Ortho position

Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

As can be seen, this algorithm additionally identifies constitutionally equivalent atoms. These are atoms with the same EC value (3) in the final iteration, such as the two carbon atoms in the ortho positions of the phenyl ring with an EC value of 9 (Figure 2-44). 

It is also to he noted that ortho and para substitution often occur together in the same molecule, so that the group R enters the para and also both the ortho positions thus both aniline and phenol on bromination readily give symmetric (2,4,6) tribromo-compounds (p. 165), while both toluene and phenol on nitration readily give the symmetric trinitro-derivatives (p. 170). 

Aniline undergoes very ready nuclear substitution by bromine even in the cold, the bromine atoms entering the two ortho positions and the para position with the formation of symmetric or 2,4.6-tribromoaniline. The presence... 

If phenol is heated with more concentrated nitric add (in the presence of sulphuric acid), nitration occurs ultimately at the para and at both the ortho positions, giving picric acid or 2,4,6-trinitrophenol. To prepare picric acid, however, it is more convenient first to heat the phenol with sulphuric acid, whereby a mixture of 0- and p-phenol sulphonic acids is readily obtained. If this mixture is now heated with concentrated nitric acid, nitration occurs at the... 

Tertiary aliphatic - aromatic amines, unlike those of the aliphatic series, react with nitrous acid with the formation of G-nitroso compounds the nitroso group enters almost exclusively in the para position if available, otherwise in the ortho position. Thus dimethylaniline yields />-nitrosodiniethylaniline ... 

The production of diacetyl derivatives is facilitated by the presence of substituents in the ortho position thus an excellent yield of diacetyl-o-toluidine results when o-toluidine is heated with an excess of the reagent ... 

Oxygenation takes place with peracids. The cyclopalladated benzylamine complex 466 is converted into the salicylaldamine complex 504 by the treatment with MCPBA[456] or /-BuO H[457]. Similarly, azobenzene is oxidized with MCPBA at the ortho position[458]. 

The major influence of the methyl group is electronic The most important factor IS relative carbocation stability To a small extent the methyl group sterically hinders the ortho positions making attack slightly more likely at the para carbon than at a single ortho carbon However para substitution is at a statistical disadvantage because there are two equivalent ortho positions but only one para position... 

The ortho position between the two methyl groups is less reactive because it is more sterically hindered... 

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

FIGURE I.I6 Isomers of biphenyl compounds with bulky groups attached at the ortho positions. 

Numerous studies have probed how novolac microstmcture influences resist hthographic properties. In one example, a series of resists were formulated from novolacs prepared with varying feed ratios ofpara-jmeta-cmso. These researchers found that the dissolution rate decreased, and the resist contrast increased, as thepara-jmeta-cmso feed ratio increased (33). Condensation can only occur at the ortho position ofpara-cmso but can occur at both the ortho- and i ra-positions of meta-cmso. It is beheved that increased steric factors and chain rigidity that accompany increasedpara-cmso content modify the polymer solubihty. 

The para and ortho positions of phenols condense at the carbonyl group of acetone to make bisphenols, eg, bisphenol A, 4,4 -(l-methylethyhdene)bisphenol [80-05-07]). If the H atom is activated, CICH— compounds add to the carbonyl group in the presence of strong base chloroform gives chloretone (l,l,l-trichloro-2-methyl-2-propanol [57-15-8]). 

The accepted configuration of naphthalene, ie, two fused benzene rings sharing two common carbon atoms in the ortho position, was estabUshed in 1869 and was based on its oxidation product, phthaUc acid (1). Based on its fused-ring configuration, naphthalene is the first member in a class of aromatic compounds with condensed nuclei. Naphthalene is a resonance hybrid ... 

Reactions of the Carboxyl Groups. Carboxyl groups in the ortho position spontaneously form a strainless ftve-membered ring when heated to give anhydrides as shown for (8). Salts and esters (4) are readily formed as shown for (4) and (5), respectively. 

