Question

The preferred reagent for the preparation of pure alkyl chloride from alcohol is

• A. HCl + ZnCl\(_2\)
• B. PCl\(_5\)
• C. SOCl\(_2\)
• D. PCl\(_3\)

Hint:

Methods for converting Alcohols (R-OH) to Alkyl Chlorides (R-Cl): - Lucas Reagent (HCl + anhy. ZnCl\(_2\)): Good for 3\(^\circ\)>2\(^\circ\) alcohols. 1\(^\circ\) alcohols react slowly upon heating. - PCl\(_5\): R-OH + PCl\(_5\) \( \rightarrow \) R-Cl + POCl\(_3\) + HCl. - PCl\(_3\): 3R-OH + PCl\(_3\) \( \rightarrow \) 3R-Cl + H\(_3\)PO\(_3\). - SOCl\(_2\) (Thionyl Chloride): R-OH + SOCl\(_2\) \( \rightarrow \) R-Cl + SO\(_2\)(g) + HCl(g). Preferred for pure product as byproducts are gases. Often done with pyridine (Darzen's process).

Answer 1

The Correct Option is C

Alkyl chlorides (R-Cl) can be prepared from alcohols (R-OH) using several reagents.
The question asks for the "preferred" reagent for "pure" alkyl chloride.
1.
HCl + ZnCl\(_2\) (Lucas Reagent): \( \text{R-OH} + \text{HCl} \xrightarrow{\text{ZnCl}_2} \text{R-Cl} + \text{H}_2\text{O} \).
This reaction works well for tertiary and secondary alcohols.
For primary alcohols, it requires heating and is slower.
The product water needs to be removed.
Purity can be an issue due to side reactions or incomplete reaction, especially for primary alcohols.
2.
PCl\(_5\) (Phosphorus pentachloride): \( \text{R-OH} + \text{PCl}_5 \rightarrow \text{R-Cl} + \text{POCl}_3 + \text{HCl} \).
Both byproducts, phosphoryl chloride (POCl\(_3\)) and hydrogen chloride (HCl), are volatile but POCl\(_3\) can be difficult to separate completely if it has a similar boiling point or reacts further.
3.
SOCl\(_2\) (Thionyl chloride): \( \text{R-OH} + \text{SOCl}_2 \rightarrow \text{R-Cl} + \text{SO}_2\text{(g)} + \text{HCl(g)} \).
This method is often preferred because the byproducts, sulfur dioxide (SO\(_2\)) and hydrogen chloride (HCl), are gases and escape from the reaction mixture, leaving behind a relatively pure alkyl chloride.
This makes purification easier.
The reaction is often carried out in the presence of a base like pyridine to neutralize the HCl formed.
4.
PCl\(_3\) (Phosphorus trichloride): \( 3\text{R-OH} + \text{PCl}_3 \rightarrow 3\text{R-Cl} + \text{H}_3\text{PO}_3 \).
The byproduct, phosphorous acid (H\(_3\)PO\(_3\)), is a non-volatile liquid/solid and needs to be separated from the alkyl chloride, which can sometimes be difficult.
Considering the ease of purification and purity of the product, thionyl chloride (SOCl\(_2\)) is generally the preferred reagent for preparing alkyl chlorides from alcohols because the gaseous byproducts are easily removed.
This matches option (3).