Which of the following is incorrect

Options

A. `FeCl_(3)` is used in the detection of phenols
B. Fehling solution used in the detection of glucose
C. Tollen's reagent is used in detection of unsaturation
D. `"NaHCO"_(3)` is used in the detection of carbonyl compounds

Explanation

To solve the question of identifying which statement is incorrect regarding the detection methods for various organic compounds, we will analyze each option one by one.

### Step-by-Step Solution:

1. **Option 1: FeCl3 is used in detection of phenol.**
- **Analysis:** Ferric chloride (FeCl3) is indeed used to detect phenols. When phenols react with FeCl3, they form colored complexes (purple, green, or red) due to the formation of a phenol-iron complex.
- **Conclusion:** This statement is **correct**.

2. **Option 2: Failing solution is used in detection of glucose.**
- **Analysis:** The Fehling's solution contains copper(II) sulfate and is used to test for reducing sugars, including glucose. When glucose (which has an aldehyde group) is present, it reduces the copper(II) ions to copper(I) oxide, forming a red precipitate.
- **Conclusion:** This statement is **correct**.

3. **Option 3: Tollens reagent is used in detection of unsaturation.**
- **Analysis:** Tollens' reagent (ammoniacal silver nitrate) is used to detect aldehydes, not unsaturation. When aldehydes react with Tollens' reagent, they reduce the silver ions to metallic silver, resulting in a silver mirror. Therefore, this statement incorrectly states that it is used for unsaturation detection.
- **Conclusion:** This statement is **incorrect**.

4. **Option 4: NaHCO3 is used in detection of carbonyl compound.**
- **Analysis:** Sodium bicarbonate (NaHCO3) is typically used to detect carboxylic acids, not carbonyl compounds like aldehydes or ketones. When a carboxylic acid reacts with NaHCO3, it produces carbon dioxide gas, leading to effervescence. Hence, this statement is also incorrect.
- **Conclusion:** This statement is **incorrect**.

### Final Conclusion:
Both **Option 3** and **Option 4** are incorrect. However, since the question asks for which one is incorrect, the first incorrect statement identified is **Option 3**.