Silica proton-donor function

The possibility exists that a surface hydroxyl group on silica can itself function as a proton donor (12),... 

Silica gel is also the support of choice for the activation of V-halosuccinimides. The silica functions both as a proton donor which increases the electrophilic nature of the reagent, and as a support with geometrical constraints which contributes to the stereoselectivity. Alkyl and aryl sulfoxides are readily halogenated at the a position with yields of48-80%91. The reactions are carried in the solid state on the surface of TLC plates. The conversion of the optically active alkyl 4-methylphenyl sulfoxide into 1-haloalkyl 4-methylphenyl sulfoxide is accompanied by inversion of configuration at the S-atom. The stereoselectivity in these reactions is much higher than that observed in liquid-phase halogenation. 

Adsorption TLC selection of the mobile phase is conditioned by sample and stationary-phase polarities. The following polarity scale is valid for various compound classes in NPTLC in decreasing order of K values carboxylic acids>amides>amines>alcohols>aldehydes > ketones > esthers > nitro compounds > ethers > hal-ogenated compounds > aromatics >olefins > saturated hydrocarbons > fluorocarbons. For example, retention on silica gel is controlled by the number and functional groups present in the sample and their spatial locations. Proton donor/acceptor functional groups show the greatest retention, followed by dipolar molecules, and, finally, nonpolar groups. 

As an example of the effect of solution pH on anion adsorption, chromium solutions at various pH levels are contacted with two amine functionalized silica gels. The results show that the extraction efficiency increases as the pH increases up to a maximum value for each extractant and then starts to decrease as shown in Figure 7.16. This behavior could be a result of (1) the protonation of amine groups on the multidonor atom ligand (see Figure 7.22 in Section 7.4), (2) preferential adsorption of multicharged anionic species, and (3) steric hinderance factors between the anionic species and the adjacent protonated amine donor atoms. However, sufficient information is not yet available at the present to analyze this behavior conclusively. 

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