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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.
Both the regioselectivity—the diverse arene substitution patterns—and the speed of an electrophilic aromatic substitution are affected by the substituents already attached to the benzene ring. In terms of regioselectivity, some groups promote substitution at the ortho or para positions, whereas other groups favor substitution at the meta ...
Halogen substituents are an exception: they are resonance donors (+M). With the exception of the halides, they are meta directing groups. Halides are ortho, para directing groups but unlike most ortho, para directors, halides mildly deactivate the arene. This unusual behavior can be explained by two properties:
Monosubstituted phenyl groups (that is, disubstituted benzenes) are associated with electrophilic aromatic substitution reactions and the products follow the arene substitution pattern. So, a given substituted phenyl compound has three isomers, ortho (1,2-disubstitution), meta (1,3-disubstitution) and para (1,4-disubstitution). A disubstituted ...
The metal–arene product can also add to another aryne, leading to chain-growth polymerization. Using copper(I) cyanide as the initiator to add to the first aryne yielded polymers containing up to about 100 arene units. [22] When leaving group (LG) and substituent (Y) are mutually ortho or para, only one benzyne intermediate is possible.
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The Gatterman reaction describes arene reactions with hydrocyanic acid. [19] [20] The Houben–Hoesch reaction describes arene reactions with nitriles. [21] A reaction modification with an aromatic phenyl ester as a reactant is called the Fries rearrangement. In the Scholl reaction two arenes couple directly (sometimes called Friedel–Crafts ...
In organic chemistry, meta indicates the positions of substituents in aromatic cyclic compounds. The substituents have the 1,3-positions, for example in resorcinol. Meta may also denote the dehydrated form of an acid, salt or organic derivative in a series. For example: metabisulfite: 2 bisulfite (HSO − 3) → 1 metabisulfite (S 2 O 2− 5 ...