Which of the following is incorrect ?

Options

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

Answer

To determine which of the statements is incorrect, let's analyze each option one by one.

### Step-by-Step Solution:

1. **Option 1: FeCl3 is used in the detection of phenol.**
- **Analysis**: This statement is correct. When phenol reacts with ferric chloride (FeCl3), it forms a violet complex. This reaction is a well-known test for the presence of phenolic compounds.
- **Conclusion**: Option 1 is correct.

2. **Option 2: Felling solution is used in the detection of glucose.**
- **Analysis**: This statement is also correct. Felling solution is used to test for reducing sugars, which include glucose. Glucose contains an aldehyde group, and when it reacts with Felling solution, it gets oxidized to a carboxylic acid, resulting in the formation of a red precipitate of cuprous oxide (Cu2O).
- **Conclusion**: Option 2 is correct.

3. **Option 3: Tolens reagent is used in the detection of unsaturation.**
- **Analysis**: This statement is incorrect. Tolens reagent (Ag(NH3)2^+) is specifically used to detect aldehydes, not unsaturation. It forms a silver mirror when aldehydes are present, indicating a positive test. Therefore, this statement does not hold true.
- **Conclusion**: Option 3 is incorrect.

4. **Option 4: NaHSO3 is used in the detection of carbonyl compounds.**
- **Analysis**: This statement is correct. Sodium bisulfite (NaHSO3) reacts with carbonyl compounds (such as aldehydes and ketones) to form a white precipitate of bisulfite adducts. This reaction is a common method for identifying carbonyl compounds.
- **Conclusion**: Option 4 is correct.

### Final Conclusion:
- **Incorrect Statement**: Option 3 is the only incorrect statement. It incorrectly states that Tolens reagent is used for detecting unsaturation, whereas it is used for detecting aldehydes.