Amine oxides tetraphenylborate

Titration with an anionic surfactant At low pH, long-chain amine oxides and the free amines can be determined by titration with an anionic surfactant or tetraphenylborate, using either the two-phase indicator method or the single-phase ion selective electrode method, as described in Chapter 16. The free amine can be separated from the aqueous sample solution by extraction at pH 10.5-12.0 with petroleum ether. The amine content is then determined by stripping off the petroleum ether, dissolving the residue in water, and titrating at pH 2 with an anionic. The amine oxide content is determined by difference (153). 

Amine oxides have cationic properties at low pH and so can be titrated with anionic reagents according to the procedures developed for cationic surfactants. Titrations with tetraphenylborate (81) and with dodecylsulfate (82) have been demonstrated, using a potentiometric end point. A turbidimetric end point has also been demonstrated (28). An am-perometric end point may be used in a two-phase system with dodecylsulfate titrant, if a suitable cationic indicator is added such as the iron(II) 1,10-phenanthroline complex (83). Long-chain amines, including any unreacted amine from synthesis of the amine oxide, will quantitatively interfere. 

A range of structurally different chiral primary amines was converted into the corresponding iminium tetraphenylborate salts (Fig. 5.3) and tested in the asymmetric epoxidation of a standard test substrate, 1-phenylcyclohexene, using Oxone (4 equiv) as the stoichiometric oxidant, sodium carbonate (8 equiv) as base, in acetonitrile/water (2 1) at 0 °C (Table 5.1) [19,21]. 

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