Silicon

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

Silanes: SinH2n+2

Same structure as alkanesAll 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 358Si-Si 222 Si-H 328 Si-O 452

Silica and Silicates

Silica: SiO2

Si = +4O = -2

Crystalline = quartzNoncrystalline = vitreous

Silicates: Si-O anions + metal cations

General Rules:

1.Si always makes 4 Si-O bonds

2.Can be terminal (Si-O) or bridging (Si-O-Si)

3.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 structureSubstitute some Si atoms with Al

Another representation

Ball and stick Tetrahedral connectivity

Sodalite Cages

Formation of Zeolites from Sodalite Cages

Different arrangementslead to different pore spaces.

Pure SiO2, 0% Al replacement 10% Al replacement

Charge and counter ions

Pure SiO2, 0% Al replacement 10% Al replacement

Affinity for water

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

Solution

Catalyze reaction inside a zeolite pore that only accommodates thepara isomer.

Silicones

Synthesis of Silicone Precursors

General Reaction

Polydimethylsiloxane is the most common and available silicone.

Silicone Structure/Properties/Reactant Relationships