Many of the pigments carry a nitro group ia the diazonium component, usually ia the ortho position (R = NO2). Among the acetoacetaryUde components the o-methoxy derivative (R2 = OCH, R3 = H) is one of the most important ia the production of azo pigments. The colors of these pigments range from red to green-shade yellows. Commercially important products ate shown ia Table 3. 

Significant quantities of the diphenoquinone are also produced if the ortho substituents are methoxy groups (36). Phenols with less than two ortho substituents produce branched and colored products from the reactions that occur at the open ortho sites. It is possible to minimize such side reactions in the case of o-cresol oxidation by using a bulky ligand on the copper catalyst to block the open ortho position (38). 

Trilialophenols can be converted to poly(dihaloph.enylene oxide)s by a reaction that resembles radical-initiated displacement polymerization. In one procedure, either a copper or silver complex of the phenol is heated to produce a branched product (50). In another procedure, a catalytic quantity of an oxidizing agent and the dry sodium salt in dimethyl sulfoxide produces linear poly(2,6-dichloro-l,4-polyphenylene oxide) (51). The polymer can also be prepared by direct oxidation with a copper—amine catalyst, although branching in the ortho positions is indicated by chlorine analyses (52). 

Halophenols without 2,6-disubstitution do not polymerize under oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or intermpt the propagation step of the chain process. To prepare poly(phenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmaim ether synthesis. A cuprous chloride—pyridine complex in 1,4-dimethoxybenzene at 200°C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1) ... 

Phtha/k anhydride is the most important type of dibasic acid derivative ki alkyd preparation because of its low cost and the excellent overall properties it imparts to the reski. The anhydride stmcture allows a fast esterification to form half-esters at relatively low reaction temperatures without hberatkig water, thereby avoiding the danger of excessive foaming ki the reactor. However, skice the two carboxyl groups of phthaUc anhydride are ki the ortho position to each other on the benzene ring, cycHc stmctures may and do occur ki the reski molecules. 

The aromatic ring of alkylphenols imparts an acidic character to the hydroxyl group the piC of unhindered alkylphenols is 10—11 (2). Alkylphenols unsubstituted in the ortho position dissolve in aqueous caustic. As the carbon number of the alkyl chain increases, the solubihty of the alkah phenolate salt in water decreases, but aqueous caustic extractions of alkylphenols from an organic solution can be accomphshed at elevated temperatures. Bulky ortho substituents reduce the solubihty of the alkah phenolate in water. The term cryptophenol has been used to describe this phenomenon. A 35% solution of potassium hydroxide in methanol (Qaisen s alkah) dissolves such hindered phenols (3). 

There is a health benefit associated with hindering hydrogen bonding. Alkylphenols as a class are generally regarded as corrosive health hazards, but this corrosivity is eliminated when the hydroxyl group is flanked by bulky substituents in the ortho positions. In fact, hindered phenols as a class of compounds are utilized as antioxidants in plastics with FDA approval for indirect food contact. 

In 1957 a procedure was described that selectively alkylated phenol in the ortho position (7). This approach, using aluminum catalysis, made a variety of 2,6-dialkylphenols accessible. The mechanism proposed for this ortho alkylation is outlined as follows ... 

V-Alkylaniline and /V,/V-di alkyl aniline hydrochlorides can be rearranged to C-alkyl anilines by heating the salts to 200—300°C. In this reaction, known as the Hofmann-Martius rearrangement, the alkyl group preferentially migrates to the para position. If this position is occupied, the ortho position is alkylated. 

Diphenylamine can be alkylated exclusively in the ortho positions by reacting with an olefin in the presence of aluminum diphenylamide (7), which can be readily obtained by heating DPA with powdered aluminum, or more easily by treating sodium diphenylamide with AIQ. - Ethylene is more reactive than propylene, which in turn is more reactive than isobutylene. Eor a typical reaction, a small amount of the amide is generated in a DPA melt and ethylene is introduced under pressure (5 —30 MPa) at 200—400°C. The absorption of ethylene stops after about 30 min and 2,2 -diethyldiphenylamine is obtained in 95% yield. With propylene only a 25% yield of the 2,2 -diisopropyldiphenylamine is obtained. 

