How can 6th carbon atom in glucose be oxidized to give glucouronic acid?

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Glucose can be oxidized at the first carbon atom to form D-gluconic acid, at the sixth carbon atom to yield D-glucuronic acid, and at both the first and sixth carbon atoms to yield D-glucaric acid.
Monosaccharide oxidation can yield three different types of oxidation products. The oxidizing agent used determines the product.
Weak oxidizing agents, such as Tollens, Fehling's, and Benedict's solutions, oxidize the carbonyl group end of а monosaccharide to give an -onic acid. Oxidation of the aldehyde end of glucose produces gluconic acid, and oxidation of the aldehyde end of galactose produces galactonic acid. The structures involved in the glucose reaction are 
Because monosaccharides act as reducing agents in such reactions, they are called reducing sugars. With Tollens solution, glucose reduces Ag+ ion to Ag, and with Benedict's and Fehling's solutions, glucose reduces Cu2+ ion to Cu+ ion. А reducing sugar is a carbohydrate that gives a positive test with Tollens, Benedict's and Fehling's solutions. All monosaccharides are reducing sugars.
Tollens, Fehling's, and Benedict's solutions can be used to test for glucose in urine, а symptom of diabetes. For example, using Benedict's solution, we observe that if no glucose is present in the urine (а normal condition), the Benedict's solution remains blue.
The presence of glucose is indicated by the formation of а red precipitate. Testing for the presence of glucose in urine is such а common laboratory procedure that much effort has been put into the development of easy-to-use test methods.
Strong oxidizing agents can oxidize both ends of а monosaccharide at the same time (the carbonyl group and the terminal primary alcohol group) to produce а dicarboxylic acid. Such polyhydroxy dicarboxylic acids are known as -aric acids. For glucose, such an oxidation produces glucaric acid.

Although it is difficult to do in the laboratory, in biological systems enzymes can oxidize the primary alcohol end of an aldose such as glucose, without oxidation of the aldehyde group, to produce а -uronic acid. For glucose, such an oxidation produces D-glucuronic acid.
D-Glucose D-Glucuronic acid

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