What molecule am I?
exo-Tetrahydrodicyclopentadiene (exo-THDC; aka exo-tricyclo[5.2.1.02,6]decane or simply exo-tricyclodecane) is a tricyclic saturated hydrocarbon with a prominent place in the jet fuel industry, where it is known as JP-10.
exo-THDC appeared in the chemical literature in 1957, when Paul von R. Schleyer at Princeton University (NJ) isolated it as an intermediate in the synthesis of adamantane1 from dicyclopentadiene2. Three years later, Schleyer, along with Malcolm M. Donaldson, discussed exo-THDC, along with its endo isomer and adamantane, in an article on the relative stability of bridged hydrocarbons.
Now about Jet fuel: In 1968 US Patent 3,381,046 to Esso Research and Engineering Co. (Linden, NJ; now part of ExxonMobil), inventors Charles A. Cohen and Clifford W. Muessig described the preparation of exo-THDC and isomers of its derivative exo-tetrahydrodimethyldicyclopentadiene3 from their respective diene precursors by hydrogenating the diene mixture, then treating the saturated hydrocarbons with 99–100% sulfuric acid at 85–95 °C for 26 h. The purified product was described as “an improved fuel which may be used in jet and rocket engines”, but no fuel performance data were claimed.4
In subsequent years, many oil companies were awarded patents for the use of exo-THDC in “missile fuels”, “high energy fuels”, “high density turbine fuels”, “ramjet fuels”, and the like. In the 2020s, two articles from Zhejiang University and Zhejiang Gongshang University (both in Hangzhou, China) described methods for accelerating the endothermic cracking of hydrocarbon fuels, including JP-10. Faster cracking is desirable for cooling the engines in hypersonic aircraft.
In 2021, Hujun Xie, Wenjun Fang, and colleagues introduced the use of macromolecular initiators, specifically, a hyperbranched polyester (HPE), to generate radicals and increase the rate of endothermic cracking. The polyester was composed of 1,1,1-tris(hydroxymethyl)propane5 and 2,2-bis(hydroxymethyl)propionic acid6 units. The authors used palmitic acid7 to modify its end hydroxyl groups.
In 2023, Yongsheng Guo, Yitong Dai, and co-workers expanded on the advantages of HPE. They studied the evaporation and autoignition characteristics of JP-10 droplets with or without HPE. Their results showed that the puffing and micro-explosion phenomena of HPE-blended JP-10 droplets accelerate fuel evaporation and autoignition.
exo-Tetrahydrodicyclopentadiene
hazard information*
| Hazard class** | GHS code and hazard statement | |
|---|---|---|
| Flammable liquids, category 3 | H226—Flammable liquid and vapor |  |
| Aspiration hazard, category 1 | H304—May be fatal if swallowed and enters airways |  |
| Skin corrosion/irritation, category 2 | H315—Causes skin irritation |  |
| Skin sensitization, category 1 | H317—May cause an allergic skin reaction | |
| Serious eye damage/eye irritation, category 2A | H319—Causes serious eye irritation | |
| Acute toxicity, inhalation, category 3 | H331— Toxic if inhaled |  |
| Short-term (acute) aquatic hazard, category 2 | H400—Very toxic to aquatic life | |
| Long-term (chronic) aquatic hazard, category 2 | H410—Very toxic to aquatic life with long-lasting effects | |
*Compilation of multiple safety data sheets.
**Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.
Molecule in the News
For years, chemists have attempted to make allotropes of nitrogen other than the ubiquitous one, N2. Dozens of Nx molecules have appeared in the literature, but all have been too unstable to isolate. Included in these are four linear allotropes of N6, or hexanitrogen (see table).
| Sample N6 literature reports | |||
| Year | Electronic structure | CAS Reg. No. | Comments |
| 1982 | N–=N+=N–N=N+=N– | 81373-20-8 | Same as newly reported N6, but not isolated. |
| 2000 | N•=N=N–N=N=N• | 265992-66-3 | Theoretical calculations only |
| 2001 | N≡N+–N=N–N+≡N | 342587-30-8 | Dication; theoretical only |
| 2016 | N–=N+=N–N=N+=N– | 924705-91-9 | Prediction of newly reported N6 |
Earlier this month, Weiyu Qian, Artur Mardyukov*, and Peter R. Schreiner* at Justus Liebig University Giessen (Germany) reported a stable form of linear hexanitrogen, albeit only stable at liquid nitrogen temperature (77 K). The chemists used a room-temperature gas-phase reaction of molecular chlorine or bromine with silver azide1 (AgN3) to produce N6, which was trapped in an argon matrix at 10 K. They also made films of neat N6 at 77 K.
The authors used IR and UV-vis spectroscopy and 15N-labelling, along with theoretical calculations to confirm their findings. They also determined that the electronic structure of the somewhat bent molecule is N–=N+=N–N=N+=N–, or a dimer of two azido radicals. Upon heating, the molecule decomposes explosively to ordinary N2.
The authors believe that the advent of metastable hexanitrogen could lead the way for future opportunities in energy storage.
1. CAS Reg. No. 13863-88-2.
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exo-Tetrahydrodicyclo-
pentadiene fast facts
| CAS Reg. No. | 2825-82-3 |
| SciFinder name | 4,7-Methano-1H-indene, octahydro-, (3aR,4S,7R,7aS)-rel- |
| Empirical formula | C10H16 |
| Molar mass | 136.23 g/mol |
| Appearance | Colorless liquid |
| Boiling point | 187 ºC |
| Water solubility | Immiscible |
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