The reductive dechlorination of chlorinated aromatics is more compHcated in that the initial dechlorination of more highly chlorinated compounds may be either chemical or enzymatic, eg, PGP, whereas the dechlorination of less chlorinated compounds or dechlorinated products is typically enzymatic. For example, the first dechlorination of 2,4-dichlorophenol (ortho position) can occur either chemically or enzymatically the second dechlorination (para position) is enzymatic (eq. 10). 

The solubility in alkali sulfide solutions stems from the presence of disulfide groups —S—S— in ortho position to the terminal amino groups. These disulfide groups are reduced to the mercapto groups —SH, which are soluble in alkaU. 

For attack at either of the two ortho positions or the para position, three such stmctures can be written. 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

The ratio between the isomers obtained in coupling with 1,3- and 1,5-naphtholsulfonic acids depends on the reactivity of the diazo component. Energetic ones, such as the 2,4-dinitrobenzenediazonium compound, essentially couple only with l-naphthol-3-sulfonic acid [3771-14-0] in the para position, but 4-chloro-benzenediazonium salt (a weaker diazo) attacks the ortho position. Both isomers result when mononitrobenzenediazonium compounds are used. The tendency to couple para is greater in l-naphthol-5-sulfonic acid [117-59-9] C QHgO S (21). For the combination of... 

Other chlorinating agents, such as pentachlorocyclohexadienone, have been subjected to laboratory study to make it possible to select each of the isomers (19). The use of 2,3,4,5,6,6-hexachlorocyclohexa-2,4-dien-l-one [21306-21 -8] makes chlorination possible in the ortho position. 

In the chlorination of 2,4-dichlorophenol it has been found that traces of amine (23), onium salts (24), or triphenylphosphine oxide (25) are excellent catalysts to further chlorination by chlorine ia the ortho position with respect to the hydroxyl function. During chlorination (80°C, without solvent) these catalysts cause traces of 2,4,5-trichlorophenol ( 500 1000 ppm) to be transformed iato tetrachlorophenol. Thus these techniques leave no 2,4,5-trichlorophenol ia the final product, yielding a 2,4,6-trichlorophenol of outstanding quaUty. The possibiUty of chlorination usiag SO2CI2 ia the presence of Lewis catalysts has been discussed (26), but no mention is made of 2,4,5-trichlorophenol formation or content. 

In the first case (22), almost stoichiometric amounts of sulfuric acid or chlorosulfonic acid are used. The amine sulfate or the amine chlorosulfate is, first, formed and heated to about 180 or 130°C, respectively, to rearrange the salt. The introduction of the sulfonic acid group occurs only in the ortho position, and an almost quantitative amount of l-aminoanthraquinone-2-sulfonic acid is obtained. On the other hand, the use of oleum (23) requires a large excess of SO to complete the reaction, and inevitably produces over-sulfonated compound such as l-amino-anthraquinone-2,4-disulfonic acid. Addition of sodium sulfate reduces the byproduct to a certain extent. Improved processes have been proposed to make the isolation of the intermediate (19) uimecessary (24,25). 

The shade may be varied by choosing amines. For aromatic amines, the steric effect of substituents in the ortho position reduces the conjugation of the anibno group with the anthraquinone moiety, and the result is a hypsochromic shift and brighter shade. Thus Cl Acid Blue 129 (120) has a more reddish and brighter shade than Cl Acid Blue 25 (118). Cycloalkylamines have a similat effect on the shade. Cl Acid Blue 62 [5617-28-7] (125) Cl 62045) is an example. 

Substitutions that displace electrons toward the carboxyl group of aromatic acids diminish the rate of the reaction (16). The substitution of fluoromethoxy or ethoxy groups in the ortho position has an accelerating action, whereas iodo, bromo, nitro, or methyl groups produce retardation. The influence of groups in the meta and para positions is not nearly so marked (17). 

AHyl phenyl ethers rearrange cleanly at high temperatures, producing o-aHyl phenols or -aHylphenols if both ortho positions are blocked. This reaction is called the Claisen rearrangement (10). 

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