Liquid Chromatography - Bonded Phase Synthesis by Reaction in a Solvent 1
Bonded Phase Synthesis by Reaction in a Solvent.
The solvents normally used in bonded phase synthesis are aromatic hydrocarbons e.g., toluene that boils at 110˚C or mixed xylenes that boil 138-140˚C. The procedure varies a little depending on the size of the batch and the type of silanizing reagent. An example of a laboratory scale synthesis using a chlorosilane is as follows.
10 g of the chosen silica is dried at 250˚C for about 2 hours and dispersed in a flask containing 100 ml of toluene dried over sodium. If a mono-chlorosilane reagent is used a small trace of water in the toluene can be tolerated and can be eventually eliminated by the use of excess chlorosilane reagent. Under such circumstances the toluene need not be dried over sodium. If, a dichlorosilane is used however, for example in the initial step in the synthesis of an oligomeric phase, the presence of water may cause linear polymerization. Consequently, stringent precautions must be taken to eliminate all traces of water. A slight excess of the chlorosilane is then added to the silica dispersion together with 5 ml of pyridine. The pyridine acts as scavenger for the hydrochloric acid released during the reaction. The mixture is refluxed for about 5 hours and the product is then filtered on a sintered glass filter, washed sequentially with toluene, tetrahydrofuran (THF), methanol, methanol water (50:50 v/v) and finally with methanol and dried under suction. The bonded phase now needs end-capping; that is, any unreacted silanol groups are treated with a small molecular weight silanizing reagent to react with those hydroxyl groups that were stearically unavailable to the larger reagent due to exclusion. To end-cap the product, the bonded phase is refluxed for two hours in a mixture of 100 ml of toluene and 25 ml of hexamethyldisilazane. The product is again filtered free of the reaction liquid mixture and washed sequentially with, toluene tetrahydrofuran, methanol, methanol water (50:50 v/v) and finally with methanol and then dried under vacuum. It should be pointed out that end-capping cannot eliminate the hydroxyl groups that are stearically hindered by the bonded moiety, or at best, only a small proportion of them will be removed. It can, however, react with any readily available hydroxyl groups, particularly those contained in pores that the original reagent could not enter but to which the smaller capping reagent has access. Capping can also eliminate any hydroxyl groups attached to the bonded moiety resulting from the presence of dichloro or tricoloro impurities in the silanizing reagent. After capping, the carbon content of the product can then be determined to estimate the extent of reaction.