Ortho substitution, definition

The intermediate 1,5-dicarbonyl compounds of type 24 (Scheme 1) can be constructed not only on the basis of meta-alkoxy-substituted benzyl ketones (C4 + Ci synthesis, Section II,C), but also under definite conditions starting from aryl ketones (C2 + C3 synthesis). Thus, in a molecule of acylveratrole derivatives of type 79, the excess of 7r-electron density due to the presence of two ortho-methoxy groups allows such compounds to be involved in electrophilic substitutions with benzoin (73URP2 74KGS1575). 

Di-substitution Products.—Usually however the di-substitution products are designated by numbers as first indicated. The names ortho, meta and para are also sometimes used exactly as in the benzene products together with other similar names applying to definite pairs of positions. By examining the formula we shall find that ten isomeric di-substitution products of naphthalene are possible in case the two substituents are the same. These ten with their numerical designations and names are as follows ... 

The D values in Table 4 display some interesting and remarkable trends in the spin-delocalizing ability of the heteroaryl substituents. The three regio-isomeric pyridyl-substituted triplet diradicals 12g ( — 0.01), 12i ( — 0.05), and 12k (—0.06) delocalize spin worse than the phenyl reference system in the order para > ortho > meta phenyl. Especially in the ortho and para isomers delocalization is definitely less effective than for phenyl. Thus, the pyridyl derivatives act as spin donors by enhancing the spin density at the radical site, although the effects are relatively small. The small effects are mainly due to destabilizing aminyl-type radical structures with unfavorable spin accumulation at the nitrogen atom (Fig. 20) [61]. 

In general, the Perkin reaction is limited to aromatic aldehydes. The activity of substituted benzaldehydes in the Perkin reaction is similar to the trends observed in other reactions involving the carbonyl group. A halogen (28) or nitro (34) group in any position increases the rate of reaction and the yield a methyl group (26) in any position decreases the rate and yield, and this effect falls off in the order ortho > meta > para A methoxy group in the ortho position (30) has a small favorable influence, but in the para position (32) it has a definite unfavorable effect on the rate and yield. 

Because a is defined by a similar model the action of meta or ortho substituents is tantamount to a single interaction pathway. When the transmission path in the ionisation of benzoic acids differs from that in the unknown reaction special treatment is required [42]. In the case of substituted aspirins the carboxyl group is involved in catalysis. Thus two pathways exist for substituent transmission not comparable with the simple Hammett definition of a an alternative approach is to look at the substituents and realise that a problem exists in assigning a values (Eqn. 56). 

FIGURE 7.10 Typical examples of the H-NMR spectra of CBA-w in the nematic LC state and the definition of the molecular axis (see the text) (a) n = 9 (b) n = 10. In the upper diagrams, the quadrupolar splittings (Av ) are assigned to the individual C-D bonds of the spacer. The dipolar ( >hd) quadrupolar splittings (AVo) of the mesogenic core (lower diagrams) are due to the deuteron substituted at the ortho position. (From Abe and Furuya [1988].)... 

Unlike resoles, which show a definite preference for methylolation and condensation at the para position, ring positions in novolacs are less differentiated. The normal ratio of o,p-, and /j,p-linkages in a novolac will be 1 2 1. This may be affected by the choice of catalyst, and much work has been done to control this aspect of novolac synthesis, with the emphasis on producing highly or/Zio-Iinked resins. In some cases, the judicious choice of protic acid may lead to the desired result. More commonly, a Lewis acid salt is chosen as the catalyst. These are usually divalent metal salts of acetates or similar small carboxylates. Zinc acetate is probably the most common example. Often resins made using these salts cannot be cleanly characterized as resole or novolac. They may have a resole molar ratio and a novolac pH or they may be made near neutral conditions. As mentioned before, commercial phenolic polymers showing 85% ortho linkage are available. Solvent choices may also be important to determination of substitution patterns. 

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