Arene substitution pattern
Ortho, meta, and para substitution
- In ortho-substitution, two substituents occupy positions next to each other, which may be numbered 1 and 2. In the diagram, these positions are marked R and ortho.
- In meta-substitution the substituents occupy positions 1 and 3 (corresponding to R and meta in the diagram).
- In para-substitution, the substituents occupy the opposite ends (positions 1 and 4, corresponding to R and para in the diagram).
The toluidines serve as an example for these three types of substitution.
Synthesis
Electron donating groups, for example
Properties
Although the specifics vary depending on the compound, in simple disubstituted arenes, the three isomers tend to have rather similar boiling points. However, the para isomer usually has the highest melting point, and the lowest solubility in a given solvent, of the three isomers.[1]
Separation of ortho and para isomers
Because electron donating groups are both ortho and para directors, separation of these isomers is a common problem in synthetic chemistry. Several methods exist in order to separate these isomers:
- polarthan the para in general.
- Fractional crystallisation can be used to obtain pure para product, relying on the principle that it is less soluble than the ortho and thus will crystallise first. Care must be taken to avoid cocrystallisation of the ortho isomer.[2]
- Many diazonium salts and used to prepare other pure ortho or para compounds.[3]
Ipso, meso, and peri substitution
-
ipso- substitution.
-
meso- substitution.
-
peri- substitution.
- Ipso-substitution describes two substituents sharing the same ring position in an intermediate compound in an isopropyl groups can form stable carbocations, hence are ipso directing groups.
- Meso-substitution refers to the substituents occupying a benzylic position. It is observed in compounds such as calixarenes and acridines.
- Peri-substitution occurs in naphthalenes for substituents at the 1 and 8 positions.[citation needed]
Cine and tele substitution
- In cine-substitution, the entering group takes up a position adjacent to that occupied by the leaving group. For example, cine-substitution is observed in aryne chemistry.[4]
- Tele-substitution occurs when the new position is more than one atom away on the ring.[5]
Origins
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
Examples
Examples of the use of this nomenclature are given for isomers of cresol, C6H4(OH)(CH3):
-
o-cresol
-
m-cresol
-
p-cresol
There are three arene substitution isomers of
There are three arene substitution isomers of benzenedicarboxylic acid (C6H4(COOH)2) – the ortho isomer phthalic acid, the meta isomer isophthalic acid, and the para isomer terephthalic acid:
These terms can also be used in six-membered
-
niacin
See also
- Descriptor (chemistry)
- Isomer
- Structural isomerism
References
- ^ Morrison and Boyd, Organic Chemistry, Allyn and Bacon Inc, Boston, 1959. Ch.9, p. 250.
- ^ Morrison and Boyd, Organic Chemistry, Allyn and Bacon Inc, Boston, 1959. Ch. 10, p. 290.
- ^ Morrison and Boyd, Organic Chemistry, Allyn and Bacon Inc, Boston, 1959. Ch. 21, pp. 573-574.
- ^ Wilhelm Körner (1867) "Faits pour servir à la détermination du lieu chimique dans la série aromatique" (Facts to be used in determining chemical location in the aromatic series), Bulletins de l'Académie royale des sciences, des lettres et des beaux-arts de Belgique, 2nd series, 24 : 166-185; see especially p. 169. From p. 169: "On distingue facilement ces trois séries, dans lesquelles les dérivés bihydroxyliques ont leurs terms correspondants, par les préfixes ortho-, para- et mêta-." (One easily distinguishes these three series – in which the dihydroxy derivatives have their corresponding terms – by the prefixes ortho-, para- and meta-.)
- ^ Hermann von Fehling, ed., Neues Handwörterbuch der Chemie [New concise dictionary of chemistry] (Braunschweig, Germany: Friedrich Vieweg und Sohn, 1874), vol. 1, p. 1142.
- ^ Graebe (1869) "Ueber die Constitution des Naphthalins" (On the structure of naphthalene), Annalen der Chemie und Pharmacie, 149 : 20-28; see especially p. 26.
- ^ Victor Meyer (1870) "Untersuchungen über die Constitution der zweifach-substituirten Benzole" (Investigations into the structure of di-substituted benzenes), Annalen der Chemie und Pharmacie, 156 : 265-301; see especially pp. 299-300.
- ^ William B. Jensen (March 2006) "The origins of the ortho-, meta-, and para- prefixes in chemical nomenclature," Journal of Chemical Education, 83 (3) : 356.