Silicon Tetrachloride
Properties
| State | Liquid (colorless, fuming, with sharp odor) |
| Color | Colorless |
| Solubility | Reacts with water (hydrolysis to silicic acid and HCl); miscible with benzene, ether, chloroform |
| Melting Point | -68°C |
| Boiling Point | 57.6°C |
About Silicon Tetrachloride
Silicon tetrachloride is a colorless, fuming, sharp-smelling liquid with the formula SiCl4 and a molar mass of 169.898 g/mol. The molecule is tetrahedral with Si-Cl bond lengths near 2.02 angstroms, and the compound is the workhorse intermediate of two enormous industries: semiconductor-grade polysilicon and synthetic fused silica for optical fibers. In the Siemens process, metallurgical-grade silicon (about 98 percent pure, made by carbothermic reduction of quartz) reacts with HCl at 300 °C in a fluidized bed to give a mixture of trichlorosilane (HSiCl3, the main product) and SiCl4. The chlorosilanes are then fractionally distilled to the absurd purity required for chip-grade silicon — better than 99.9999999 percent, the so-called nine-nines or 9N grade — because the volatility difference between the chlorosilanes and most metal-chloride impurities is what makes that purification possible. The purified HSiCl3 is then reduced with hydrogen at about 1100 °C on a heated silicon rod, depositing pure polysilicon for crystal pulling into single-crystal ingots and wafer slicing. In the optical-fiber industry, SiCl4 is fed into a hydrogen-oxygen flame in the OVD or MCVD process to deposit ultra-pure SiO2 soot onto a rotating substrate, building up the cylindrical glass preform from which kilometers of single-mode fiber are drawn. SiCl4 also makes fumed silica when burned in flame: ultra-fine 7-40 nm SiO2 particles used as a thixotrope in paints and reinforcing filler in silicone rubber.
Where you'll encounter it
If you've ever walked through a polysilicon plant — the upstream end of a solar-cell or chip-wafer supply chain — the loudest sensory impression is the eye-watering acid mist where SiCl4 leaks meet humid air, hydrolyzing instantly to silicic acid colloid plus HCl fumes. The plant runs SiCl4 through a triple-distillation tower train that takes weeks of steady-state operation to reach equilibrium, because separating SiCl4 from the boron and phosphorus chloride dopants that ruin chip grade requires hundreds of theoretical plates. In a fiber-optics MCVD lab, SiCl4 vapor is bubbled into a heated silica tube along with germanium tetrachloride to dope the core with GeO2, which raises the refractive index by 1 to 2 percent and creates the index gradient that traps light by total internal reflection. The third encounter most working chemists have with SiCl4 is as a Lewis acid in organic synthesis — it's used catalytically with chiral diamines to activate aldehydes for asymmetric allylation, the Denmark allylation chemistry.
Common Uses
- Siemens-process intermediate for nine-nines polysilicon used in chip wafers and solar cells
- Optical-fiber preform deposition via MCVD and OVD with GeCl4 dopant for core/cladding index gradient
- Fumed-silica production by flame hydrolysis for paint thickeners and silicone rubber filler
- Lewis acid catalyst for Denmark asymmetric allylation in organic synthesis
- Precursor for organosilicon compounds and silicone polymers via Grignard or alkylation routes
Safety Information
GHS: Skin corrosion Category 1B (H314), STOT SE Category 3 respiratory (H335), reacts violently with water releasing HCl. The hazard profile is dominated by hydrolysis: contact with skin moisture or pulmonary fluid generates HCl fumes that cause severe burns and pulmonary edema, with delayed onset of respiratory symptoms up to 48 hours after exposure. No OSHA PEL is set for SiCl4 itself, but the HCl release falls under OSHA PEL for HCl of 5 ppm ceiling. Handle in a fume hood with full PPE — neoprene gloves rated for HCl, full face shield, lab coat, and ideally an air-purifying respirator with acid-gas cartridges. Store under dry nitrogen with PTFE-sealed valves; ground glass joints will fuse with hydrolysis products.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.