Ortho position, definition

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. 

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].)... 

Methods are indicated for producing esters directly from the acid chlorides in this case the reaction is conducted in a solvent (for example, in nitrobenzene) or without it [68, 69]. However, the synthesis of acid chlorides containing substituents with labile hydrogen atoms in the ortho-position to the carboxyl group involve definite difficulties. Protection of the OH group in salicylic acid, for example, by methoxyla-tion, followed by its saponification, complicates the technology and leads to considerable losses of the finished product. 

Schenck and Steinmetz [26] have found that the addition of maleic anhydride to benzene can be photosensitized by benzophenone. The photosensitized reaction also works with toluene, o-xylene, and chlorobenzene. Hardham and Hammond [27,28] have also studied the photosensitized addition and they obtained definitive evidence that this reaction proceeds via the triplet state. They also proposed an ortho adduct as intermediate but were unable to isolate it. Bryce-Smith and Vickery [29] found that phenanthrene reacts with maleic anhydride at the 9,10-positions to give an adduct that does not further react with a molecule of maleic anhydride. This reaction can also be sensitized by benzophenone. 

A number of reactions have close similarities to ortho- and lateral lithiation, even if they do not fall under the more rigid definition of the terms.233 For example, vinylic protons with nearby directing groups can frequently be lithiated readily. Some examples are shown - 479-483 are all lithiated as though the double bond were part of an aromatic ring.233 The importance of coordination in these reactions is shown by 483, which lithiates at the more acidic position a to S if HMPA is added to disrupt Li-0 interactions. Other similar lithiations are known in the cyclopropyl and cyclobutyl series.233... 

We used diphenylmethanol to examine this point and found that in this compound also additivity does not hold. A definitive answer cannot therefore be given as to whether there is any hydrophobic driving force which causes Benadryl to exist in a folded conformation in aqueous solution. Moreover, with such strong interaction between the two aromatic rings, one cannot expect ir values for any substituents in the ortho ring positions to bear any relation to ir values in a simple benzene derivative. 

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 ... 

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. 

If the lateral chain is attached at the 3-position, i.e. ortho to an other alkyloxy chain, the behaviour changes. The properties of these substances are comparable to those of the swallow-tailed compounds. However, mesogens bearing three alkyloxy chains linked directly to the outer aromatic rings are grouped as tricatenar substances, according to the definition given by Malthete et al. [42] (see Chap. XII of this volume). 

---