The Use of Silver Nitrate Complexing to Separate Olefin Containing Compounds

In Organic Synthesis Coll. Vol. III a mixture of cis and trans cyclooctene is separated by mixing somewhat more than two molar equivalents of an aqueous silver nitrate solution with a pentane solution of the cis and trans compounds. The cis compound does not form any adduct and so remains in the pentane solvent. With vigorous stirring the trans compound forms a complex and dissolves in the aqueous phase. Although silver is an expensive reagent, , at least in principle, it is recoverable, so it can be considered for use at scale. Winstein and Lucas studied the complexes formed between silver nitrate and unsaturated hydrocarbons and found that in some cases these are solids useful for isolation and purification.[J. Am Chem. Soc. 60, 836 (1938)]. Complexes that are solids can be recrystallized, often from hot alcohol. It is not clear whether functional groups besides double bonds interfere with separation in this way, although only hydrocarbons have been described in the literature. It is not apparent why a number other functional groups would be incompatible with the method. It may just be that, when other functionalities are present, there are better known options for separations.

The Diels-Alders adduct between norbornadiene and cyclopentadiene contains two double bonds and forms a 2:3 hydrocarbon/silver nitrate adduct [Am. Soc. 81, 4273 (1959)]. The Diels-Alder adduct between norbornene and cyclopentadiene contains only one olefin group and was purified using its 1:1 adduct [Am. Soc. 86, 2188 (1964).].From work with the mixtures of 1,3; 1,4; and 1,5- cyclooctadiene, it has been shown that silver nitrate forms complexes with each of these, but they have different stabilities, and can be separated by using different temperatures. The weak complexes are only isolable at low temperatures [J. Chem. Soc. 312 (1954)]. The natural triene humulene was purified as a silver nitrate complex containing 2 molar equivalents of silver nitrate [Australian J. Chem. 14,272 (1961)][Tet. Let. 1977 (1965)].

It is interesting to speculate whether an aqueous solution of silver nitrate could be used to treat a mixture of olefin and the dihalocarbene adduct to remove the olefin. The dihalocarbene adduct would be expected to be reactive with silver nitrate if they were both in a homogenous solution but if the dihalocarbene adduct was in a saturated hydrocarbon solvent and the silver nitrate was in water, they might not come into sufficient contact to react. It might also be a problem for complex formation if the olefin that one sought to complex was itself not sufficiently soluble in water to allow reaction.

Use in Column Chromatography

Olefin containing compounds are separated on reverse phase columns when silver nitrate is dissolved in the mobile phase. It would be interesting to see whether the reverse phase HPLC mobility of unsaturated compounds in an aqueous silver nitrate eluate might give an indication of their complexing ability.
 

Liquid-liquid Extraction

Silver nitrate in methanol improves the separation of saturated fatty acids from unsaturated acids that can be held in solution better when silver nitrate is added. This suggests the possibility of liquid-liquid extraction between pentane or hexane and aqueous silver nitrate.