Question

Laboratory method for the preparation of acetyl chloride is.
(a) \[C{H_3}COOH + SOC{l_2} \to C{H_3}COCl\]
(b) \[C{H_3}COOH + PC{l_3} \to C{H_3}COCl\]
(c) \[C{H_3}COONa + PC{l_3} \to C{H_3}COCl\]
(d) All of these

Answer 1

Hint: The acylation involves the removal of \[ - OH\] group of carboxylic acids through chlorine (\[ - Cl\]) to form acyl chlorides or acid chlorides(\[RCOCl\]).

Complete step-by-step answer:
Sulphur dichloride oxide or thionyl chloride (\[SOC{l_2}\])
It is a colourless liquid form at room temperature.
In the reaction of thionyl chloride with carboxylic acid, the formation of an acyl chloride occurs with the liberation of gaseous byproducts such as sulphur dioxide (\[S{O_2}\]) and hydrogen chloride (\[HCl\] ) gases. In this method, the separation of acid chlorides (\[RCOCl\]) is simplified to an extent because the by-products are both gases. Therefore, this method is preferable for laboratory use.
 \[C{H_3}COOH + SOC{l_2} \to C{H_3}COCl + S{O_2} + HCl\]
Phosphorus trichloride (\[PC{l_3}\])
                          It is a colourless liquid and reacts with water violently with the formation of hydrogen chloride (\[HCl\] ) gas.
 Similar to thionyl chloride the \[PC{l_3}\]also reacts with carboxylic acids and forms acid chlorides. The byproduct of this reaction is phosphoric acid (\[{H_3}P{O_4}\]) which is solid in nature and cannot be separated easily from the reaction product (\[RCOCl\]).
                               \[C{H_3}COOH + PC{l_3} \to C{H_3}COCl + {H_3}P{O_4}\]
When \[PC{l_3}\]react with sodium salt acetic acid (\[C{H_3}COONa\]) it forms \[C{H_3}COCl\] and \[NaCl\] as a byproduct. Due to poor yield, this method is not acceptable for laboratory use.
                               \[C{H_3}COONa + PC{l_3} \to C{H_3}COCl + NaCl\]

The above discussion indicates that thionyl chloride is a suitable candidate for laboratory use because the by-products are gases which can easily escape. Hence, answer (a) is correct.

Note: The thionyl chloride and phosphorus trichloride need to be handled very carefully because when they encounter water they react vigorously.