For the reaction `R- Br to R- O - N =O`
the suitable reagent is

Options

A. `NaNO_(2) + HCI`
B. `HNO_(2)`
C. `AgNO_(3)`
D. `KNO_(2)`

Answer

To convert an alkyl bromide (R-Br) to a nitroso compound (R-O-N=O), we need to identify the suitable reagent that will facilitate this transformation through a nucleophilic substitution mechanism.

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
The reaction involves the substitution of bromine (Br) in the alkyl bromide (R-Br) with a nitroso group (R-O-N=O). This indicates a nucleophilic substitution reaction.

2. **Determine the Nucleophile**:
The nitroso group (NO2) can act as a nucleophile. However, it can be ambident, meaning it can attack from different sites. In this case, we need to focus on the reagent that will allow the oxygen to be the attacking site.

3. **Select the Suitable Reagent**:
The reagent that can provide the nitroso group in the desired manner is potassium nitrite (KNO2). This reagent allows the oxygen atom to attack the carbon atom of the alkyl bromide, leading to the formation of the desired product (R-O-N=O).

4. **Write the Reaction**:
The reaction can be summarized as follows:
\[ R-Br + KNO2 \rightarrow R-O-N=O + KBr \]
Here, the bromine atom is substituted by the nitroso group, resulting in the formation of the nitroso compound.

5. **Conclusion**:
Therefore, the suitable reagent for the conversion of R-Br to R-O-N=O is **KNO2**.