Phenol, weak acid

The phenols are in many ways similar to alcohols of aliphatic structures where the hydroxyl group is attached to a chain of carbons. The phenolic hydroxyl group, however, is influenced by the presence of the aromatic ring. Because of the aromatic ring, the hydrogen of the phenolic hydroxyl is labile, which makes phenols weak acids. We will deal with their chemical properties separately in Chapter 2. 

In many cases the main substance will separate out in crystalline form as the extract cools. Separation of carboxylic acids, other rather strong acids (e.g., halogenated phenols), weak acids, and neutral compounds can be achieved by shaking the water-immiscible extract consecutively with aqueous solutions of sodium hydrogen carbonate, sodium carbonate, and sodium hydroxide. In some cases buffer solutions are best used. The aqueous solutions are then acidified and extracted with ether. The operations should be carried out rapidly using ice-cooled solutions. Acetone extracts may be fractionated in the same way if first diluted with 4-5 times their volume of ether. 

Note. Phenols (unless nitrated) do not liberate CO, from Na,CO, solution. (e) Weakly alkaline. Alkali salts of some weak acids, pyridine. 

The imides, primaiy and secondary nitro compounds, oximes and sulphon amides of Solubility Group III are weakly acidic nitrogen compounds they cannot be titrated satisfactorily with a standard alkaU nor do they exhibit the reactions characteristic of phenols. The neutral nitrogen compounds of Solubility Group VII include tertiary nitro compounds amides (simple and substituted) derivatives of aldehydes and ketones (hydrazones, semlcarb-azones, ete.) nitriles nitroso, azo, hydrazo and other Intermediate reduction products of aromatic nitro compounds. All the above nitrogen compounds, and also the sulphonamides of Solubility Group VII, respond, with few exceptions, to the same classification reactions (reduction and hydrolysis) and hence will be considered together. 

Separations based upon differences in the chemical properties of the components. Thus a mixture of toluene and anihne may be separated by extraction with dilute hydrochloric acid the aniline passes into the aqueous layer in the form of the salt, anihne hydrochloride, and may be recovered by neutralisation. Similarly, a mixture of phenol and toluene may be separated by treatment with dilute sodium hydroxide. The above examples are, of comse, simple apphcations of the fact that the various components fah into different solubihty groups (compare Section XI,5). Another example is the separation of a mixture of di-n-butyl ether and chlorobenzene concentrated sulphuric acid dissolves only the w-butyl other and it may be recovered from solution by dilution with water. With some classes of compounds, e.g., unsaturated compounds, concentrated sulphuric acid leads to polymerisation, sulphona-tion, etc., so that the original component cannot be recovered unchanged this solvent, therefore, possesses hmited apphcation. Phenols may be separated from acids (for example, o-cresol from benzoic acid) by a dilute solution of sodium bicarbonate the weakly acidic phenols (and also enols) are not converted into salts by this reagent and may be removed by ether extraction or by other means the acids pass into solution as the sodium salts and may be recovered after acidification. Aldehydes, e.g., benzaldehyde, may be separated from liquid hydrocarbons and other neutral, water-insoluble hquid compounds by shaking with a solution of sodium bisulphite the aldehyde forms a sohd bisulphite compound, which may be filtered off and decomposed with dilute acid or with sodium bicarbonate solution in order to recover the aldehyde. 

Vinylation. Acetylene adds weak acids across the triple bond to give a wide variety of vinyl derivatives. Alcohols or phenols give vinyl ethers and carboxyHc acids yield vinyl esters (see Vinyl polymers). 

This reaction is useful in the preparation of anionic derivatives from the chlorides when the nucleophilic displacement route is unsatisfactory. Even weak acids, eg, phenols, mercaptans, and cycHc nitrogen compounds, can be made to undergo reaction with triorganotin hydroxides or bisoxides if the water of reaction is removed a2eotropicaHy as it forms. 

Because phenols are weak acids, they can be freed from neutral impurities by dissolution in aqueous N sodium hydroxide and extraction with a solvent such as diethyl ether, or by steam distillation to remove the non-acidic material. The phenol is recovered by acidification of the aqueous phase with 2N sulfuric acid, and either extracted with ether or steam distilled. In the second case the phenol is extracted from the steam distillate after saturating it with sodium chloride (salting out). A solvent is necessary when large quantities of liquid phenols are purified. The phenol is fractionated by distillation under reduced pressure, preferably in an atmosphere of nitrogen to minimise oxidation. Solid phenols can be crystallised from toluene, petroleum ether or a mixture of these solvents, and can be sublimed under vacuum. Purification can also be effected by fractional crystallisation or zone refining. For further purification of phenols via their acetyl or benzoyl derivatives (vide supra). 

