UN 2646 — Hexachlorocyclopentadiene
Placard: Toxic. ERG Guide 151. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
UN 2646 is Hexachlorocyclopentadiene, a highly toxic persistent chlorinated liquid assigned to ERG Guide 151. Dense vapor and toxic fire products require control.
Hazard overview: HIGHLY TOXIC chlorinated liquid; inhalation, ingestion or skin absorption may be fatal. Vapors are very heavy and may collect in low or confined areas. Liquid is dense and persistent, spreading contamination on surfaces, soil or water.
Response guidance: For UN 2646, isolate the area, avoid skin contact and use SCBA where dust, vapor, mist or fire is present. Prevent spread of contaminated runoff, cool containers from protection and verify controls with SDS and ERG 151.
Firefighter training notes: Training for UN 2646 should emphasize toxic exposure routes, skin absorption, SCBA use, dust/vapor control, decontamination, runoff containment and SDS verification. Use ERG 151, SDS and local SOP.
Regulatory context: Hexachlorocyclopentadiene is regulated as a hazardous material for transport and emergency response. Storage, exposure, spill reporting, waste and fire-code duties depend on quantity, concentration and jurisdiction; verify shipping papers, SDS and local authority requirements.
Storage & handling: Hexachlorocyclopentadiene should be stored in tightly closed compatible containers with ventilation, secondary containment, restricted access and separation from heat, oxidizers and incompatible materials.
UN 2646 Quick Details
Common Hazards of UN 2646
- HIGHLY TOXIC chlorinated liquid; inhalation, ingestion or skin absorption may be fatal.
- Vapors are very heavy and may collect in low or confined areas.
- Liquid is dense and persistent, spreading contamination on surfaces, soil or water.
- Non-flammable, but heating or fire may produce hydrogen chloride, phosgene-type gases and toxic chlorinated smoke.
- Runoff may be toxic, persistent and environmentally harmful.
- Containers may rupture or fail when heated.
- Avoid all skin contact and prevent secondary contamination.
Chemical Identity & Physical Properties
Yellow to amber oily liquid with a pungent, acrid odor. Dense and readily volatilizes at room temperature.
| Also known as | HCCPDPerchlorocyclopentadieneC-56Hexachloro-1,3-cyclopentadiene |
| CAS Number | 77-47-4 |
| Appearance | Yellow to amber oily liquid with a pungent, acrid odor. Dense and readily volatilizes at room temperature. |
| Flash Point | Not flammable or combustible |
| Boiling Point | 239C (462F) |
| Vapor Density | 9.42 (much heavier than air) |
| Water Reactivity | Hydrolyzes slowly in water, producing toxic and corrosive gases |
Fireground Response Guidance — UN 2646
Extinguishing Media
PPE Requirements
Use positive-pressure SCBA and full chemical protective clothing. Level A may be needed for heavy vapor, dust, splash risk or unknown concentrations; prevent all skin contact.
Isolation & Evacuation
First Actions for a UN 2646 Incident
- Call 911 and the emergency response number on the shipping paper, if available.
- Keep unauthorized personnel away and establish incident command.
- Stay upwind, uphill and upstream.
- Avoid breathing vapor, dust, mist, smoke or fumes and avoid skin or eye contact.
- Do not touch damaged containers or spilled material without proper training and PPE.
- Prevent contaminated dust, liquid, runoff and decontamination waste from spreading.
- Ventilate confined spaces only after monitoring and only if properly trained and equipped.
- Use ERG Guide 151, SDS, shipping papers and monitoring to set isolation, evacuation and entry decisions.
📋 Copy & Share Field Card
UN 2646 — HexachlorocyclopentadieneUse for: Quick radio or face-to-face size-up. Short, structured, field-ready.
Use for: Incident command briefing, staging area whiteboard, or pre-entry team brief.
Use for: Quick text to command or incoming units. Fits in a single SMS.