Silicon Silanes Silica, silicates, and zeolites Silicones Pure silicon Computer chips

Silanes: SinH2n+2 Same structure as alkanes All unstable, pyrophoric: SiH4 + 2 O2  SiO2 + H2O Generally not stable beyond Si8 or so. No “silenes” because … __________________

Why would silanes be so much less stable than alkanes? Bond Energies, kJ/mol: C-C 346 C-H 413 C-O 358 Si-Si 222 Si-H 328 Si-O 452

Silica and Silicates Silica: SiO2 Si = +4 O = -2 Crystalline = quartz Noncrystalline = vitreous

Silicates: Si-O anions + metal cations General Rules: Si always makes 4 Si-O bonds O can be terminal (Si-O) or bridging (Si-O-Si) Charge comes from Si = +4, O = -2

Structure depends on O/Si ratio SiO4 Predict Si2O7 Predict Si3O9

O/Si Ratio: 4 3.5 3

O/Si Ratio: 2.5 2.75 2

Silicates: General Trend As O/Si ratio decreases:

Tetrahedral structure depiction:

Aluminosilicates: Zeolites SiO2 base structure Substitute some Si atoms with Al

Another representation Ball and stick Tetrahedral connectivity

Sodalite Cages

Formation of Zeolites from Sodalite Cages

Different arrangements lead to different pore spaces.

Charge and counter ions Pure SiO2, 0% Al replacement 10% Al replacement

Affinity for water Pure SiO2, 0% Al replacement 10% Al replacement

Zeolites and Catalysis Purpose of a catalyst: increase rate, selectivity Desired: react para isomer to form terephthalic acid Problem: under reaction conditions, three isomers can interchange So, might end up with wrong isomers of C6C4(CO2H)2

polyester

Solution Catalyze reaction inside a zeolite pore that only accommodates the para isomer.

Silicones

Synthesis of Silicone Precursors

General Reaction

Polydimethylsiloxane is the most common and available silicone.

Silicone Structure/Properties/Reactant Relationships