Step 1: Understanding the Question:
The question asks for the specific reagent that yields a nitroalkane ($R-NO_2$) when it reacts with an alkyl halide (haloalkane).
Step 2: Detailed Explanation:
1. Nitrite ion ($NO_2^-$) is an ambident nucleophile; it can attack via the oxygen atom to form alkyl nitrites ($R-O-N=O$) or via the nitrogen atom to form nitroalkanes ($R-NO_2$).
2. Reagents like $KNO_2$ or $NaNO_2$ are ionic. In solution, the negatively charged oxygen atom is more nucleophilic, leading primarily to the formation of alkyl nitrites.
3. Silver nitrite ($AgNO_2$) is largely covalent. The bond between $Ag$ and $O$ is strong, making the lone pair on nitrogen the primary nucleophilic site.
4. Therefore, the reaction $RX + AgNO_2 \rightarrow R-NO_2 + AgX$ predominantly yields the nitroalkane.
Step 4: Final Answer:
Silver nitrite is used to prepare nitroalkanes from haloalkanes.