Chemistry · Ch 3 — p-Block Elements-II
Phosphorous trichloride and pentachloride
Phosphorous trichloride and pentachloride
Phosphorous trichloride (PCl3):
Preparation: when a slow, controlled stream of chlorine gas is passed over white phosphorus, phosphorous trichloride is formed. It can also be prepared by treating white phosphorus with thionyl chloride (P4 + 8SOCl2 -> 4PCl3 + 4SO2 + 2S2Cl2).
Properties: cold water hydrolyses phosphorous trichloride to phosphorous acid and hydrochloric acid (PCl3 + 3H2O -> H3PO3 + 3HCl). Mechanistically, this hydrolysis proceeds by the water oxygen first coordinating to phosphorus through phosphorus's vacant 3d orbital (forming an intermediate adduct, PCl3.H2O), which then eliminates HCl in a stepwise fashion (PCl3.H2O -> P(OH)Cl2 + HCl; HPOCl2 + H2O -> H2PO2Cl + HCl; H2PO2Cl + H2O -> H3PO3 + HCl) — a mechanism analogous to the hydrolysis of silicon tetrachloride, SiCl4. Similar substitution reactions occur with alcohols and carboxylic acids in place of water, giving alkyl/acyl chlorides together with phosphorous acid (3C2H5OH + PCl3 -> 3C2H5Cl + H3PO3; 3C2H5COOH + PCl3 -> 3C2H5COCl + H3PO3).
Uses: phosphorus trichloride is used as a chlorinating agent and in the preparation of phosphorous acid, H3PO3.
Phosphorous pentachloride (PCl5):
Preparation: treating PCl3 with excess chlorine gas gives phosphorous pentachloride (PCl3 + Cl2 -> PCl5). …
PCl3 is pyramidal (sp3 hybridised phosphorus, one lone pair occupying the fourth tetrahedral position) with three equivalent P-Cl bonds, structurally analogous in shape to NH3 and PH3. The P-Cl bond length is about 204 pm and the Cl-P-Cl bond angle is about 100 degrees, compressed below the ideal tetrahedral 109.5 degrees by the lone pair's greater repulsion. PCl3 is prepared by passing dry chlorine over heated white phosphorus, and it hydrolyses readily …