Acetone Peroxide - Chemistry

Chemistry

"Acetone peroxide" most commonly refers to the cyclic trimer TCAP (tri-cyclic acetone peroxide, or tri-cyclo, C9H18O6) obtained by a reaction between hydrogen peroxide and acetone in an acid-catalyzed nucleophilic addition. The dimer (C6H12O4) and open monomer are also formed, but under proper conditions the cyclic trimer is the primary product. A tetrameric form was also described. In mildly acidic or neutral conditions, the reaction is much slower and produces more monomeric organic peroxide than the reaction with a strong acid catalyst. Due to significant strain of the chemical bonds in the dimer and especially the monomer, they are even more unstable than the trimer.

At room temperature, the trimeric form slowly sublimes, reforming as larger crystals of the same peroxide.

Acetone peroxide is notable as one of the few high explosives not containing nitrogen. This is one reason it has become popular with terrorists, as it can pass through scanners designed to detect nitrogenous explosives.

TCAP generally burns when ignited, unconfined, in quantities less than about 4 grams. More than 4 grams will usually detonate when ignited; smaller quantities might detonate when even slightly confined. Completely dry TCAP is much more prone to detonation than the fresh product still wetted with water or acetone. The oxidation that occurs when burning is:

2 C9H18O6 + 21 O2 → 18 H2O + 18 CO2

Theoretical examination of the explosive decomposition of TCAP, in contrast, predicts "formation of acetone and ozone as the main decomposition products and not the intuitively expected oxidation products." This result is in good agreement with the results of 60 years of the study of controlled decompositions in various organic peroxides. It is the rapid creation of gas from a solid that creates the explosion. Very little heat is created by the explosive decomposition of TCAP. Recent research describes TCAP decomposition as an entropic explosion.

The high sensitivity to shock, heat and friction are due to the instability of the molecule. Big crystals, found in older mixtures, are more dangerous, as they are easier to shatter—and initiate—than small ones.

Due to the low cost and ease with which the precursors can be obtained, acetone peroxide can be manufactured by those without the resources needed to manufacture or buy more sophisticated explosives. When the reaction is carried out without proper equipment the risk of an accident is significant. Simply mixing sulfuric acid, hydrogen peroxide, and acetone can create the substance. Crystals of AP soon precipitate out.

There is a common myth that the only "safe" acetone peroxide is the trimer, made at low temperatures:

The mixture must be kept below 10 degrees Celsius. If the crystals form at this temperature, it forms the isomer called tricycloacetone peroxide, which is relatively stable and safe to handle. If the crystals form above this temperature, the dimeric form, called dicycloacetone peroxide. This isomer is much more unstable, and could go off at the touch, making it not safe enough to be considered a practical explosive. As long as the temperature is kept below 10 degrees Celsius, then there is little to worry about.

The trimer is the more stable form, but is not much more so than the dimer. All forms of acetone peroxide are sensitive to initiation. Organic peroxides are sensitive, dangerous explosives; due to their sensitivity they are rarely used by well funded militaries. Even for those who synthesize explosives as a hobby there are far safer explosives with syntheses nearly as simple as that of acetone peroxide.

Acetone peroxide is commonly combined with nitrocellulose by dissolving the nitrocellulose in acetone and then mixing in the acetone peroxide and letting it dry, which results in a mixture that is both more stable and somewhat more powerful than acetone peroxide by itself. This mixture is commonly referred to as APNC.

Tetrameric acetone peroxide is more chemically stable (heating to 120 °C for 4 hours), although it is still a very dangerous primary explosive. It can be prepared using tin(IV) chloride (without acid present) as a catalyst with up to 40% yield if a radical inhibitor such as hydroquinone, or a chelator such as EDTA is added.

Acetone peroxide evaporates 6.5% in 24 hours at 14–18 °C. In open air at 25 °C it has a loss by sublimation of 68.6% in 14 days. Many accidents have resulted from the fact that acetone peroxide detonates due to sublimation. Keeping it wet stops the sublimation and can prevent this type of accident.

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