Secondary Phosphinic Acids and Esters

Secondary phosphinic acids (6.254) may be prepared by the oxidation of secondary phosphine oxides (6.26) or by thermal decomposition of the latter (6.133). Air, oxygen, hydrogen peroxide, dilute nitric acid or bromine water may be used for such oxidations. Many phosphinic acids can be prepared by refluxing their alkyl esters with 20% aqueous HCl (6.255). Sulphur dioxide and phosphines may also be used (6.268). 

Phosphinic acids or their sodium salts can be obtained from the action of NaOH on some tertiary phosphine oxides (6.122,6.124) or by the hydrolysis of phosphinyl halides (6.200). 

Phosphinic esters can be obtained by the oxidation of the corresponding phosphinite ester (6.245) or by the isomerisation of phosphonous diesters (6.230) and by reactions (6.232,6.253). 

Some secondary phosphinic acids are obtained by cyclisation of phosphonic acids. 

Phosphinic acids can be esterifled by heating with trialkyl phosphites. Phosphinic acids or their esters are converted into the corresponding acid chlorides by the action of thionyl chlorides or phosphorus pentachloride. 

---