Why the ortho,meta,para positions are named

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Discussion Overview

The discussion centers on the etymology of the terms ortho, meta, and para as they relate to the positions of substituents on a benzene ring. Participants explore the origins and meanings of these terms, providing insights into their Greek roots and implications in chemistry.

Discussion Character

  • Conceptual clarification

Main Points Raised

  • One participant notes that the term "meta" comes from Greek, meaning 'among' or 'in between', indicating its position relative to ortho and para.
  • Another participant discusses the term "ortho", suggesting it implies symmetry or perpendicularity, which may seem contradictory to its application in disubstituted molecules.
  • The same participant explains that "para" translates to 'beside' or 'beyond', which aligns with its meaning of being on the opposite side of the benzene ring.
  • It is proposed that "ortho" refers to neighboring substituents, "meta" to separated but not opposed substituents, and "para" to opposed substituents.

Areas of Agreement / Disagreement

Participants generally agree on the Greek origins of the terms and their meanings, but there is some ambiguity regarding the implications of the definitions, particularly for "ortho" and "para".

Contextual Notes

The discussion highlights potential confusion in the definitions and their applications, particularly regarding the implications of symmetry and opposition in the context of molecular structures.

poolwin2001
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Is there a reason why the ortho,meta,para positions of substituents in the enzene ring named that way?
 
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Yes there is a reason, but it is a bit confusing.All three terms come from Greek.

meta is easy - coming from Greek meaning 'among' or in between (as in metastable), is the state in between ortho- and para-.

But the other two seem a little contradictory. Looking at the definition of ortho, one finds:

ortho - straight, upright, vertical (which would see to imply symmetry of a disubstitutional molecule). Or it can refer to perpendicular or right (which is more consistent with the convention).

para - beside, along side of, beyond, aside from.

The last term goes well with the meaning of 'parallel'. However it seems that the third meaning 'beyond' would be consistent with the fact the para- means on the opposite side of.

By convention -

ortho refers to two neighboring substituents
meta refers to two substituents which are separated, but not opposed. It is the state between ortho and para.
para refers to two opposed substituents.
 
Thanks.
poolwin2001
 
Thanks indeed, Astronuc. This is a great explanation. I had no source on hand to clarify what they really meant.
 

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