Question

Which of the following reagents cannot be used to prepare an alkyl halide?
A. $NaCl$
B. $HCl + ZnC{l_2}$
C. $SOC{l_2}$
D. $PC{l_5}$

Answer 1

Hint: In organic chemistry, the term "reagent" denotes a chemical ingredient (a compound or mixture, typically of inorganic or small organic molecules) introduced to cause the desired transformation of an organic substance.

Complete step by step answer:
The mixture of $HCl$ and $ZnC{l_2}$ is called the Lucas Reagent.
$HCl + ZnC{l_2}$ is used in the Groove's process for the preparation of alkyl halides from alcohols.
The reaction can be written as:
It is also used to distinguish primary, secondary and tertiary alcohols.
The reaction can be accomplished in the presence of only $HCl$ in the same way, although often $ZnC{l_2}$ which is a Lewis acid is added to help compensate for the lower nucleophilicity of chloride ion.
The most common methods for converting $1^0$ and $2^0$ alcohols to the corresponding chloro and bromo alkanes (i.e. replacement of the hydroxyl group) are treatments with thionyl chloride and phosphorus tribromide, respectively.
Hence we can say Thionyl chloride, $SOC{l_2}$ is also used as a reagent to convert alcohols into alkyl chlorides.
\[R - OH\xrightarrow{{SOC{l_2}}}\; {\text{ }}RX + HCl + S{O_2}\]
The reaction of alcohols \[R - OH\] with $PC{l_5}$ and $PC{l_3}$ yields an alkyl halide $R - Cl$.
With$PC{l_5}$, the reaction is quite simple leading to the formation of $R - Cl$ and \[POC{l_3}\].
Hence options (B), (C) and (D) can be eliminated.
And $NaCl$ cannot be used for the preparation of alkyl chlorides from alcohols.

Hence option (A) will be considered as a correct option.

Note: Lucas' reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid. This solution is used to classify alcohols of low molecular weight. The reaction is a substitution in which the chloride replaces a hydroxyl group.