OH → CH2OH(CHOH)4COOH
Glucose Gluconic acid
As Gluconic Acid is a monocarboxylic acid, it shows that the carbonyl group is present in glucose as an aldehydic group.
Oxidation of Gluconic Acid
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When gluconic acid and glucose are oxidised with nitric acid, they both lead to the formation of a dicarboxylic acid, called saccharic acid.
Things to Remember
- Gluconic acid is produced from D-glucose by the oxidation of its aldehyde group (C1) to a carboxyl group.
- Commonly found in salts with sodium and calcium. Gluconic acid or gluconate is used to maintain the cation-anion balance on electrolyte solutions.
- Gluconic acid is an organic compound with molecular formula C6H12O7 and condensed structural formula HOCH2(CHOH)4COOH.
- The most studied and widely used fermentation process involves the fungus Aspergillus niger. Gluconic acid and its deriva- tives, the principal being sodium gluconate, have wide applications in food and pharma- ceutical industry.
Sample Questions
Ques. Which of the following reactions of glucose can be explained only by its cyclic structure? (2 Marks)
(i) Glucose forms pentaacetate.
(ii) Glucose reacts with hydroxylamine to form an oxime.
(iii) Pentaacetate of glucose does not react with hydroxylamine.
(iv) Glucose is oxidised by nitric acid to gluconic acid.
Ans. (iii)- Pentaacetate of glucose does not react with hydroxylamine.
Explanation: Pentaacetate does not form an open-chain structure. Therefore, its reaction with Hydroxylamine can only be explained by its cyclic structure.
Ques. Optical rotations of some compounds along with their structures are given below. which of them have D configuration? 
(2 Marks)
(i) I, II
(ii) I, II, III
(iii) II, III
(iv) III
Ans. (ii)
Explanation: The given compounds represent Glucose, Gluconic acid, and Saccharic acid respectively. As each compound has a hydroxyl group(-OH), placed to the right of the last stereocenter in Fischer’s projection, all the three compounds have a D configuration.
Ques. Which of the following B group vitamins can be stored in our body? (2 Marks)
(i) Vitamin B2
(ii) Vitamin B12
(iii) Vitamin B6
(iv)Vitamin B1
Ans. (ii)
Explanation: Vitamin B12 can be stored in our body as it is insoluble in water
Ques. Under what conditions glucose is converted to gluconic and saccharic acid? (2 Marks)
Ans. a) When Glucose is reacted with mild oxidizing agents like bromine water (Br2), it gets converted to Gluconic Acid.
b) On the other hand, when glucose is reacted with String oxidizing agents like nitric acid (HNO3), it gets converted to saccharic acid.
Ques.How do you explain the presence of all the six carbon atoms in glucose in a straight chain? (2 Marks)
Ans. When glucose is heated for a prolonged time with HI, it forms n-hexane, which tells us that all the six carbon atoms are linked in a straight chain.
Ques. The letters ‘D’ or ‘L’ before the name of a stereoisomer of a compound indicates the correlation of the configuration of that particular stereoisomer. This refers to their relationship with one of the isomers of glyceraldehyde. Predict whether the following compound has a ‘D’ or ‘L’ configuration. (2 Marks)
Ans. The given compound has a ‘L’ configuration as the –OH group is linked on the left side of the C5 carbon atom.
Ques. How do you explain the presence of five —OH groups in the glucose molecule? (2 Marks)
Ans. When a glucose molecule is treated with acetic anhydride (CH3CO)2O, in the presence of ZnCl2, it undergoes acetylation to form glucose pentaacetate which confirms the presence of five –OH groups.
Ques. Which sugar is called invert sugar? Why? (2 Marks)
Ans. Sucrose is called invert sugar. As the hydrolysis of sucrose brings about a change in the sign of rotation, from Dextro (+) to laevo (–) it is called invert sugar.
Ques. Why must vitamin C be supplied regularly in diet? (2 Marks)
Ans. Vitamin C is water soluble therefore it is readily excreted in urine and can’t be stored in our body. Thus, it is important that vitamin C be supplied regularly in diet.
Ques. Name the linkage connecting monosaccharide units in polysaccharides (2 Marks)
Ans. Glycosidic linkage.
Ques. How do you explain the presence of an aldehydic group in a glucose molecule? (2 Marks)
Ans. a) Glucose reacts with hydroxylamine to form a monoxime and adds one molecule of hydrogen cyanide to give cyanohydrin.
b) So, it contains a carbonyl group which can be an aldehyde or a ketone.
c) On mild oxidation with bromine water, glucose gives gluconic acid which is a six-carbon carboxylic acid. This indicates that the carbonyl group present in glucose is an aldehydic group.
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