Borderline chemical compound

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In chemistry, organic chemical compounds are classified as compounds that have a C-H bond or C-C bond, or both. However, this definition is not absolute, and many other organizations or groups have different classifications. IUPAC does not offer a definition of "inorganic" or "inorganic compound" but does define inorganic polymer as "...skeletal structure that does not include carbon atoms."[1] The Inorganic Crystal Structure Database (ICSD) defines as "inorganic" carbon compounds, compounds that may contain either C-H or C-C bonds, but not both.[2]

For historical reasons discussed below, a few types of carbon-containing compounds, such as carbides, carbonates, simple oxides of carbon (for example, CO and CO2), and cyanides are considered inorganic. Allotropes of carbon, such as diamond, graphite, fullerenes, and carbon nanotubes are also excluded because they are simple substances composed of only a single element and therefore allotropes are not generally considered to be true chemical compounds.

Specific cases

Guanidine

Like urea, guanidine lacks C-H or C-C bonds and only has C-NH bonds, but it's often classified as organic.

Hydrogen cyanide

The exact classification of HCN as organic or inorganic remains unresolved. Going by the C-H bond definition, HCN would be organic, but in aqueous solution it dissociates into H+ and CN-, and since the cyanide ion is a pseudohalogen, this makes it inorganic.

Metal carbonyls

Although carbon monoxide is universally classified as inorganic, metal carbonyls are often classified as organometallic compounds and are always referenced and researched in organic chemistry, but not in inorganic chemistry.

Mellitic acid

Like oxalic acid, this compound lacks C-H bonds, but it has an aromatic (benzene) ring made of C-C bonds, which holds 6 carboxyl groups.

Mellitic anhydride

Mellitic anhydride lacks any C-H bonds, but it has a benzene ring and 3 organic carboxylic anhydride bound to the aromatic ring. Hydrolysis of this compound gives mellitic acid, which is an organic compound, which, like oxalic acid, lacks any C-H bonds. Since this compound is often classified as oxocarbon or carbon oxide, mellitic anhydride is considered inorganic by most definitions.

Oxalic acid

Oxalic acid lacks C-H bonds, but it does have a C-C bond.

Perhalocarbons

Perhalocarbon compounds are hydrocarbons where the hydrogen atoms have been replaced by halogen atoms. Examples include carbon tetrachloride, perfluorocarbons, etc. Perhalocarbons have very different properties from their hydrocarbon parent.

Perhalocarboxylic acids, like trifluoroacetic acid are sometimes classified as inorganic, though more often they're classified as organic.

Urea

Urea lacks any C-H or C-C bonds, however, it does have C-NH2 bonds, which are omnipresent in many organic compounds.

References

  1. https://goldbook.iupac.org/html/I/IT07515.html
  2. https://web.archive.org/web/20170830061100/https://icsd.fiz-karlsruhe.de/search/resources/content/sci_man_ICSD_v1.pdf

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