During halogenation of alkanes the halogens and alkane show a specific treand. Which of the following statements is not correct?

Options

A. The reactivity of halogens is in the order `F_(2) gt Cl_(2) gt Br_(2) gt I_(2)`
B. For a given halogen the reactivity of hydrocarbon is in the order of `3^(@) gt 2^(@) gt 1^(@)`
C. Bromine is less reactive than chlorine towards a particular alkane.
D. On chlorination monosubstituted product is formed while on bromination disubstituted products are formed.

Explanation

To solve the question regarding the halogenation of alkanes and identify which statement is not correct, we will analyze each statement step by step.

### Step 1: Understand the Halogenation Process
Halogenation of alkanes occurs when alkanes react with halogens (X2) in the presence of light (hν). This reaction follows a free radical mechanism which involves three main steps:
1. **Chain Initiation**: Formation of halogen radicals through homolytic cleavage of the halogen molecule.
2. **Chain Propagation**: Reaction of the alkane with the halogen radicals to form haloalkanes.
3. **Chain Termination**: Combination of radicals to form stable products.

### Step 2: Analyze Each Statement
Now, let's evaluate the given statements one by one.

**Statement 1**: The reactivity of halogen is in the order of F2, Cl2, Br2, and I2.
- **Analysis**: This statement is correct. Fluorine (F2) is the most reactive, followed by chlorine (Cl2), bromine (Br2), and iodine (I2). The reactivity decreases down the group due to increasing atomic size and decreasing bond strength.

**Statement 2**: For a given halogen, the reactivity of hydrocarbon is in the order of tertiary, secondary, and primary.
- **Analysis**: This statement is also correct. Tertiary radicals are more stable and reactive than secondary and primary radicals due to the greater number of electron-donating alkyl groups.

**Statement 3**: Bromine is less reactive than chlorine towards a particular alkane.
- **Analysis**: This statement is correct. Chlorine is more reactive than bromine due to its lower activation energy and ability to react with all types of hydrogen atoms (primary and secondary) in a similar manner.

**Statement 4**: In chlorination, monosubstituted products are formed while in bromination, disubstituted products are formed.
- **Analysis**: This statement is incorrect. Both chlorination and bromination primarily yield monosubstituted products when a single hydrogen atom is abstracted. While bromination can be more selective and lead to different ratios of products, it does not inherently lead to disubstituted products.

### Conclusion
The statement that is not correct is **Statement 4**: "In chlorination, monosubstituted products are formed while in bromination, disubstituted products are formed."