Silicon

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Silicon

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

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Silanes SinH2n2Same structure as alkanesAll
unstable, pyrophoric SiH4 2 O2 ? SiO2
H2OGenerally not stable beyond Si8 or so. No
silenes because __________________
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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
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Silica and Silicates
Silica SiO2 Si 4 O -2 Crystalline
quartz Noncrystalline vitreous
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Silicates Si-O anions metal cations

• General Rules
• Si always makes 4 Si-O bonds
• Can be terminal (Si-O) or bridging (Si-O-Si)
• Charge comes from Si 4, O -2

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Structure depends on O/Si ratioSiO4Predict
Si2O7Predict Si3O9
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O/Si Ratio 4 3.5 3
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O/Si Ratio 2.5 2.75 2
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Silicates General TrendAs O/Si ratio
decreases
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Tetrahedral structure depiction
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Aluminosilicates ZeolitesSiO2 base
structureSubstitute some Si atoms with Al
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Another representation
Ball and stick
Tetrahedral connectivity
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Sodalite Cages
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Formation of Zeolites from Sodalite Cages
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Different arrangements lead to different pore
spaces.
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Charge and counter ions
Pure SiO2, 0 Al replacement
10 Al replacement
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Affinity for water
10 Al replacement
Pure SiO2, 0 Al replacement
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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
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Solution
Catalyze reaction inside a zeolite pore that only
accommodates the para isomer.
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Silicones
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Synthesis of Silicone Precursors
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General Reaction
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Polydimethylsiloxane is the most common and
available silicone.
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Silicone Structure/Properties/Reactant
Relationships