[CH2:1]([P:3]([OH:5])[OH:4])[CH3:2].[CH:6](=[O:9])[CH:7]=[CH2:8].[O-]S(OOS([O-])(=O)=O)(=O)=O.[Na+].[Na+]>O>[CH2:1]([P:3]([CH2:8][CH2:7][CH:6]=[O:9])(=[O:5])[OH:4])[CH3:2]564 g (6 mol) of ethylphosphonous acid (produced as in Example 1) are dissolved in 860 g of water and initially charged to a 5 l five-neck flask equipped with thermometer, reflux condenser, high-performance stirrer and dropping funnel. The reaction mixture is heated to 100° C. and 392 g (7 mol) of 2-propenal and 500 g of a 5% strength sodium peroxodisulfate solution (1.5 mol % based on 2-propenal) is added dropwise at atmospheric pressure over 1 h. Then, the water is distilled off in vacuo. The residue is taken up in tetrahydrofuran and the insoluble salts are filtered off. The solvent of the filtrate is removed in vacuo to leave 675 g (75% of theory) of ethyl(2-formylethyl)phosphinic acid as oil.
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