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I was just turning the pages of my chemistry dictionary, when I found the structure of TNT (Trinitrotoluene):

Wiki/TNT

...there after I got interested in its symmetry as an explosive, and started looking for structures of other explosive compounds and found that almost all of them have very symmetrical structures.... Here are some that I found:

  1. RDX

    RDX

  2. HMX: High Molecular Weight RDX (cyclotetramethylene-tetranitramine)

    HMX

  3. PETN [Pentaerythritol tetranitrate (PETN)]

    PETN

  4. Heptanitrocubane

    heptan.cubane

  5. HHTDD (hexanitrohexaazatricyclododecanedione)

HHTD

  1. TATB, triaminotrinitrobenzene or 2,4,6-triamino-1,3,5- trinitrobenzene

TATB

There are many others which can be found here: wikipedia/chemical-explosives

I wonder why they generally tend to be so symmetrical (as compared to any other random non-explosive molecule)?

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    $\begingroup$ Most of the small molecules are symmetrical. $\endgroup$ – Ivan Neretin May 14 '19 at 16:09
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    $\begingroup$ What those compounds have in common is a lot of $\ce{NO2}$ groups. They are trying to repeat the same structural element as often (and densely) as possible. So there is a lot of repetition which kind of implies high symmetry. $\endgroup$ – Feodoran May 14 '19 at 16:24
  • $\begingroup$ I did up voted the answer by @Poutnik but it can also be that there is no a straight answer to your Q, at least as formulated. Do not identify symmetry in itself as a source of instability. I up voted the answer because it goes point by point indeed. $\endgroup$ – Alchimista May 15 '19 at 8:37
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    $\begingroup$ Along with symmetry, steric hindrance is also a major similarity to all of the above mentioned molecules. It seems the compounds tend to be highly reactive in order to relieve their steric strains. $\endgroup$ – glucose May 15 '19 at 17:35
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Before discussing symmetrical nature of explosives, we might ask, "Why do they contain so many nitro groups?"

The answer is in estimation of energetic properties that relies on the empirically derived Kamlet and Jacobs equations 1 .

In these equations the heat released by the decomposition, the number of moles of gas produced, and the molecular weight of these gases are all critical factors. Density too is crucial. The more molecules of a high-energy material that can be packed into the limited volume of a shell or rocket the better.

E.g.: Nitrocubanes carrying five or more nitro groups contain enough oxygen to oxidize all constituent carbon and hydrogen atoms to gaseous CO, CO2, or H2O. Each of these, along with N2, "explodes" from the solid to 12 gaseous molecules. The expansion from the dense solid to a lot of gas (much expanded by the released heat) produces the desired effect in propellants and explosives.

If one examines the synthetic route, then

  • RDX is synthesized with treating Hexamethylenetetramine or methenamine, also known as hexamine or urotropin with fuming nitric acid. Urotropin is a SYMMETRIC molecule synthesized from formaldehyde and ammonia.2

enter image description here

  • PETN (Pentaerythritol tetranitrate) production is by the reaction of pentaerythritol with concentrated nitric acid. Pentaerythritol is prepared via a base-catalyzed poly-addition reaction (aldol) between acetaldehyde and 3 equivalents of formaldehyde, followed by a Cannizzaro reaction with a fourth equivalent of formaldehyde to give the final product.3

    A SYMMETRICAL precursor pentaerythritol is used. enter image description here

  • HMX is obtained by a two-stage nitrolysis of hexamethylenetetramine (HMTA) in acetic acid, using the solution of ammonium nitrate in nitric acid. A SYMMETRICAL precursor is used.4

enter image description here

  • Heptanitrocubane is obtained from nitration of TNC. TNC is synthesized from cubane, which is SYMMETRIC.5

enter image description here

  • HHTDD (hexanitrohexaazatricyclododecanedione) is essentially an open analogue of the cyclic nitroamine cage compounds such as CL-20.[6]

enter image description here

Summarizing

Synthetic routes used in production lead to symmetrical compounds in which more molecules of a high-energy material can be packed into the limited volume of a shell or rocket leading to greater density. For a given explosive, the detonation pressure is proportional to the square of its density.

References

1 http://www.ch.ic.ac.uk/local/projects/wang/discussion1.html

2 https://www.organic-chemistry.org/namedreactions/delepine-reaction.shtm

3 https://en.wikipedia.org/wiki/Pentaerythritol

4 http://www.chemikinternational.com/pdf/2012/01_2012/CHEMIK_2012_p58-63.pdf

5 http://www.ch.ic.ac.uk/local/projects/wang/nitro5.html

[6] https://en.wikipedia.org/wiki/HHTDD

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The main reason to synthesize symmetric molecules are related to their synthesis:

  • Some symmetry is due to the nature of the molecular structure itself, like RDX and HMX. This is related to tricyclic structure of hexamethylenetetramine. Similarly, for Tri/tetramers of peroxyacetone. For these structures, they are created rather spontaneously (HMX with some help) than by directed multistep synthesis.

