Explosives and Explosions The Chemistry of High Energy Organic Compounds

What is an Explosion? Rapid burning of a material resulting in a sudden build-up and release of heat and gas pressure.

Explosions Which Rely on Oxygen from the Air Combustion of gasoline in the engine of your car. How easy is it to make gasoline explode?

Hexanes: Among the many hydrocarbons found in gasoline, they are representative of the volatility and explosivity of gasoline.

Hexanes burn rapidly in air to form carbon dioxide and water. Hexanes will not explode in a closed container and neither will gasoline. Too much fuel Not enough oxygen The gas tank on your car is not an explosion hazard. Vapor phase concentration of hydrocarbons is above upper explosive limit (UEL) of 8%.

Ethanol also burns in air. Ethanol is highly explosive in a closed container optimum balance of fuel and oxygen in the vapor Vapor phase concentration of ethanol at room temperature falls between the LEL and UEL.

What about other types of hydrocarbon fuels and fuel tanks? Diesel and Jet fuel tanks have a higher risk of explosion than automobile fuel tanks. TWA Flight 800 exploded in mid air in 1996, probably due to a vapor phase fuel tank explosion.

Surprisingly, explosivity of jet airplane fuel tanks is not well studied or understood. “The July 17, 1996, crash of TWA flight 800, a Boeing 747 airplane, was blamed on a fuel-air explosion within the center wing tank, with the ignition source still unidentified. As a consequence of the accident, the Federal Aviation Administration (FAA) is evaluating improved safety requirements for the fuel tanks on commercial aircraft. One technique, recommended by the National Transportation Safety Board (NTSB), is to maintain sufficient fuel in the center wing tanks of transport aircraft to limit the liquid fuel temperature rise and evaporation, thus keeping the vapor fuel/air ratio below the explosive limit. Initial attempts to determine the benefit of additional fuel in the center tank were frustrated by the lack of an acceptable method for determining the explosive hazard in the tank under varying conditions.” - FAA final report, TWA Flight 800 crash investigation.

How can we make explosives without the limitation of needing oxygen from the air? Make the “oxygen” (oxidizing agent) part of the chemical structure. Example: Nitrocellulose used in gun powder.

Cellulose (cotton) burns slowly in air.

Nitrocellulose (gun cotton) burns very rapidly even without air.

High Explosives Burn at Supersonic Speeds Conflagration: rapid burning with a flame front traveling through the material at 1 m/sec to 300 m/sec. Detonation: “instantaneous” burning with flame front traveling through the material at 1000 m/sec to 3000 m/sec resulting in a supersonic shock wave.

Primary and Secondary High Explosives Primary High Explosives - detonate very easily - minimal activation energy. Secondary High Explosives - do not detonate easily - high activation energy

Early Examples of Primary High Explosives

Nitroglycerine Nitroglycerine detonates by rapidly rearranging to a collection of small stable gas molecules releasing a huge quantity of heat and pressure. Pure Nitroglycerine is way too “sensitive” to be a useful explosive. It was the invention of dynamite by Alfred Nobel that converted nitroglycerine into a useful commercial and military explosive by mixing nitroglycerine with clay (diatomaceous earth) and forming the mixture into dynamite sticks.

Nitrogen triiodide NI3 also detonates by rearrangement to a collection of small stable gas molecules.

Secondary High Explosives Compounds which are not easily (accidentally) detonated but which can be detonated intentionally to cause very high energy explosions. Secondary explosives require a small amount of a primary explosive to set them off.

Examples of Secondary High Explosives

Predicting the Products of Organic High Explosive Reactions Carbon combines with oxygen to form CO to maximum extent possible. Hydrogen combines with any additional oxygen to form H2O to maximum extent possible. CO combines with any additional oxygen to form CO2. Nitrogen forms N2. Excess oxygen forms O2. Excess hydrogen forms H2.

Oxygen Balance: A useful concept for evaluating high explosives. Oavail - Oneeded %OB = --------------------------- (100) mass of comp.

Oxygen Balance of Some Representative High Explosives Explosive %OB TNT -74 RDX -43 Nitroglycerine +7.0 Ammonium Nitrate +20

Mixing Explosives to Achieve Optimum %OB Amatols = mixtures of ammonium nitrate and TNT ANFO = mixtures of ammonium nitrate and fuel oil

ANFO – A crude “low tech” high explosive that has been used by terrorists with devastating results. Sterling Hall Bombing Here at UW: “…. In the early morning hours of August 24, 1970, the New Years Gang loaded about 2,000 pounds of ammonium nitrate soaked in aviation fuel into a stolen Ford. The group parked the van below the Army Mathematics Research Center, in a driveway of Sterling Hall. At 3:42 A.M. the bomb exploded. It was powerful enough to knock out windows six blocks away, and police found pieces of the Ford van on top of an eight-story building nearby….” - www.sit.wisc.edu/~psohandbook

Organic Peroxides – A very different and less predictable class of potentially explosive compounds.

Acetone Peroxide Formed from acid catalyzed reaction of acetone with hydrogen peroxide. Formed as a mixture of dimer and trimer structures.

Acetone Peroxide Extremely dangerous and unpredictable in it’s detonation behavior. Has been used by terrorists. - easily prepared from common chemicals which are not regulated. - not detected by bomb-sniffing dogs.