Correct answer is 2. we use this reagent in finkelstein.reaction in which 1st we have to convert an alcohol to any alky halide by its reaction with HX and then after forming alkyl halide we can convert it into alkyl iodide

Why reagents can not convert alcohol into alkyl halide?

Alcohol is a functional group that contains a hydroxyl (-OH) group attached to a carbon atom. Alkyl halides, on the other hand, are organic compounds in which a halogen atom (fluorine, chlorine, bromine, or iodine) is attached to a carbon atom. The conversion of alcohols to alkyl halides is of great importance in organic synthesis. However, not all reagents can convert alcohols into alkyl halides.

Red P/Br2

Red phosphorous and bromine are not suitable reagents for the conversion of alcohols to alkyl halides. The reaction between red phosphorus and bromine is a radical reaction that requires heat or light. The reaction is not very selective and can lead to the formation of a mixture of products. Moreover, the reaction can be highly exothermic and explosive.

KI/Acetone

The reaction between iodine and potassium iodide in the presence of acetone is often used for the conversion of primary and secondary alcohols to alkyl iodides. However, this reagent is not suitable for the conversion of alcohols to alkyl halides other than iodides. Moreover, the reaction can be slow and requires heating.

SOCL2

Thionyl chloride (SOCl2) is a common reagent for the conversion of alcohols to alkyl chlorides. However, it is not suitable for the conversion of alcohols to alkyl halides other than chlorides. Moreover, the reaction can be highly exothermic and can generate toxic gases such as HCl.

HCL/ZnCl2

Hydrochloric acid (HCl) in the presence of zinc chloride (ZnCl2) is often used for the conversion of tertiary alcohols to alkyl chlorides. However, it is not suitable for the conversion of primary and secondary alcohols to alkyl halides. Moreover, the reaction can be highly exothermic and can lead to the formation of a mixture of products.

In conclusion, the conversion of alcohols to alkyl halides requires suitable reagents that can selectively convert the hydroxyl group to a halogen atom. Different reagents are suitable for different types of alcohols and halogens. It is important to choose the right reagent for a specific reaction to ensure a high yield and selectivity.