Among the following, that does not form an ideal solution is:

Options

A. `C_(6)H_(6) and C_(6)H_(5)CH_(3)`
B. `C_(2)H_(5)Cl and C_(2)H_(5)OH`
C. `C_(6)H_(5)Cl and C_(6)H_(5)Br`
D. `C_(2)H_(5)Br and C_(2)H_(5)I`

Answer

To determine which of the given pairs does not form an ideal solution, we need to understand the concept of ideal solutions and Raoult's law.

### Step-by-Step Solution:

1. **Understanding Ideal Solutions**: An ideal solution is one that follows Raoult's law, which states that the vapor pressure of a solution is directly proportional to the mole fraction of the solvent. Ideal solutions are formed when the components have similar molecular sizes, structures, and intermolecular forces.

2. **Analyzing the Options**:
- **Option 1: Benzene (C6H6) and Toluene (C6H5CH3)**: Both benzene and toluene have similar molecular structures and sizes. They are both non-polar and have similar intermolecular forces (London dispersion forces). Therefore, they form an ideal solution.

- **Option 2: Chloroethane (C2H5Cl) and Ethanol (C2H5OH)**: Chloroethane has a halogen (Cl) and ethanol has a hydroxyl (OH) group. These groups lead to different types of intermolecular forces (dipole-dipole interactions in ethanol and van der Waals forces in chloroethane). Thus, they do not have similar interactions and do not form an ideal solution.

- **Option 3: Chloroethane (C2H5Cl) and Bromoethane (C2H5Br)**: Both chloroethane and bromoethane are haloalkanes and have similar molecular structures. They exhibit similar intermolecular forces, which means they form an ideal solution.

- **Option 4: Bromoethane (C2H5Br) and Iodoethane (C2H5I)**: Similar to the previous pair, bromoethane and iodoethane are also haloalkanes with similar structures and intermolecular forces, thus they form an ideal solution.

3. **Conclusion**: The pair that does not form an ideal solution is **Chloroethane and Ethanol** (Option 2).

### Final Answer:
The correct option is **Option 2: Chloroethane and Ethanol**.