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I'm wondering what is the meaning of tetrahydro when used as a prefix. For example, THC (tetrahydrocannabinol) and also THF folic acid (tetrahydro folate) or THF (tetrahydrofuran).

I couldn't recognize any detail in their structure that implies a "TETRA HYDRO" character and could not determine in which position in the molecule those "hydro"'s should appear.

Does "hydro" here means water? Or hydrogen? Or maybe a complex with four molecules of water?
Sorry for the lack of knowledge, I couldn't find any information on the web about this type of nomenclature....

THC and tetrahydro folic acid

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    $\begingroup$ Hydrogen. It means the hydrogenated version of an aromatic with 4 hydrogens added. Generally used where the hydrogenated version does not have a separate name though there is some crossover e.g. tetrahydronaphthalene/tetralin. To be completely systematic the name should specify which bonds are saturated. e.g. tetralin/1,2,3,4-tetrahydronaphthalene $\endgroup$
    – Waylander
    Aug 30, 2021 at 11:24
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    $\begingroup$ It is partly because many compounds were not originally named in an entirely systematic way. A very simple example is the popular solvent tetrahydrofuran. Named as a tetra-hydrogenated derivative of furan (a five membered ring, C4H4O). The simple names make sense if you know the history and survive because the full systematic names are often very unwieldy. $\endgroup$
    – matt_black
    Aug 30, 2021 at 17:55
  • $\begingroup$ Specifically in this case: the 'proper' name for tetrahydrofuran is 'oxolane', which is not really unwieldy per se, but definitely obscure. Other systematic names can be pretty clunky. $\endgroup$ Aug 30, 2021 at 21:54

2 Answers 2

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The chemical prefix "hydro" is a confusing one, because the Greek root "hydro" in English words usually refers to water, as in "hydrophilic" (hydro=water; philic=loving) for example. Another example relevant to this question is "hydrogen" (hydro=water and gen=making).

In chemical names, however, "hydro" is a shortened form of "hydrogen" that retains the meaning "hydrogen" rather than its root meaning of "water". Thus, it means that a hydrogen atom has been added to the molecule whose name follows the prefix. Greek root multipliers like "tetra" are used if more than one hydrogen atom has been added, so "tetrahydroX" means that four atoms of hydrogen have been added to X.

As a simple example, tetrahydrofuran (top) and its root compound furan (bottom) are shown below (images from wikicommons). The white balls represent hydrogen atoms, black are carbon and red is oxygen. Tetrahydrofuran

Furan

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According to the Gold Book and IUPAC 1988 Recommendations (Ref.1):

Hydron: The general name for the cation $\ce{H+}$; the species $\ce{H-}$ is the hydride anion and $\ce{H}$ is the hydro group. These are general names to be used without regard to the nuclear mass of the hydrogen entity, either for hydrogen in its natural abundance or where it is not desired to distinguish between the isotopes.

Thus, the following names comprise general names, to be used without regard to the nuclear mass of the hydrogen entity, either for hydrogen in its natural abundance or where it is not desired to distinguish between the isotopes:

  1. The atom, $\ce{H}$: hydrogen
  2. The cation, $\ce{H+}$: hydron
  3. The anion, $\ce{H-}$: hydride
  4. The group, $\ce{-H}$: hydro

The specific names pertaining to specific isotope of the forth category (hydro) are protio for $\ce{^1H}$, deuterio for $\ce{^2H}$, and tritio for $\ce{^3H}$ (Ref.1).

In IUPAC nomenclature, R-3.1.2 states that:

If the name of the parent hydride implies the presence of the maximum number of noncumulative double bonds (see R-2.4.1), other states of hydrogenation can usually be indicated by use of the prefix "hydro-" together with an appropriate numerical prefix signifying the addition of hydrogen atoms. This operation is regarded as the reduction of double bonds; thus, hydrogen atoms can only be added in pairs (by use of "dihydro-", "tetrahydro-", etc.). "Indicated hydrogen" (see R-1.3) is included if required by the parent hydride.

R-1.3 Indicated Hydrogen: Under certain circumstances it is necessary to indicate in the name of a ring, or ring system, containing the maximum number of noncumulative double bonds, one or more positions where no multiple bond is attached. This is done by specifying the presence of an "extra" hydrogen atom at such positions by citation of the appropriate numerical locant followed by an italicized capital $H$.

Some examples for these rules are given below:

IUPAC 1988 Recommendations

References:

  1. J. F. Bunnett and R. A. Y. Jones, "Names for hydrogen atoms, ions, and groups, and for reactions involving them (Recommendations 1988)," Pure & Appl. Chem. 1988, 60(7), 1115-1116 (DOI: https://doi.org/10.1351/pac198860071115).
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