Silica surface mediated additions

Surface-mediated addition of HC1 or HBr can be carried out in the presence of silica or alumina.150 The hydrogen halides can be generated from thionyl chloride, oxalyl chloride, oxalyl bromide, phosphorus tribromide, or acetyl bromide. The kinetic products from HC1 and 1-phenylpropyne result from syn addition, but isomerization to the more stable Z-isomer occurs upon continued exposure to the acidic conditions. 

The surface-mediated addition of HC1, HBr, and HI to 1-phenylpropyne in the presence of silica or alumina yields initially the corresponding E isomer as a result of syn addition.141 Rapid isomerization in excess reagents allows the formation of the thermodynamically more stable Z isomers. 

Some organisms actively participate in the precipitation of minerals for example, sulfide-oxidizing microbes mediate the crystallization of native sulfur crystals, and microbes are also thought to participate in the precipitation of marcasite and iron oxide minerals. Additionally, the surfaces of organisms provide favorable sites for nucleation and growth of amorphous silica, metal sulfide and metal oxide crystals, and this facilitates mineral precipitation and fossilization of vent fauna (Figure 3). 

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