As described in detail below, any definition of organic compound that uses simple, broadly applicable criteria turns out to be unsatisfactory, to varying degrees. The modern, commonly accepted definition of organic compound essentially amounts to any carbon containing compound, excluding several classes of substances traditionally considered as 'inorganic'. However, the list of substances so excluded varies from author to author. Still, it is generally agreed upon that there are (at least) a few carbon containing compounds that should not be considered organic. For instance, almost all authorities would require the exclusion of
alloys that contain carbon, including
steel (which contains
cementite, Fe3C), as well as other metal and semimetal carbides (including "ionic" carbides, e.g, Al4C3 and
CaC2 and "covalent" carbides, e.g. B4C and SiC, and graphite intercalation compounds, e.g.
KC8). Other compounds and materials that are considered 'inorganic' by most authorities include: metal
carbonates, simple
oxides (CO, CO2, and arguably, C3O2), the
allotropes of carbon,
cyanides excluding those containing an organic residue (e.g., KCN, (CN)2, BrCN, CNO−, etc.), and heavier analogs thereof (e.g., CP− '
cyaphide anion', CSe, COS; although CS2 '
carbon disulfide' is often classed as an
organic solvent). Halides of carbon without hydrogen (e.g., CF4 and CClF3),
phosgene (COCl2),
carboranes,
metal carbonyls (e.g., nickel carbonyl),
mellitic anhydride (C12O9), and other exotic
oxocarbons are also considered inorganic by some authorities.
A slightly broader definition of organic compound includes all compounds bearing C-H or C-C bonds. This would still exclude urea. Moreover, this definition still leads to somewhat arbitrary divisions in sets of carbon-halogen compounds. For example,
CF4 and
CCl4 would be considered by this rule to be "inorganic", whereas
CF3H and
CHCl3 would be organic, though these compounds share many physical and chemical properties.