Glucose gets oxidized to gluconic acid on reaction with bromine water. This indicates the presence of:

Aldehyde group

The correct answer is option 1. Aldehyde group.

Bromine water (a solution of bromine in water) is used as a reagent to test for the presence of aldehyde groups. Aldehydes are easily oxidized by bromine water. In this oxidation reaction, the aldehyde group (-CHO) is converted to a carboxylic acid group (-COOH), while bromine is reduced to bromide ions (Br⁻).

Reaction of Glucose with Bromine Water:

Structure of Glucose:

Glucose is a monosaccharide with the molecular formula \(C_6H_{12}O_6\). In its open-chain form, glucose contains an aldehyde group at one end of the molecule (specifically, at the first carbon atom). The rest of the molecule has hydroxyl groups (-OH) attached to the other carbon atoms.

Oxidation Reaction:

When glucose reacts with bromine water, the aldehyde group in glucose is oxidized. The reaction can be summarized as:

Here, glucose \((C_6H_{12}O_6)\) is oxidized to gluconic acid (C₆H₁₂O₇), and bromine (Br₂) is reduced to bromide ions (Br⁻). In the oxidation reaction, the aldehyde group of glucose is converted into a carboxylic acid group, resulting in gluconic acid. The reaction demonstrates that the aldehyde group has been oxidized.

Why This Indicates an Aldehyde Group:

Specific Reaction with Bromine Water:

Bromine water selectively oxidizes aldehyde groups, converting them into carboxylic acids. This reaction does not occur with ketones or other functional groups under the same conditions. Since glucose reacts with bromine water and forms gluconic acid, it confirms that glucose contains an aldehyde group, which is the site of the reaction.

Other Functional Groups:

Ketone Groups: Ketones generally do not react with bromine water. They are not oxidized to carboxylic acids in this reaction.

Carboxylic Acids: Carboxylic acids are not further oxidized by bromine water. If glucose were already a carboxylic acid, it would not show this reaction.

Anhydride Groups: Anhydrides do not specifically react with bromine water in the same way. The reaction described is specific to aldehyde groups.

Conclusion:

The positive reaction of glucose with bromine water, leading to the formation of gluconic acid, specifically indicates the presence of an aldehyde group in glucose. This is because the aldehyde group is oxidized to a carboxylic acid by bromine water, and this is a characteristic test for aldehydes. Thus, the presence of the aldehyde group is confirmed by the positive result in this reaction.