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Definition

The ortho position refers to the two adjacent positions on a benzene ring.

The meta position refers to the positions separated by one carbon atom on a benzene ring.

The para position refers to the opposite position (separated by two carbon atoms) on a benzene ring.

Background

The dictionary.com pages for ortho, meta, and para state that they are independent from the prefixes ortho- (correct), meta- (after; beyond; a higher level), para- (alongside) respectively.

However, I suspect that the meta position is just after the ortho position, so it is named meta ("after" in Greek).

Question

How did these words become affiliated with the positions on a benzene ring?

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    $\begingroup$ lost in translation, apparently ... en.wikipedia.org/wiki/Arene_substitution_pattern $\endgroup$
    – Buttonwood
    Commented Oct 17, 2016 at 21:49
  • $\begingroup$ Buttonwood the info on that link would be a great basis for an answer! $\endgroup$
    – Curt F.
    Commented Oct 17, 2016 at 21:52
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    $\begingroup$ Related: chemistry.stackexchange.com/q/40506 $\endgroup$
    – Jan
    Commented Oct 17, 2016 at 23:44
  • $\begingroup$ @CurtF. Your suggestion is convincing, especially after recognizing how some of them (ortho) are used in inorganic chemistry (the indicator set by @Jan). $\endgroup$
    – Buttonwood
    Commented Oct 18, 2016 at 9:44

1 Answer 1

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The designation of an ortho-, meta-, or para-substitution pattern was coined when spectroscopic analysis was just starting (Robert Bunsen, for example). Some of the spectroscopic techniques to assign substitution patterns (IR-, and more importantly, NMR spectroscopy) were simply not available. To quote wikipedia's entry about origines of arene substitution pattern:

The prefixes ortho, meta, and para are all derived from Greek, meaning correct, following, and beside, respectively. The relationship to the current meaning is perhaps not obvious. The ortho description was historically used to designate the original compound, and an isomer was often called the meta compound. For instance, the trivial names orthophosphoric acid and trimetaphosphoric acid have nothing to do with aromatics at all. Likewise, the description para was reserved for just closely related compounds. Thus Berzelius originally called the racemic form of aspartic acid paraaspartic acid (another obsolete term: racemic acid) in 1830. The use of the prefixes ortho, meta and para to distinguish isomers of di-substituted aromatic rings starts with Wilhelm Körner in 1867, although he applied ortho prefix to a 1,4 isomer and the meta prefix to a 1,2-isomer. It was the German chemist Karl Gräbe who, in 1869, first used the prefixes ortho-, meta-, para- to denote specific relative locations of the substituents on a di-substituted aromatic ring (viz, naphthalene). In 1870, the German chemist Viktor Meyer first applied Gräbe's nomenclature to benzene. The current nomenclature was introduced by the Chemical Society in 1879.

*ortho-* *meta-* and *para*-dichlorobenzene

Besides many among the chemists are familiar of this designation, it is limited to describe the relationship of two substituents of a benzene. Hence frequently chemical suppliers and data bases prefer to describe the position of them by numbers. IUPAC's Blue Book still allows the use of o, m, and p (rule A-12.3), yet the consent moves into the direction of a consistent numbering that is more adaptable in terms of number of (potentially different) substituents and their relative arrangement to (hetero)aromatic nuclei.

To illustrate this, think of examples like lutidine (systematically 2,6-dimethylpyridine), a frequently used mild, sterically hindered base that does not fit into the other system.

enter image description here

In the end, we've to learn understand both dialects + some of non-systematic names, too.

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