Which one is a reducing sugar ?

Options

A. Galactose
B. Gluconic acid
C. Sucrose
D. `beta`-methyl galactoside.

Answer

To determine which of the given options is a reducing sugar, we need to understand what a reducing sugar is. A reducing sugar is defined as a sugar that has a free aldehyde or keto group, which allows it to reduce other compounds, such as cupric ions to cuprous ions.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are:
- Galactose
- Gluconic acid
- Sucrose
- Beta methyl galactoside

2. **Understand Reducing Sugars**: Reducing sugars are characterized by the presence of a free aldehyde or keto group. This is crucial for their ability to act as reducing agents.

3. **Analyze Each Option**:
- **Galactose**:
- Galactose is a monosaccharide and specifically an aldohexose, meaning it has a free aldehyde group. Therefore, galactose is a reducing sugar.
- **Gluconic Acid**:
- Gluconic acid is formed from the oxidation of glucose, which means its aldehyde group has been oxidized. As a result, gluconic acid does not have a free aldehyde or keto group and is not a reducing sugar.
- **Sucrose**:
- Sucrose is a disaccharide made of glucose and fructose. The glycosidic bond between these two sugars involves their reducing groups, which means neither of them is free. Hence, sucrose is not a reducing sugar.
- **Beta Methyl Galactoside**:
- In beta methyl galactoside, the methyl group is attached at the anomeric carbon, which means the aldehyde group is not free. Therefore, it is also not a reducing sugar.

4. **Conclusion**: Based on the analysis, the only option that qualifies as a reducing sugar is **galactose**.

### Final Answer:
The reducing sugar among the options is **galactose**.

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