Monosaccharides are polyhydric aldehydes and ketones which cannot be hydrolysed into simpler carbohydrates. The monosaccharides containing `-CHO` group are called aldoses while those containing `C=O` group are called ketoses. The aldehyde group is always present at `C_(1)` while keto group is generally present at `C_(2)`. All monosaccharides are oxidised by Tollen's reagent and Fehling solution and are called reducing sugars.
The monosaccharide molecules may be assigned D and L-configurations depending upon whether the configuration of the molecule is related to D- or L-glyceraldehyde. If the `-OH` group is attached to the carbon adjacent to the `-CH_(2)OH` group (last chiral carbon) is on the right hand side, it is assigned D-configuration. The molecule is assigned L-configuration if the -OH group attached to the carbon adjacent to the `-CH_(2)OH` group is on the left. The monosaccharides contain one or more chiral carbon atoms. Pentoses and hexoses have cyclic structures furanose (five membered) and pyranose (six membered). During cyclization, `C_(1)` in aldohexoses and `C_(2)` in fructose become chiral and the newly formed -OH group may be either on the left or on the right in Fischer projection formulae. These monosaccharides, therefore, exist in two stereoisomeric forms called `alpha`-anomer and `beta`-anomer while `C_(1) and C_(2)` are called glycosidic or anomeric carbon. The bonds joining glycosidic carbon are called glycosidic linkages. `D(+)` glucose exists in two stereoisomeric forms, `alpha`-D- glucose and `beta`-D-glucose. When either of these two forms of glucose i.e., `alpha-D-`glucose are dissolved in water and allowed to stand, these get slowly converted into other form and an equilibrium mixture of both is formed. This process is called mutarotation.
Which of the following statements is not correct?

Options

A. Monosaccharides reduce Tollen's reagent.
B. On dissolving `alpha`-D-glucose in water having specific rotation `111^(@)`, its specific rotation decreases
C. Glucose is aldohexose while fructose is ketohexose
D. In D-glucose, -OH group is present to left at 5th carbon atom.

Answer

To determine which statement is not correct regarding monosaccharides, we will analyze each option provided in the question based on the information given in the transcript.

### Step-by-Step Solution:

1. **Understanding Monosaccharides and Their Properties**:
- Monosaccharides are the simplest carbohydrates and can be classified as aldoses (containing an aldehyde group) or ketoses (containing a ketone group).
- They are reducing sugars, meaning they can reduce Tollen's reagent and Fehling's solution due to the presence of a free aldehyde or hemiacetal group.

2. **Analyzing Option 1**:
- **Statement**: "Monosaccharides reduce Tollen's reagent."
- **Evaluation**: This statement is correct as all monosaccharides are reducing sugars.

3. **Analyzing Option 2**:
- **Statement**: "On dissolving α-D-glucose in water having specific rotation 111°, its specific rotation decreases."
- **Evaluation**: This statement is also correct. The phenomenon of mutarotation occurs, leading to an equilibrium between α-D-glucose and β-D-glucose, resulting in a change in specific rotation.

4. **Analyzing Option 3**:
- **Statement**: "Glucose is aldohexose while fructose is ketohexose."
- **Evaluation**: This statement is correct. Glucose has an aldehyde group and is a hexose, while fructose has a ketone group and is also a hexose.

5. **Analyzing Option 4**:
- **Statement**: "In D-glucose, OH is present to the left of the 5th carbon."
- **Evaluation**: This statement is incorrect. In the Fischer projection of D-glucose, the -OH group on the fifth carbon is indeed on the right side.

### Conclusion:
The statement that is not correct is **Option 4**: "In D-glucose, OH is present to the left of the 5th carbon."