Why the ortho,meta,para positions are named

[poolwin2001]

Is there a reason why the ortho,meta,para positions of substituents in the enzene ring named that way?

 

[Astronuc]

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.

 

[poolwin2001]

Thanks.
poolwin2001

 

[chem_tr]

Thanks indeed, Astronuc. This is a great explanation. I had no source on hand to clarify what they really meant.