Ortho-activating substituents

All alkyl groups not just methyl are activating substituents and ortho para direc tors This IS because any alkyl group be it methyl ethyl isopropyl tert butyl or any other stabilizes a carbocation site to which it is directly attached When R = alkyl... 

Some of the most powerful activating substituents are those m which an oxygen atom IS attached directly to the nng These substituents include the hydroxyl group as well as alkoxy and acyloxy groups All are ortho para directors... 

Because we have come to associate activating substituents with ortho para directing effects and deactivating substituents with meta the properties of the halogen substituents appear on initial inspection to be unusual... 

A hydroxyl group is a very powerful activating substituent and electrophilic aro matic substitution m phenols occurs far faster and under milder conditions than m ben zene The hrst entry m Table 24 4 for example shows the monobrommation of phenol m high yield at low temperature and m the absence of any catalyst In this case the reac tion was carried out m the nonpolar solvent 1 2 dichloroethane In polar solvents such as water it is difficult to limit the brommation of phenols to monosubstitution In the fol lowing example all three positions that are ortho or para to the hydroxyl undergo rapid substitution... 

Ionkin has reported a similar series of Ni(II) catalysts 1.50a and b bearing ortho-difuryl substituents that are noteworthy for their high thermal stability [127], The bulkier benzofuranyl-substituted catalyst 1.50b possesses the most attractive catalytic properties (Table 5, entry 9) the ability to form high molecular weight polymers (albeit in high polydispersity) and reasonable activity even at 150 °C. Even under these harsh conditions, the polymer branching density is still relatively low. 

These computational predictions have been tested experimentally. Kinetic measurements confirmed that both ortho-N02 and ortho-CHO substituents substantially decrease activation energies for the Bergman cyclization supporting earlier computational predictions.51... 

In a study aimed at the identification of products of free radical reactions with polystyrene- and aromatic-based PEMs using model compounds, Hiibner and Roduner observed the addition of free radicals to the aromatic rings, preferentially in the ortho position to alkyl- and RO-substituents (in polystyrene- and aromatic-based PEMs, the para position is blocked by the presence of the sulfonic acid group). This study demonstrated the combined ortho-activation by these substituents and the meta-directing effect... 

Should the activating substituent be at position 2, further substitution will be almost exclusively at position 1 this follows from consideration of resonance structures, where the 2-substituent has minimal effect if attack occurs at position 4. Of course, this would equate to meta attack, which we know is unfavourable for an ortho and para director (see Section 8.4.3). 

Any substituent whose atom attached to the benzene contains a lone pair of electrons is ortho-para directing (but not necessarily a ring activator). Substituents without a lone pair on the atom attached to the ring are likely meta directors (with the exception of alkyl groups and aromatic rings, which turn out to be ortho-para directors). 

Problem 16.21 (a) Account for the fact that ArCOOH. with a strongly activating substituent ortho or para to COOH, loses CO, during attempted electrophilic substitution, (b) Write the equation for the reaction of p-aminobenzoic acid and Br,. ... 

More important is the substitution of leaving groups in ortho positions to the diazonium moiety by water when other activating substituents are present on the aromatic ring. For example, 2-bromo-3,4,6-trifluoro- or 3,4,6-trifluoro-2-nitroaniline cannot be diazotized in water without extensive hydrolysis of the 2-substituent giving 4.193 This drawback can be overcome, to give 5, by performing the diazotization in acetone at — 20°C.3 193 194... 

If 2-chloropyridine and 2-bromopyridine could be made to react with phenylacetonitrile derivatives under phase transfer conditions, then the method would be of interest because the compounds thus obtained are starting materials for a large series of pharmaceuticals.217 Unfortunately, the reaction can be carried out only if an activating substituent such as a nitro group is located ortho or para to the leaving group.218,219... 

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