What do “ortho-“, “meta-” and “para-” mean in organic chemistry?

The prefixes ortho-, meta-, and para- represent an ancient nomenclature system that represented the three possible substitution patterns in disubstituted monocyclic aromatic compounds derived from benzene. Regardless of which of the substituted ring carbons is taken as parent carbon, disubstitution can only give rise to three distinct positional isomers with substituents at positions 2, 3, or 4 relative to the first carbon. Each of these isomers is called an ortho, meta, and para isomer, respectively.

ortho substitution pattern

The prefix ortho- is used to represent the pattern of disubstitution in which both substituents are adjacent to each other. That is, ortho- represents the 1,2 substitution pattern of the disubstituted aromatic ring, as shown below:

Ortho substitution can also be represented by the symbol o- which substitutes for the numbers 1,2- used as prefixes to indicate the positions of the substituents. The latter correspond to the systematic numbering recommended by the International Union of Pure and Applied Chemistry (IUPAC).

Examples of ortho-substituted compounds

The image below shows three examples of disubstituted benzenes that exhibit either ortho or 1,2 substitution. As can be seen in all cases, both substituents are on adjacent carbons. The first two examples include the common names by which these aromatic derivatives are known and in all cases the names are shown using both the ortho prefix (or its symbol o-) and the numbering recommended by IUPAC.

meta- substitution pattern

The prefix meta- indicates a 1,3 substitution in which the substituent groups are separated by one carbon, as shown below:

As in the case of ortho substitution, meta substitution can also be represented by the letter m-, as well as by the numbering 1,3 recommended by the IUPAC.

Examples of meta-substituted compounds

The image below shows the meta – isomers of the same compounds shown above. In this case, the 1,3 substitution pattern can be observed in which the ring substituents are separated by one carbon. Again, common and systematic names are shown using both the prefix meta – (or its symbol m-) and the IUPAC recommended numbering.

Substitution pattern for –

The prefix para – indicates a 1,4 substitution pattern in which the substituents are in opposite positions on the ring, as shown below:

As before, the substitution for can also be represented by the letter p-, as well as by the numbering 1,4 recommended by the IUPAC.

Examples of para-substituted compounds

The following image shows the para – isomers of the same compounds shown in the two previous examples. In this case, the 1,4 substitution pattern can be observed with the substituents at opposite positions on the ring. Again, common and systematic names are shown using both the prefix for – (or its symbol p-) and the IUPAC recommended 1,4 numbering.

Additional uses of the prefixes ortho- , meta- and para-

As mentioned at the beginning of the article, this prefix system is mainly used in the nomenclature of disubstituted aromatic compounds. However, being a relative positioning system, it is also often used to indicate the position of one or two carbons of the benzene ring with respect to a substituent of interest.

In these cases, the prefixes ortho- , meta- and para- do not necessarily indicate the absolute 1,2, 1,3 and 1,4 positions of the ring, but rather both 2, 3 positions and the unique 4 position with respect to a particular substituent or carbon on the ring, as if that carbon were carbon 1 (although it may not be).

In this situation, it is irrelevant whether the compound has two substituents or not, since the ortho, meta, and para positions are only used to identify relative positions on the ring and not necessarily the actual position of substituents.

Examples of additional applications of ortho- , meta- and para-

Activating groups and ortho-para directors: in a benzene ring, the presence of certain functional groups such as the hydroxyl group and the amino group, causes the carbons that are adjacent and in the opposite position of the ring with respect to said groups to be More reagents for electrophilic aromatic substitution reactions.

This means that if an aromatic compound that has an –OH group undergoes electrophilic substitution, it is more likely to occur on the carbons that are in the ortho position (the two adjacent carbons) and on the carbon that is in the ortho position. for with respect to the –OH group than on the two meta carbons. So, these types of substituents are called activators (because they make the reaction faster) and ortho-para directors, because the reaction is directed mainly at those carbons.

In compounds with more than two substituents: If we have a ring with three or more substituents, we can use the prefixes ortho- , meta- and para- to indicate the relative positions of some with respect to others, regardless of the fact that the compound cannot be named as an ortho, meta, or para isomer (since, in nomenclature, these prefixes only apply to disubstituted rings). For example, in 2,4,6-trinitrotoluene, whose structure is shown below, we can say that all nitro groups (–NO ₂) are in meta position with respect to each other. We can also say that two of them (those in positions 2 and 6) are in the ortho position with respect to the methyl group, while the other is in the para position (the one in carbon 4).

Origin of the prefixes ortho- , meta- and para-

The prefixes ortho- , meta- and para- come from Greek prefixes. Ortho- comes from the Greek orthos -, which means right or correct. The prefix meta- means next, which makes sense, since it is the position that follows the ortho position. Lastly, para- in Greek means adjacent to, outside of, or against. In the present case, the last meaning (against) is being used, since, as we have seen, it is the opposite carbon in the benzene ring.

These prefixes were originally introduced into organic chemistry by Wilhelm Körner in 1867, although with a different meaning than what is given today. It was not until 1879 that the Royal Society of Chemistry (the Royal Society of British Chemistry) adopted by consensus ortho, meta and para to indicate the substitution described above.

Despite being a considerably old nomenclature system, the IUPAC still allows its use. However, it is not recommended, particularly because of the ambiguity that can arise with heterocyclic aromatic compounds.

Limitations of the prefixes ortho- , meta- and para-

This is a relative positioning system that can only be applied to compounds derived from benzene by substitution of their hydrogen atoms. As such, it cannot be applied to polycyclic aromatic compounds such as those derived from naphthalene, anthracene, and phenanthrene, to name a few.

Neither can the nomenclature ortho, meta and para be used in heterocyclic aromatic compounds, even if they have six-membered aromatic rings as in the case of pyridine. The reason for this limitation is that, in these cases, the different positions in which substituents can be connected are not equal to each other due to the presence of the heteroatom.

Finally, ortho-, meta- and para- cannot be used in the nomenclature of disubstituted aromatic rings that do not have 6-carbon rings.

References

ACD Labs. (nd). Rule A-12. Substituted Aromatic Compounds (MONOCYCLIC HYDROCARBONS) . ADC Labs. https://www.acdlabs.com/iupac/nomenclature/79/r79_61.htm#a_12_3

Carey, F. (2021). Organic Chemistry (9th ^ed .). MCGRAW HILL EDUCATION.

Naming the Benzenes. (2015, July 18). https://chem.libretexts.org/@go/page/32469

Origin of «ortho», «meta», «para» . (2016, October 17). Chemistry Stack Exchange. https://chemistry.stackexchange.com/questions/61153/origin-of-ortho-meta-para