A -Methylation of the NH of heterocycles using 1 is also known as exemplified by the methylation of indole/ The interesting mechanism is delineated below. O-methylation of weak acids such as phenols, carboxylic acids and oximes as well as 5-methylation such as A -phenylisorhodanine, certain thioketones, and dithiocarboxylic acids have also been reported." ... 

The values of the 1-hydroxy moiety of (5)-(- -)-iVb-acetyl- 1-hydroxytrypto-phan methyl ester (32), methyl l-hydroxyindole-3-butylate (33), iVb-methoxy-carbonyl-l-hydroxytryptamine (34), 1-hydroxymelatonm (19), l-hydroxy-6-nitroindole (35), and l-hydroxy-5-nitroindole (36) are determined to be 9.8, 8.4, 8.2, 8.1, 6.9, and 6.8, respectively (Fig. 2) (2000H1881). Thus, 1-hydroxyindoles are weak acids, stronger than phenol and weaker than succinimide. Therefore,... 

Oxygen compounds in crude oils are more complex than the sulfur types. However, their presence in petroleum streams is not poisonous to processing catalysts. Many of the oxygen compounds found in crude oils are weakly acidic. They are carboxylic acids, cresylic acid, phenol, and naphthenic acid. Naphthenic acids are mainly cyclopentane and cyclohexane derivatives having a carboxyalkyl side chain. 

Cresylic acid is a commercial mixture of phenolic compounds including phenol, cresols, and xylenols. This mixture varies widely according to its source. Properties of phenol, cresols, and xylenols are shown in Table 4-5 Cresylic acid constitutes part of the oxygen compounds found in crudes that are concentrated in the naphtha fraction obtained principally from naphthenic and asphaltic-based crudes. Phenolic compounds, which are weak acids, are extracted with relatively strong aqueous caustic solutions. 

Phenol, a white crystalline mass with a distinctive odor, becomes reddish when subjected to light. It is highly soluble in water, and the solution is weakly acidic. 

The 20 common amino acids can be further classified as neutral, acidic, or basic, depending on the structure of their side chains. Fifteen of the twenty have neutral side chains, two (aspartic acid and glutamic acid) have an extra carboxylic acid function in their side chains, and three (lysine, arginine, and histidine) have basic amino groups in their side chains. Note that both cysteine (a thiol) and tyrosine (a phenol), although usually classified as neutral amino acids, nevertheless have weakly acidic side chains that can be deprotonated in strongly basic solution. 

It may be noted that very weak acids, such as boric acid and phenol, which cannot be titrated potentiometrically in aqueous solution, can be titrated conductimetrically with relative ease. Mixtures of certain acids can be titrated more accurately by conductimetric than by potentiometric (pH) methods. Thus mixtures of hydrochloric acid (or any other strong acid) and acetic (ethanoic) acid (or any other weak acid of comparable strength) can be titrated with a weak base (e.g. aqueous ammonia) or with a strong base (e.g. sodium hydroxide) reasonably satisfactory end points are obtained. 

Phenol, formaldehyde, and urea have been copolymerized to achieve resins and subsequent networks with improved flame retardance and lower cost relative to phenol-formaldehyde analogues. The condensation of a phenolic methylol group with urea (Fig. 7.32) is believed to be the primary reaction under the weakly acidic conditions normally used. 

Phenols differ from alcohols in that they are weak acids. Similar to the resonance in phenol itself (Section 18.8), the resonance in the anion... 

Phenols are weak acids as a result of delocalization and stabilization of the... 

C17-0090. Alcohols generally are not acidic, yet phenol (Cg H5 OH) is a weak acid. Draw the Lewis... 

The permanganate oxidation of phenols is complicated by the intervention of lower oxidation states of manganese, (c/. the oxidation of toluene, p. 298). For example, the oxidation of 2,6-dinitrophenol in weakly acidic solution displays an induction period, following second-order kinetics thereafter. However, addition of potassium fluoride inhibits reaction almost completely, but manganous ions strongly accelerate it. 

Phenols are weak acids and hence can be present in the forms of neutral molecule (AH) and univalent anion (A ). We consider the transfer of acid across the interface between an organic solvent (O) and water (W) in the forms of a neutral molecule and a univalent ion, respectively. 

Accelerating Effect due to Phenols on the Rupture of Ether Linkages. Phenols are weak acids and polar solvent, and so often observed to enhance the thermal decomposition of covalent bond, but we could not observe any accelerating effect due to phenol on the decomposition of dibenzyl. 

Phenol, CsH5OH, is a weak acid for which K., 1.7 X 10 l0, whereas aliphatic alcohols such as... 

The [MoOI(prP4)] precursor 3 was finally converted to the corresponding dinitrogen complex by electrochemical reduction with a Hg pool electrode in the presence of dinitrogen and phenol. The latter reagent was added as a weak acid to induce protonation of the oxo group and subsequent elimination as water. The blue solution of the Mo oxo complex thereby turned... 

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