Which of the following pair of isomers cannot be separated by fractional crystallization or distillation or chromatography ?

Options

A. Maleic acid and Fumaric acid
B. `(+)-` Tartaric acid and meso - tartaric acid
C. `CH_(3)-underset(NH_(2))underset(|)(C)H-COOH` and `H_(2)N - CH_(2) -CH_(2) -COOH`
D. (+) - lactic acid (-) - lactic acid

Answer

To solve the question, we need to identify which pair of isomers cannot be separated by fractional crystallization, distillation, or chromatography.

### Step-by-Step Solution:

1. **Understand the Types of Isomers**:
- Isomers can be classified into two main categories: **enantiomers** and **diastereoisomers**.
- **Enantiomers** are mirror images of each other and have the same physical properties, making them difficult to separate using physical methods.
- **Diastereoisomers** are not mirror images and have different physical properties, allowing them to be separated by physical methods.

2. **Identify the Given Options**:
- The options provided include:
1. Malic acid and fumaric acid (diastereoisomers)
2. Tartaric acid and mesotartaric acid (diastereoisomers)
3. CH3-CH-COH-NH2 (not specified, but assumed to be diastereoisomers)
4. Lactic acid (+ lactic acid and - lactic acid) (enantiomers)

3. **Analyze Each Pair**:
- **Malic acid and fumaric acid**: These are diastereoisomers and can be separated by physical methods.
- **Tartaric acid and mesotartaric acid**: These are also diastereoisomers and can be separated by physical methods.
- **CH3-CH-COH-NH2**: Assuming this is a pair of diastereoisomers, they can also be separated by physical methods.
- **Lactic acid (+ and - lactic acid)**: These are enantiomers and cannot be separated by fractional crystallization, distillation, or chromatography because they have the same physical properties.

4. **Conclusion**:
- The pair of isomers that cannot be separated by fractional crystallization, distillation, or chromatography is the pair of enantiomers: **lactic acid (+ and - lactic acid)**.

### Final Answer:
The correct option is **4. Lactic acid (+ lactic acid and - lactic acid)**, as they are enantiomers and cannot be separated by physical methods.

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