PCl5 / SOCl2 and carboxylic acids

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SUMMARY

The discussion focuses on the reactions of carboxylic acids with phosphorus pentachloride (PCl5) and thionyl chloride (SOCl2). The equations presented are R-COOH + PCl5 ⇌ R-COCl + POCl3 + HCl and R-COOH + SOCl2 ⇌ R-COCl + SO2 + HCl. It is established that the reaction with SOCl2 goes to completion in an open system due to the gaseous nature of the products, while the reaction with PCl5 does not reach completion despite HCl being a gas, primarily due to the presence of phosphoryl chloride (POCl3) as a liquid. The use of triethylamine and Schlenk conditions is also noted as a common practice in these reactions.

PREREQUISITES
  • Understanding of carboxylic acid reactivity
  • Knowledge of phosphorus pentachloride (PCl5) and thionyl chloride (SOCl2) chemistry
  • Familiarity with reaction equilibrium concepts
  • Experience with Schlenk line techniques in organic synthesis
NEXT STEPS
  • Research the mechanism of carboxylic acid chlorination with SOCl2
  • Study the properties and applications of phosphoryl chloride (POCl3)
  • Explore the use of triethylamine as a base in organic reactions
  • Learn about Schlenk line techniques and their applications in air-sensitive reactions
USEFUL FOR

Chemists, organic synthesis practitioners, and students studying reaction mechanisms involving carboxylic acids and chlorinating agents.

trollcast
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In my chemistry notes it gives the equations,

##R-COOH + PCl_5 \rightleftharpoons R-COCl + POCl_3 + HCl##

##R-COOH + SOCl_2 \rightleftharpoons R-COCl + SO_2 + HCl##

It then says that in an open system the reaction with thionyl chloride will go to completion as all the products except the acyl chloride can escape as they are gases, so the equilibrium keeps moving right. However why does the reaction with PCl5 not go to completion since the HCl will escape as its a gas?
 
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trollcast said:
In my chemistry notes it gives the equations,

##R-COOH + PCl_5 \rightleftharpoons R-COCl + POCl_3 + HCl##

##R-COOH + SOCl_2 \rightleftharpoons R-COCl + SO_2 + HCl##

It then says that in an open system the reaction with thionyl chloride will go to completion as all the products except the acyl chloride can escape as they are gases, so the equilibrium keeps moving right. However why does the reaction with PCl5 not go to completion since the HCl will escape as its a gas?

Phosphoryl chloride is a liquid, ergo...

In reality we as a base, usually triethylamine, to these reactions and perform them under schlenk condition.
 

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