Question

Write a reaction that indicates the presence of the aldehyde group in glucose.

Answer 1

Hint: To answer this question, you should recall the structure of glucose as well as the different reactions to prove the presence of functional groups like carbonyl and hydroxyl groups. Bromine water and cyanohydrins can be used for aldehyde test of glucose.

Complete step by step answer:
Glucose is also known as dextrose. It is referred to as aldohexose as it contains 6 carbon atoms and an aldehyde group. It can open in two forms, open-chain or ring structure.
The properties shown by glucose due to the presence of aldehyde are:
When glucose reacts with hydroxylamine, oxime is formed and cyanohydrins are formed on the addition of hydrogen cyanide to it. This reaction can confirm the presence of a carbonyl group in glucose.
And also on the reaction of glucose with a mild oxidising agent like bromine water, the glucose gets oxidised to gluconic acid (carboxylic acid-containing six carbon atoms). This indicates that the carbonyl group is present as an aldehyde group.
These reactions can be represented by the equation:
$HOC{H}_2-(CHOH{)}_4-CHO+H_{2}NOH\to HOC{H}_2-(CHOH{)}_4-CH=NOH$ and $HOC{H}_2-(CHOH{)}_4-CHO+B{r}_2\to HOC{H}_2-(CHOH{)}_4-COOH$

Note:
You should know that if you have seen drawings of sugars before, you might not have noticed the carbonyl. The reason is that carbonyl is usually "masked" as a hemiacetal structure. The hemiacetal structure is generated when a hydroxyl group along the carbon chain reaches back and bonds to the electrophilic carbonyl carbon. We can see from the structure of fructose that the keto group is present on the second carbon. Now two possibilities for cyclization exist: If attacked by the $$\text{ - C}{\text{H}}_{\text{2}}\text{OH}$$ on the 6th carbon it will generate pyranose (6-membered) ring and second that, attacked by the $$\text{ - CH}\left(\text{OH}\right)\text{ - }$$ group on the 5th carbon it will result in a furanose. In general, we can conclude that pyranose rings are more stable than furanose rings, because there are more conformational isomeric structures available for pyranose vs. furanose.