Tricresyl phosphate (TCP), an important plasticiser, is produced by reacting cresylic acid (cresols) with which phosphorus reagent?

Introduction / Context:
Tricresyl phosphate is an aryl phosphate ester widely used as a plasticiser and flame retardant. Understanding its synthesis route clarifies esterification chemistry of phenols with phosphorus halides and oxyhalides.

Given Data / Assumptions:

• Reactant phenols: cresylic acid (o-, m-, p-cresols).
• Target ester: tricresyl phosphate (TCP).
• Common phosphorus reagents: POCl3, P2O5, phosphates.

Concept / Approach:
Phenolic hydroxyls react with phosphorus oxychloride (POCl3) to form triaryl phosphate esters, with HCl as a by-product, often in the presence of a base or catalyst. P2O5 is a dehydrating agent and does not directly furnish the triaryl phosphate ester in the same way; inorganic phosphates (ammonium/calcium) are unsuitable reagents for direct ester formation.

Step-by-Step Solution:

Verification / Alternative check:
Industrial routes to triaryl phosphates typically employ POCl3 or similar reagents (e.g., PCl3 with oxidation), confirming the choice.

Why Other Options Are Wrong:

• P2O5: primarily a dehydrating agent; not the standard reagent for TCP synthesis.
• Ammonium/calcium phosphate: salts, not reactive acylating agents for ester formation.
• H3PO4 only: insufficient for direct triaryl phosphate esterification under practical conditions.

Common Pitfalls:
Assuming any phosphorus-containing compound can produce phosphate esters; reactive oxyhalides are required.

Final Answer:
Phosphorus oxychloride (POCl3)