Bonded Phases - Characteristics of Bonded Phases
An entirely different procedure for bonding organic groups to the silica gel surface utilizes the Grignard reaction (4-7), a process also pioneered by Halasz et al. The silica is first treated with bromine or preferably with thionyl chloride to replace the surface hydroxyl groups with halogen atoms.
The reaction must proceed in an anhydrous environment. Consequently, to remove the strongly bound water on the silica surface, the silica must be heated to about 350oC prior to reaction. After reaction with thionyl chloride, the hydrochloric acid, sulfur dioxide and excess reagent must be completely removed before subsequent treatment with the Grignard reagent. The resulting chlorinated silica is then reacted with an appropriate Grignard reagent in dry diethyl ether and the alkyl group then replaces the chlorine on the silica in the expected manner.
Employing the Grignard reagent the organic group is attached directly to the surface silicon atom with the more stable Si-C bond. As a single hydrocarbon chain can be attached devoid of the base dimethyl groups involved with the mono-chlorsilane reagents, the interactive dispersion group will be significantly less sterically hindered. The efficiency of the Grignard reaction does not appear to have been reported and it would be useful to know if a larger proportion of silanol groups was derivatized by the this method than by the chlorsilane procedure. The more efficient the reaction, the greater will be the stability of the bonded phase. Halasz et al (5) also considered the direct reaction of amino groups to the hydroxyls on the silica surface. They treated silica with thionyl chloride using the standard procedure and replaced the hydroxyl groups with chlorine atoms as depicted below. The chlorinated silica was then treated with ethylene diamine. The amine reacts with the chlorsilane with the elimination of hydrochloric acid leaving the ethylenediamine group attached to the silica as the hydrochloride. The ethylene diamine-silica exhibited interesting properties as a stationary phase with weak ion exchange characteristics.
In addition, Halasz et al used the ethylenediamine-silica as a starting material for the formation of a different bonded phases. They reacted the ethylene diamine-silica (as the free base) with p-nitrobenzyl chloride. This reaction was carried out in dioxane as the solvent and a p-nitrophenyl tail was attached to the ethylene diamine silica. The product behaved as a polar bonded phase with a polarity about halfway between that of a reverse phase and that of native silica gel.