Orientation rules

The entry of additional substituents into aromatic molecules, which have already been substituted, is affected by the existing substituents. Thus electron donors such as methoxy-, amino- or alkyl groups lead mainly to increased o-/p-substitu-tion. In this substitution mode, which is governed by the stability of the rc-com-plex, particular attention should be given to steric and rearrangement effects, which can cause marked variations within the orientation rule. 

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It has generally been assumed that phosphorous oxychloride-pyridine dehydrations, the elimination of sulfonates, and other base catalyzed eliminations (see below) proceed by an E2 mechanism (see e.g. ref. 214, 215, 216). Concerted base catalyzed eliminations in acyclic systems follow the Saytzelf orientation rule i.e., proceed toward the most substituted carbon), as do eliminations (see ref 214). However, the best geometrical arrangement of the four centers involved in 2 eliminations is anti-coplanar and in the cyclohexane system only the tran -diaxial situation provides this. 

The principle involved in the discussion mentioned above appears to be most general in nature, governing almost all kinds of chemical interaction, including intermolecular and intramolecular, as well as unicentric and multicentric. If the principle is applied to a unicentric reaction, it behaves as an orientation rule, and if it is employed to treat the multicentric reaction, as already mentioned in the discussion of Eq. (3.20), the stereoselection rule results 56,63,64)... 

The reactivity indices derived from the theory which has been developed in Chap. 3 are the frontier-electron density, the delocalizability, and the superdelocalizability, as has been mentioned in Chap. 2. These indices usually give predictions which are parallel with the general orientation rule mentioned in Chap. 5. The superdelocalizability is conventionally defined for the jr-electron systems on the basis of Eq. (3.21) and Eq. (3.24) as a dimensionless quantity of a positive value by the following equations 49> ... 

The ortho/para orientation rule of ground state chemistry appears to be followed in the photosubstitution reactions of nitrobenzene derivatives in liquid ammoniaa40) ... 

Generated from diacetyl peroxide, methyl radicals attack 2-methylfuran at position 5 preferentially if both 2- and 5-positions are occupied as in 2,5-dimethylfuran there is still little or no attack at the 3(4)-position. If there is a choice of 2(5)-positions, as in 3-methylfuran, then that adjacent to the methyl substituent is selected.249 These orientation rules are very like those for electrophilic substitution, but are predicted for radical attack by calculations of superdelocalizability (Sr) by the simple HMO method. Radical bromination by IV-bromsuccinimide follows theory less closely, presumably because it does not occur through a pure radical-chain mechanism.249... 

HP-6 Reference to aromatic systems stability, orientation rules and their relationship with more or less unsaturated cyclohexane rings and decalin systems. 

Another peculiar feature that sparked further study was that in these seemingly nucleophilic photosubstitutions it was the meta-nitro-compounds rather than the ortho- and para-isomers, that showed the most efficient and clean reaction. This behaviour strongly contrasted with what might have been expected on the basis of the classical orientation rules for thermal reactions. 

What are the orientation rules than can be extracted from the data obtained with various aromatic compounds and nucleophiles ... 

A third orientation rule presents itself from a survey of the nucleophilic photosubstitutions of bi- and tricyclic aromatic compounds (Vink et al., 1972a Vink et aL, 1972b Lok et al., 1973). 

This preference of photoreaction with a nucleophile at position 1 of azulene and naphthalene (4 and 2 in biphenyl, 9 in phenanthrene) is also evident upon considering the products from the reactions of derivatives of these hydrocarbons (Lok, 1972). In many other cases besides those represented in Figure 10 and equations (19) amd (20), the a-reactivity can be recognized as a major orientation rule. 

Some four principles and orientation rules are now rather well established ... 

Huisgen, Szeimies, and Mobius have studied the addition reactions of aryl azides to a,/S-unsaturated esters and nitriles.1 4 Methyl acrylate (73) reacts with aryl azides to form l-aryl-4-carbomethoxy-A -triazolines in agreement with the orientation rule based on electronic effects. These A -triazolines are completely converted by base catalysis into the ring-opened isomer. Thus l-phenyl-4-carbomethoxy-A2-triazo ine (74) gives, in the presence of triethylamine at room temperature, methyl 3-aniline-2-diazopropionate (75). The A2-triazolines as well as the a-diazoesters are thermolabile. 74 is converted into l-phenyl-2-carbomethoxyaziridine (76) and 75 gives methyl 3-anilinoacrylate (77) as thermolysis product.262... 

Photochemical substitution reactions can proceed through high-energy products such as radical ions, the primary process being a dissociation or an ionization of the excited molecule. Such processes do not have to follow the orientation rules dictated by the charge distribution of the excited molecule, and in many instances the product distribution is still little understood. 

Service-Oriented Rules of Thumb Strigle (Packed Tower Design and Applications, 2d ed., Gulf Publishing, Houston, Tex., 1994) proposed a multitude of rules of thumb as a function of the service, column pressure, and physical properties. These rules are based on the extensive experience of Strigle and the Norton Company (now merged with Koch-Glitsch LP). 

The number of combinations is considerably increased if in addition to liquid deuterio-ammonia other reagents are also employed (e.g. acids) and use is made of the difference in the orientation rules for hydrogen reactions with acids and bases which was described above. 

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