UN 1242 — Methyldichlorosilane
Placard: Dangerous When Wet. ERG Guide 139. Training/quick-reference only — use current ERG + SOP/SOG for incident-specific actions.
Methyldichlorosilane is a colorless to pale yellow fuming liquid that produces flammable and toxic gases on contact with water, requiring careful handling and immediate response in case of an emergency. It has a sharp, pungent, and acrid odor, and fumes in moist air due to hydrolysis.
Hazard overview: Methyldichlorosilane may ignite on contact with water or moist air, producing HCl gas and heat, and can cause respiratory problems due to HCl vapor generation. Prolonged exposure can lead to serious health issues, including burns and eye damage.
Response guidance: In case of an emergency, isolate the spill within 50m in all directions, and consider evacuation if a tank or rail car is involved in a fire, wearing Level A or B protective gear, including SCBA and full chemical-resistant suit. Use dry chemical, CO2, dry sand, or soda ash for extinction, avoiding water contact.
UN 1242 Quick Details
Common Hazards of UN 1242
- Produce flammable and toxic gases on contact with water.
- May ignite on contact with water or moist air.
- Some react vigorously or explosively on contact with water.
- May be ignited by heat, sparks or flames.
- May re-ignite after fire is extinguished.
- Some are transported in highly flammable liquids.
- Containers may explode when heated.
- Runoff may create fire or explosion hazard.
Chemical Identity & Physical Properties
Methyldichlorosilane is a colorless to pale yellow fuming liquid with a sharp, pungent, and acrid odor, liquid at room temperature, and fumes in moist air due to hydrolysis.
| Also known as | DichloromethylsilaneMethylsilicon dichlorideSilane, dichloromethyl-MDCS |
| CAS Number | 75-54-7 |
| Appearance | Colorless to pale yellow fuming liquid with a sharp, pungent, acrid odor. Fumes in moist air due to hydrolysis. Liquid at room temperature. |
| Flash Point | -32C (-26F) |
| Boiling Point | 41C (106F) |
| Vapor Density | 4.0 (heavier than air) |
| Water Reactivity | Reacts violently with water, producing HCl gas and heat. May ignite spontaneously. Do not use water directly on material. |
Fireground Response Guidance — UN 1242
Extinguishing Media
PPE Requirements
Level A or B protective gear, including SCBA and full chemical-resistant suit, is required when handling methyldichlorosilane due to its flammable and toxic properties, as well as HCl vapor generation.
Isolation & Evacuation
First Actions for a UN 1242 Incident
- Keep unauthorized personnel away.
- Stay upwind, uphill and/or upstream.
- Ventilate closed spaces before entering, but only if properly trained and equipped.
- Isolate spill or leak area in all directions for at least 50 meters (150 feet) for liquids and at least 25 meters
- For highlighted materials: see Table 1 - Initial Isolation and Protective Action Distances.
- For non-highlighted materials: increase the immediate precautionary measure distance, in the downwind
📋 Copy & Share Field Card
UN 1242 — MethyldichlorosilaneUse 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.