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Silicon

Silicon. Silanes Silica, silicates, and zeolites Silicones Pure silicon Computer chips. Silanes : Si n H 2n+2 Same structure as alkanes All unstable, pyrophoric : SiH 4 + 2 O 2  SiO 2 + H 2 O Generally not stable beyond Si 8 or so. No “ silenes ” because … __________________.

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Silicon

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

  2. Silanes: SinH2n+2Same structure as alkanesAll unstable, pyrophoric: SiH4 + 2 O2 SiO2 + H2OGenerally not stable beyond Si8 or so. No “silenes” because … __________________

  3. 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

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

  5. 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

  6. Structure depends on O/Si ratioSiO4Predict Si2O7Predict Si3O9

  7. O/Si Ratio: 4 3.5 3

  8. O/Si Ratio: 2.5 2.75 2

  9. Silicates: General TrendAs O/Si ratio decreases:

  10. Tetrahedral structure depiction:

  11. Aluminosilicates: ZeolitesSiO2 base structureSubstitute some Si atoms with Al

  12. Another representation Ball and stick Tetrahedral connectivity

  13. Sodalite Cages

  14. Formation of Zeolites from Sodalite Cages

  15. Different arrangements lead to different pore spaces.

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

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

  18. 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

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

  20. Silicones

  21. Synthesis of Silicone Precursors

  22. General Reaction

  23. Polydimethylsiloxane is the most common and available silicone.

  24. Silicone Structure/Properties/Reactant Relationships

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