  • Some symmetry is due to reaction thermodynamics and kinetics, that does not prefer asymmetrical structures. E.g. TNT is produced easily by direct toluene nitration, as methyl and nitro groups direct coming groups to 2,4,6 positions, forming the symmetric molecule. An asymmetric 2,3,4-TNT is not created this way, its synthesis would be much more complicated, expensive and product may not be safe to produce.

    • Many asymmetric molecules are not stable enough to be synthesized, or are not chemically stable enough to be kept.

For the keeping stability, Picric acid and its salts are famous for their disintegration products making the explosive oversensitive.

  • Some symmetry is due low control over the reaction quotient, combined with the advantage of reaction undergoing all steps. See nitroglycerine being trinitrate ester, PETN being tetranitrate ester.

If they are symmetrical, their instability is evenly distributed over the molecule, so they are relatively stable, compared to eventual asymmetric variant, considering their explosive potential. That allows their synthesis, safe and cheap production, safe manipulation, safe aging.

If they were asymmetrical, their instability would be localized and concentrated to some molecule locations and the molecule would be much less stable.

Additionally, at big asymmetry of bigger molecules, there is question, why to keep the "non-explosive molecule part"?

Consider 2,3,4,5,6-pentanitrodiphenyl versus 2,3,4,5,6-pentanitrotoluene.

Another example of an asymmetrical unstable explosive is the renowned $\ce{NI3 . NH3}$

Summary

I would summarize the answer as kinetic and thermodynamic instability, together with synthesis complexity, discriminates existence and production of asymmetric explosives, comparing to much broader group of non-explosive compounds.

Additionally, the symmetry origin is often coined in symmetry of molecules chosen to be converted to explosive, or to be the structural basis for explosive.

As their symmetry makes promise to form an efficient explosive. E.g. pentaerythritol is perfectly symmetrical alkohol with 4 OH groups to be nitrated.

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    $\begingroup$ Hello Poutnik, I have a doubt. Had these symmetrical structures been adding to the stability of the molecule, then that would rather mean that the molecule being more stable would prefer not to react! ...and if that happens, it would no longer remain an explosive (explosives are supposed to react violently, and should be expected to be slightly unstable). That doesn't seem to be a valid reason. Thanks anyways for your answer. $\endgroup$ – Aakash Singh Bais May 15 '19 at 5:01
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    $\begingroup$ I have meant relatively more stable, with high activation energy, but at the same time high Gibbs energy. That means not stable in thermodynamical sense. There are 2 independent properties of explosives. Their stability against accidental explosion and their brisance when exploding. Professional explosives must be stable enough, otherwise no professional would work with them. There is an explosive I forgot its name, used for nuclear charge detonators, that is so stable it is on the edge to be called explosive at all. Additional reason for symmetrical molecules is, they are easier to synthesise $\endgroup$ – Poutnik May 15 '19 at 5:08
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    $\begingroup$ Yes, I do partially agree with you, but I was expecting a more detailed answer that could clearly illustrate why it is so, and how exactly does symmetry help, and how well does the instability balance against the stability you proposed....maybe a bit of a rigorous scientific explanation to an interesting question! Didn't mean to hurt. Thanks. $\endgroup$ – Aakash Singh Bais May 15 '19 at 5:17
  • $\begingroup$ P.S.: as reaction tvermodynzmics and kinetics usually prefer the energy rich functional groups to be evenly distributed. $\endgroup$ – Poutnik May 15 '19 at 5:17
  • $\begingroup$ You know, the question is rather broad, deserving a broad answer. Particular structures and their symmetric and asymmetric variants have to be evaluated by the case basis. Also, experimental behaviour is not easily determined theoretically. So, let wait on an explosive expert. Unfortunately, I do not know anybody at Synthesis Semtín factory near Pardubice, where Semtex was invented. $\endgroup$ – Poutnik May 15 '19 at 5:22
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SUMMARIZING

Symmetrical molecules pack most efficiently, as a result the density of a symmetrical substance is higher as compared to an asymmetrical counterpart. More over, the detonation pressure of an explosive is directly proportional to the square of its density. As a result most of the commonly used high-performance explosives have symmetrical molecular structures.

Side-observation

As Nitrogen acts as a good oxidizer helping other elements in the compound oxidize into the corresponding gaseous oxides instantaneously, most compounds combine their symmetry with large number of Nitro (NO2) groups.

Hence, explosives generally have symmetrical structures and a large number of Nitro groups.

Thanks to Chakravarty Kalyan and Poutnik for their efforts.

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