O-Nitrophenol acidity

Acetylation. Heat i g. of />-nitrophenol with 5 ml. of an acetic acid-acetic anhydride mixture under reflux for 15 minutes. Pour into water the solid acetate separates. Filter, wash with water and re-crystallise from ethanol m.p. 77 5°. This treatment usually leaves o-nitrophenol unchanged. The addition, however, of about 0 5 ml. of cone. H2SO4 to the acetylating mixture gives the o-derivative, m.p. 40°. 

Yellow. Quinones, m- and p-nitroaniline, o-nitrophenol, and many other nitrO"Compounds. [Note that some nitro-compounds often appear yellow (e.g. m-dinitrobenzene and 3, 5 -dinitro-benzoic acid), but are colourless when absolutely pure.] Iodoform. 

Cautiously add 250 g. (136 ml.) of concentrated sulphuric acid in a thin stream and with stirring to 400 ml. of water contained in a 1 litre bolt-head or three-necked flask, and then dissolve 150 g. of sodium nitrate in the diluted acid. Cool in a bath of ice or iced water. Melt 94 g. of phenol with 20 ml. of water, and add this from a separatory funnel to the stirred mixture in the flask at such a rate that the temperature does not rise above 20°. Continue the stirring for a further 2 hours after all the phenol has been added. Pour oflF the mother liquid from the resinous mixture of nitro compounds. Melt the residue with 500 ml. of water, shake and allow the contents of the flask to settle. Pour oflF the wash liquor and repeat the washing at least two or three times to ensure the complete removal of any residual acid. Steam distil the mixture (Fig. II, 40, 1 or Fig. II, 41, 1) until no more o-nitrophenol passes over if the latter tends to solidify in the condenser, turn oflF the cooling water temporarily. Collect the distillate in cold water, filter at the pump, and drain thoroughly. Dry upon filter paper in the air. The yield of o-nitrophenol, m.p. 46° (1), is 50 g. 

Phenol. The change in the orientation of substitution into phenol as a result of the superimposition of nitrosation on nitration is a well-established phenomenon. In aqueous sulphuric acid it leads to a change from the production of 73 % of o-nitrophenol under nitrating... 

The impurities present in aromatic nitro compounds depend on the aromatic portion of the molecule. Thus, benzene, phenols or anilines are probable impurities in nitrobenzene, nitrophenols and nitroanilines, respectively. Purification should be carried out accordingly. Isomeric compounds are likely to remain as impurities after the preliminary purifications to remove basic and acidic contaminants. For example, o-nitrophenol may be found in samples ofp-nitrophenol. Usually, the ri-nitro compounds are more steam volatile than the p-nitro isomers, and can be separated in this way. Polynitro impurities in mononitro compounds can be readily removed because of their relatively lower solubilities in solvents. With acidic or basic nitro compounds which cannot be separated in the above manner, advantage may be taken of their differences in pK values (see Chapter 1). The compounds can thus be purified by preliminary extractions with several sets of aqueous buffers... 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

The course of the reaction has not been fully clarified. Hydrolytic and aromatization processes are probably responsible for the formation of colored or fluorescent deriva4 tives (cf. Potassium Hydroxide Reagent). For instance, sevin is converted to a-naphthalkali metal salt of the o-hydroxycinnamic acid pro- duced by hydrolytic cleavage of the pyrone ring is converted from the non-fluorescent cis- to the fluorescent trans-form by the action of long-wavelength UV light (X = 365 nm) [2]. 

The same type of accident was also described as an effect of chlorosulphonic acid on o-nitrophenol ... 

The catalytic reduction of o-nitrophenol by the surface of a Ti(IV)/Ti(III) redox system on a Ti/Ti02 electrode has been used in a preparative-scale operation [522]. 5-Nitro-salicylic acid [523] and... 

Note that m-nitrophenol has pATa 8.4, and is a lot less acidic than o-nitrophenol or p-nitrophenol. We can draw no additional resonance structures here, and the nitro group cannot participate in further electron delocalization. The increased acidity compared with phenol can be ascribed to stabilization of resonance structures with the charge on a ring carbon through the nitro group s inductive effect. 

Other complexes described by the early workers include salicyl-aldehyde, o-nitrophenols, and acetylacetone as possible neutral ligands, HL, so that compounds of composition ML, HL or ML, HL were obtained. These were referred to as acid salts and illustrate the difficulty of deciding on a criterion for complex formation. Sfieakman (5)has reviewed the crystal structures of many acid salts of alkali metals in an investi-... 

Problem 19,14 Assign numbers from 1 for least to 4 for most to indicate the relative acid strengths in the following groups (a) phenol, m-chlorophenol, m-nitrophenol, m-cresol (b) phenol, benzoic acid, p-nitro-phenol, carbonic acid (c) phenol, p-chlorophenol, p-nitrophenol, p-cresol (d) phenol, o-nitrophenol, m-nitrophenol, p-nitrophenol (e) phenol, p-chlorophenol, 2,4,6-trichlorophenol, 2,4-dichlorophenol (/) phenol, benzyl alcohol, benzenesulfonic acid, benzoic acid. ... 

Propionic acid may also be used as catalyst however, its boiling point (141°C) is below that of the reaction temperature at the end of the reaction. The use of o-nitrophenol as catalyst resulted in a lower yield in this case. 

As the leaving group is varied from ethanol to phenol to o-nitrophenol. k3 will increase, relative to k2, and a neutral, water-catalyzed elimination of the leaving group may become important. This will result in the breakdown of the tetrahedral intermediate becoming relatively faster than its formation, and thus also in a decrease in the effectiveness of acid catalysis, since the formation of the tetrahedral intermediate is not significantly acid-catalyzed. 

SAMPLE SOLUTION (a) The problem specifies that an acid anhydride be used therefore, use acetic anhydride to prepare the acetate ester of o-nitrophenol ... 

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