It07:HMX

It07:HMX
General
Systematic name	1,3,5,7-tetranitro-1,3,5,7-tetrazocane
Other names	Cyclotetramethylene-tetranitramine Octogen, HMX
Molecular formula	C4H8N8O8
SMILES	C1N(CN(CN(CN1[N+](=O)[O-])[N+] (=O)[O-])[N+](=O)[O-])[N+](=O)[O-]
Molar mass	296.20 g/mol
Appearance	colourless crystalline solid
CAS number	2691-41-0
Properties
Density & phase	1.91 g/cm³
Solubility in water	0.0033 g/L
Melting point	277-278°C
Decomposes at:	280°C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Overview

HMX, a white crystalline solid, is an explosive nitroamine which structurally, is very similar to RDX, with the main difference being it is an 8 membered ring with 4 nitrogen atoms and 4 nitro groups. The additional bonds gives it more explosive power than RDX and is in fact the highest energy explosive under large scale production. The main disadvantage is it requires slightly more specific reaction conditions to produce and hence more expensive than TNT and RDX based explosives, but it is useful when maximum destructive power is required.

Here we can see the pure destructive power of high performance explosive compounds such as HMX as it demolishes this building with ease. Image from http://news.bbc.co.uk/1/hi/world/middle_east/4231916.stm

Name

The name HMX has more than one interpretation, including High-velocity Military eXplosive, Her Majesty's eXplosive and commonly High Melting eXplosive. It is believed that the correct name is simply High Molecular weight rdX seeing as the structure is staggeringly similar to RDX and is essentially a slightly larger version of the same structure.

History

HMX was an accidental discovery by W.E. Bachmann in 1940 as a byproduct in the Bachmann process which was primarily the synthesis of RDX. Bachmann realised that this byproduct was also a high explosive and it took him 3 years to determine the actual molecular structure. Once the structure was identified as what is now known as HMX, Bachmann designed a modified version of his existing synthesis of RDX, which allowed it to undergo mass production and be put to use in military explosives.

Synthesis

Since HMX is produced in a modified version of the Bachmann process, the same reagents are used as for the synthesis of RDX. Only the conditions are changed slightly.

The ratios of reagents are slightly different to fine tune the yield, but the main difference uses the fact that HMX is highly insoluble in water compared to RDX. After the reagents are mixed, hot water is added and the mixture is refluxed, before finally recrystallising by cooling to 20^OC with ice. The result is a 55-60% yield of HMX with RDX as a small impurity. Detailed instructions can be found here:

http://web1.caryacademy.org/chemistry/rushin/StudentProjects/CompoundWebSites/2003/hmx/Structure.htm

Usage

The main use of HMX is in the military as a maximum performance explosive. It is sometimes used pure and produces a shockwave that travels at 9,100 metres per second, although usually mixed with another explosive e.g. TNT. The idea here is that the secondary compound has a lower explosive velocity, so the HMX shockwave hits first and inflicts damage, and the shockwave of the secondary compound hits next and finishes off the target. The most common HMX mixture is Octol (See below).

In addition to explosive charges, HMX is used as a propellant in rockets and missiles, detonators for other explosives, det cord (detonator fuse), a fissioning material in nuclear warheads and in explosive nuts and bolts which allow bombs to be released from the wings of an aeroplane.

HMX has very limited civilian usage. It is sometimes used in building demolition, but from a civilian point of view, there are cheaper explosives that do the same job just as well.

Octol

This is a selection of explosive compositions. Octol is the mixed purple/white compound on the top-right. Image from http://www.army-technology.com/contractors/explosives/dyno/dyno2.html

Octol is a high explosive mixture of either:

70% HMX and 30% TNT

or

75% HMX and 25% TNT.

HMX has an explosive velocity of 9,100 metres per second, while TNT explodes at 6,900 metres per second (over 2000 m/s difference), so Octol produces two distinct shockwaves in the manner described above, which increases the destructive power.

Another HMX mixture that is sometimes used is Okfol, which is a mixture of 95% HMX and 5% wax, but this slightly reduces the shockwave's velocity to 8670 metres per second.

Mass Spectrum

Below is the observed mass spectrum for HMX.

UV Spectrum

The UV spectrum of HMX is shown below and gives a maximum at about 225 nm and the intensity drops significantly up to 290 nm. This makes sense as it is a colourless solid and thus exhibits very low absorption in the visible region.

¹H NMR Spectrum

Like RDX, HMX gives a straightforward ¹H-NMR spectrum with only one singlet. There are 4 methylenes, with 2 hydrogens on each. All the hydrogen atoms are both chemically and magnetically equivalent to each other so they do not couple with the hydrogens on the other methylenes. Hence, we only observe one singlet.

The predicted ¹H-NMR spectrum using prediction software is shown below.

The software predicts a chemical shift of 3.81ppm for the singlet and this is in good agreement with the experimentally observed chemical shift of 3.93ppm, which is recorded in DMSO solvent.