Nitrogen Triiodide Synthesis

Overview

Nitrogen Triiodide (NI3) is a brown-purple solid at room temperature that explodes when lightly perturbed. Nitrogen Triiodide is extremely sensitive to touch, shock, and even ultraviolet light, and generates large amounts of nitrogen gas (and solid iodine) upon explosion. It is not an industrial explosive (for purposes like mining or demolition) due to it’s extreme sensitivity. Transporting even grams of the substance when dry is nearly impossible.

Safety

Nitrogen triiodide is dangerous and should not be made for purposes other than demonstration. Making large amounts of the substance is unwise, and it is not stable for more than a few hours. Small quantities of Nitrogen Triiodide are safe to handle when wet, but are also not chemically stable for more than a few hours in this state.

The easiest way to dispose of Nitrogen Triiodide is simply to detonate it. If it is in an environment where detonation is not appropriate (e.g. a glass container) it is best to gently wet the explosive, taking care to to forcefully introduce the water, as this might cause detonation. Nitrogen Triiodide is soluble in large quantities of water at basic pH, and is generally safe to dispose into the sewer with a large excess of running water. However, one should check that iodine and ammonia are safe to dispose of in dilute quantities in their area.

Concentrated ammonia is dangerous, and should only be used in a fume hood. Wearing gloves, full googles, and hearing protection is important at all times during this procedure.

Materials

Beaker (Plastic Preferred)
Iodine (Solid)
Ammonia (Anhydrous or very concentrated. I have had success with concentrations in excess of 12 molar.)
Stir-Rod
Mortar and Pestle (Optional)
Filter Paper

Synthesis of Nitrogen Triiodide

Safer Route

Measure 3g of sublimated iodine crystals and 15ml of concentrated ammonia.
Transfer the crystalline iodine to a 50ml plastic container and add the ammonia. Stir for 30 seconds without attempting to break-up the crystals.
Allow the mixture to stand for 10 minutes. The supernatant will gradually turn a dark green-brown color. The nitrogen triiodide will form on the surface of the iodine crystals, so you will not be left with the pure explosive.
After 10 minutes, decant the ammonia solution to leave the crystalline iodine/triiodide crystals. Spread out these crystals onto filter paper and allow to dry.
Tapping the crystals with a meter-stick should afford some small pops and puffs of vaporized iodine.

Riskier Synthesis: Requires More Caution

Measure 3g of sublimated iodine crystals. Transfer these to a mortar and pestle, and grind until the consistency reaches a fine powder (roughly 5 minutes).
Add the iodine powder to a 50ml plastic container and add 20ml of concentrated ammonia solution. The iodine should still be mostly insoluble in the mixture, but will still turn the dark green-brown color.
Allow this mixture to stand for 5 minutes.
Decant the ammonia solution, and spread the brown solid onto filter paper. This is mostly nitrogen triiodide, and will become dangerous immediately once the drying process starts.
Once dry (approximately 10 minutes), tapping the filter paper with a meter stick will cause detonation.

Crystalline iodine vs powdered iodine. — Crystalline iodine (left) and powdered iodine (right) is the main difference between the procedures. The powder has a higher surface area to volume ration, and thus will convert more thoroughly to the explosive nitrogen triiodide.

Reaction

Synthesis Mechanism of Nitrogen Triiodide

The generally assumed reaction is:

Iodine and Ammonia Reaction to form the explosive — The formation reaction of nitrogen triiodide. Note that this is not stable in aqueous solution, and decomposes into several side products such as hydroiodic acid, hypoiodous acid, and ammonium iodide.

However, this product is not stable in aqueous solution, and the following reaction predominates.

Iodine and Ammonia React to Form the Nitrogen Triiodide Ammonia Adduct — Formation Reaction of the Nitrogen Triiodide Ammonia Adduct

This reaction forms an ammonia adduct of the triiodide, which then polymerizes in aqueous solution to form a less sterically strained molecule. This polymer is more stable and will decompose less into side products such as hydroiodic acid and ammonium iodide, but using concentrated ammonia limits the amount of degradation. This polymer is still strained enough to retain its explosive properties.

polymeric structure of nitrogen triiodide — Polymeric Structure of Nitrogen Triiodide

Decomposition of Nitrogen Triiodide

Nitrogen triiodide decomposes according to the following reaction:

Decomposition reaction of Triiodide into nitrogen gas, ammonium iodide, and iodine. — This reaction has a large positive change in entropy (5 moles of gas are generated) and large negative change in enthalpy. This reaction is very spontaneous, and is partially responsible for the sensitivity of the compound.

Structure and Properties of Nitrogen Triiodide

Nitrogen triiodide is explosive due to the large steric strain of the iodine molecule around the nitrogen. Iodine molecules are very large, and repel each other, meaning that they are not happy sharing the tight space around the nitrogen molecule.

Triiodide Ammonia Adduct Dimer shown with spacefill model to rationalize steric strain. — Steric Strain of Large Iodine Molecules Around the Central Nitrogen Atoms in a NI3-NH3 Dimer. Crystallographic file sourced from COD.

Examining bond angles also helps us rationalize the steric strain. Because the iodine molecules repel each other, they want to spread out as much as possible. VSEPR theory tells us that the expected bond angle at the nitrogen atom should be 109.5 degrees. However, this strain causes the angle to be 111.25 degrees.

Bond Angles of Non-Coordinating Iodine Molecules Around the Central Nitrogen

Additionally, this explosive strain is caused by the crowding of the nitrogen atoms. The bond length of the central nitrogen to the iodine molecule is significantly shorter (2.3 Angstrom) that the unstrained bond length of the iodine and the ammonia nitrogen (2.5 Angstrom). This crowding makes the iodine molecules want to pop-off even more, making the explosive reaction occur.