Hypothetical chemical compound
A hypothetical chemical compound is a compound that scientists have imagined but have not yet synthesized, observed, or isolated. Some impossible-to-make cases happen because of extreme crowding of atoms (steric effects) or strained bonds. Examples of the former include tetra-tert-butylmethane, ClF7, BrF7, and tetrahedrane. Others would be highly unstable and fall apart or rearrange, or exist only briefly as reactive steps or in vacuum. Some have no known way to make them, such as hypercubane. Some compounds of radioactive elements can’t be made because they decay too quickly (for example FrOH, RnF6, AtF7, PoF2). There are also “parent” compounds that can’t be isolated even though related stable structures with substituents exist (like borole).
Hypothetical compounds are often predicted from known chemistry, such as certain salts formed from a nonstable parent acid. Some imagined acids have stable salts, called phantom acids. They are useful in thought experiments. In a few cases, compounds once thought hypothetical have been made. For example, potassium trichromate has been produced on a small scale and is a strong oxidizer. Other salts like sodium trichromate and some tetrachromates have been hypothesized but not yet synthesized.
Some notable milestones include the generation of pentazole (N5H) in 2003. Other candidates once thought possible are now considered unlikely to be produced, such as polywater, oxygen tetrafluoride (OF4), chromium hexafluoride (CrF6), and osmium octafluoride (OsF8). Ethylene dione was proposed in 1913 and was said to be seen in 2015, but later the observed spectrum was explained as the oxyallyl diradical instead of a simple molecule.
Evidence for some hypothetical compounds is limited or weak. For example, tiny amounts of cyclotrioxidane (O3) might exist on surfaces, but not in bulk. Other possibilities people have discussed include xenon octafluoride (XeF8), hexazine (N6), octaazacubane (N8), nitrogen pentafluoride (NF5), and the tetrafluoroammonium fluoride [NF4]+F−. Even if a compound has not been isolated, its chemistry can still be studied or predicted.
Stability can be explored with calculations. Methods like the Born–Haber cycle help estimate a compound’s heat of formation to judge stability. But a negative formation enthalpy does not guarantee that the compound exists. For example, the calculation might suggest NaCl is stable but NeCl is not. Likewise, XePtF6 was predicted based on the stability of a related compound, O2PtF6.
This page was last edited on 2 February 2026, at 07:18 (CET).