Ortho-orienting effect

Orientation effects in benzene derivatives operate in two ways. If the substituent is inductive there are large first order charge displacements at the ortho and para positions, and these can be estimated approximately using the atom polarizabilities (which is very small at the meta position). The changes of bond order, however, and consequently of free valence, vanish in first order and hence depend on Sa. The charge g g at position s therefore increases or decreases from the value unity in the... 

But if the polar effect is not pronounced, then substitution can be powerfully assisted by the substituent. This is ortho-para direction with activation and is provided by groups such as —OH, —OR, —SR, and —NH2. A more comprehensive list of substituents and their orientation effects is provided in Table 22-6. 

Orienting Effect. The Modified Buie of Crum Brown and Gibson. If the atom attached to the aromatic nucleus is attached to some other atom by an unsaturated linkage (i.e., by any bond which we commonly write as double or triple), then the next entering group takes the meta position otherwise it takes the ortho and para positions. 

The nitration of aniline in the presence of a large amount of strong sulfuric acid results wholly in the formation of m-nitro-aniline, but the similar nitration of dimethylaniline gives principally a mixture of the ortho- and para-derivatives. Mono-methylaniline stands between aniline and dimethylaniline in respect to the orienting effect of its amino group it yields a considerable amount of the m-nitro- compound—and dimethylaniline is preferred for the preparation of tetryl. Commercial dimethylaniline contains a certain amount of monomethylaniline, from which it is extremely difficult to free it, and this in the manufacture of tetryl is converted in part into 2,3,4,6-tetranitro-phenylmethylnitramine, or m-nitrotetryl, pale yellow, almost white, crystals from benzene, m.p. 146-147.087... 

The isopropyl group is ortho, para-directing, and the nitro group is meta-directing. In this case their orientation effects reinforce each other. Electrophilic aromatic substitution takes place ... 

Hydroxylation of Benzenic Rings. Intermediate products corresponding to the monohydroxylation of the aromatic ring have been identified in many cases (43,76,77,83,84,88-90). Electron-donating substituents exert the expected orientation effects to para and ortho positions. No orientation dominates for nitrobenzene (90). Dihydroxylated intermediates have also been identified, as well as the corresponding quinones. 

Finally attention must be drawn to the fact that the orienting effect of the nitro group in nucleophile and radical reactions usually differs from that in electrophilic reactions, and instead of meta orientation, ortho or para orientation takes place. The corresponding observations are referred to in chapters dealing with nucleophile and radical substitutions of nitro compounds (pp. 204, 207 and 212 respectively). 

Only preliminary studies of the homogeneous technique have been performed and the procedure has been used to label only relatively simple molecules. However, extrapolation to some heterocyclic systems has been achieved. In general, the orientation of isotope incorporated by both homogeneous and heterogeneous procedures is similar, ortho deactivation effects being observed for positions adjacent to bulky substituents such as alkyl groups. 

The site of cyclization in the above compounds is affected by the ortho-para orienting effect (a or b attack) by the hydroxy or alkoxy substituents (Fig. 52, Chap. I). A number of alkaloids have been synthesized by this method, including sculcrinc, cor-cximinc, kikemanine, discretine and other protobcrbcrinc derivatives,-" yohim-bancs (from tetrahydroazacarbazoles),- etc. 

The activating influence of an electron-donating substituent is greatest at the 2- ortho) and 4- para) positions, whilst the deactivating effect of an electron-withdrawing substituent is least at the meta position. The orienting effects of substituents are summarized in Table 4.1. 

Why are particular orientation effects observed Why are some groups ortho, para directors and some groups meta directors ... 

To understand why particular orientation effects arise, you must keep in mind the general structures for ortho, para directors and for meta directors already given in Section 18.7. There are two general types of ortho, para directors and one general type of meta director ... 

Weisman, G. R., Johnson, V., Fiala, R. E. (1980). Tricyclic ortho amides effects of lone-pair orientation upon NMR spectra. Tetrahedron Letters 21(3 ,), 3635-8. 

Resorcinol penetrates into 3 CyD and the dichlorocarbene in the cavity attacks the para-position to the hydroxyl group easily. This position is also the ortho-position to the other hydroxyl group. A hydroxyl group has a strong orientation effect on the electrophilic substitution reaction to the ortho- and para-positions. 

Y in above case) either to ortho and para positions (both) or only to meta position. This effect of the group already present on the nucleus is known as directive influence of the group or orientation effect. On the basis of this effect all the known groups in organic chemistry have been classified into two types. 

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