Mercury(II) Chloride
Properties
| State | Solid (white crystalline) |
| Color | White |
| Solubility | Soluble in water (74 g/L at 20°C); soluble in ethanol and diethyl ether |
| Melting Point | 276°C |
| Boiling Point | 304°C |
About Mercury(II) Chloride
Mercury(II) chloride — the alchemists called it corrosive sublimate, and the name still tells you most of what you need to know. It is corrosive to living tissue at gram-scale doses (LD50 around 1 mg/kg in humans, putting it in the league of cyanide for acute oral toxicity), and it sublimes readily under heat, which is how alchemists purified it in the first place. Structurally, HgCl2 is the textbook example of a metal salt that isn't really a salt: in solid, melt, and gas it sits as discrete linear Cl-Hg-Cl molecules with sp hybridization on Hg, a Hg-Cl bond length of 2.25 Å, and zero ionic character to speak of. That covalent character is why the melting point is only 276°C (vs 801°C for NaCl), why it dissolves in ethanol and diethyl ether as well as water, and why molten HgCl2 is a poor electrolyte. Historically, surgeons in the late 19th century used 1:1000 HgCl2 solution as a surgical-instrument disinfectant after Lister's work on antisepsis — until the obvious problem with handling kg quantities of one of the most toxic salts known caught up. In modern chemistry, HgCl2 still shows up as the catalyst in oxymercuration-demercuration of alkenes (Markovnikov hydration without rearrangement), as part of Nessler's reagent (alkaline K2HgI4) for detecting NH3 down to ~0.02 ppm, and as a reagent for converting boronic acids to aryl mercurials. Most modern syntheses look for substitutes wherever possible.
Where you'll encounter it
If you have ever opened the cabinet labeled 'POISON' in an undergrad organic stockroom and seen a tiny tightly-sealed brown bottle of HgCl2 behind glass with a chain of warning tape, you have met the compound that entire courses now teach you to avoid. In a research lab that still runs oxymercuration prep — alkene → β-hydroxy mercurial via HgCl2 in THF/H2O, then NaBH4 reduction to the alcohol — the procedure runs in a fume hood with double nitrile gloves, the waste goes into a dedicated mercury sharps container, and the bench is covered with lab pads in case of a drop. The Nessler test you may have done in environmental chemistry — a yellow-orange color forming as a few drops of reagent hit a water sample with ammonia present — relies on K2HgI4 + NH3 → orange brown HgO·Hg(NH2)I, sensitive enough to flag well-water contamination from cattle operations.
Common Uses
- Catalyst for oxymercuration-demercuration of alkenes (Markovnikov hydration)
- Component of Nessler's reagent for sensitive NH3/NH4+ colorimetric detection
- Conversion of arylboronic acids to aryl mercurials in synthesis
- Photographic intensifier in historical processes (now obsolete)
- Reference compound and primary standard for trace mercury analysis
Safety Information
EXTREMELY TOXIC. GHS: H300 (fatal if swallowed), H310 (fatal in skin contact), H330 (fatal if inhaled), H373 (organ damage, kidney/CNS, repeated exposure), H410 (very toxic to aquatic life with long-lasting effects). Oral LD50 around 1-4 mg/kg in humans. OSHA PEL is 0.1 mg/m3 ceiling for Hg. Acute ingestion produces severe gastroenteritis, renal failure within 24-72 hours, and death without prompt dialysis and chelation (BAL/dimercaprol or DMSA). Skin absorption is significant — never handle without nitrile gloves and a fume hood. Vapor pressure at room temp is low but non-zero. All HgCl2 waste goes to mercury hazardous-waste stream; never to drain or general chemical waste. Restricted under the Minamata Convention on Mercury (2013).
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.