Of an amine in aqueous solution

The dissociation of an organic carboxylic acid, phenol or the conjugate acid of an amine in aqueous solution IS expressed by the equation... 

Fig. 8). There is an evident increase in both acetone and pyridine with decreasing basicity. We suggest, however, that the frequency shift is not so much a measure of the basicity of the amine in aqueous solution as of the acidity in the appropriate solvent. Thus, if the lowering of the frequency corresponds to a weakening and lengthening of the N—H bond, the limit of this process is the proton transfer ...

The Bronsted-Lowry definition of a base is an acceptor of protons. Pharmaceutical and biological sciences are concerned mainly with the behaviour of drugs in aqueous systems. Under these conditions, drugs will behave as bases only if they contain a nitrogen atom with a lone pair of electrons available for reaction with protons. The major class of compound to work in this way is the amines. An amine in aqueous solution will react with water to release hydroxide ions (OH-) as shown in equation (3.4). 

Returning to the resolution of amines, an enzymatic acylation of racemic amines in aqueous solution by a penicillin acylase (enzymes used in industry in the synthesis of penicillins) from Alicaligenes faecalis has recently been reported.40 The best acylating agent is the amide 161 of phenylacetic acid. There is a big advantage here. Unlike the ester formations and hydrolysis we discussed earlier, no amide exchange occurs with simple amides like 161. The amide (/ )-162 was formed in 45% yield and 98.5% ee. Once it has been separated from free (S)-2, it can be hydrolysed to free (R)-2 with the same enzyme This automatically perfects the ee. 

Attention has previously (see Diazonium Sails) been drawn to the fact that unless an excess of hydrochloric (or mineral) acid is used in the diazotisation process, coupling occurs between the diazonium salt and the amine to give diazoamino compounds. Thus phenyldiazonium chloride and aniline yield diazoaminobenzene. This substance may be conveniently prepared by dissolving two equivalents of aniline in three equivalents of hydrochloric acid, and adding one equivalent of sodium nitrite in aqueous solution followed by two equivalents of sodium acetate ... 

In this reaction, one of the products is OH-, which is the species of basic character in aqueous solutions. When NaOH dissolves in water, the reaction is not actually an ionization reaction because the Na+ and OH- ions already exist in the solid. The process is a dissolution process rather than an ionization reaction. Substances such as NaOH, KOH, Ca(OH)2, NH3, and amines (RH2N, R2HN, and R3N) are all bases because their water solutions contain OH-. The compounds dissolve in water to give solutions that conduct an electric current, change the colors of indicators, and neutralize acids,... 

The catalytic effect of quaternary ammonium salts in the basic liquid liquid two-phase alkylation of amines [1-3] is somewhat unexpected in view of the low acidity of most amines (pKfl>30). Aqueous sodium hydroxide is not a sufficiently strong base to deprotonate non-activated amines in aqueous solution and the hydroxide ion is not readily transferred into the organic phase to facilitate the homogeneous alkylation (see Chapter 1). Additionally, it is known that ion-pairs of quaternary ammonium cations with deprotonated amines are decomposed extremely rapidly by traces of water [4]. However, under solidrliquid two-phase conditions, the addition of a quaternary ammonium salt has been found to increase the rate of alkylation of non-activated amines by a factor of ca. 3-4 [5]. Similarly, the alkylation of aromatic amines is accelerated by the addition of the quaternary ammonium salt the reaction is accelerated even in the absence of an inorganic base, although under such conditions the amine is deactivated by the formation of the hydrohalide salt, and the rate of the reaction gradually decreases. Hence, the addition of even a weak base, such as... 

The reaction is most conveniently carried out by warming the amine in aqueous solution with the equivalent quantity of sodium cyanate and an excess of acetic acid (Expt 6.96, Method 1). 

A possible mechanism which accounts for the general features of the reaction in aqueous solution is shown in Scheme 4.7. B represents any base, in addition to OH, present in the aqueous reaction mixture, C032 or an amine, for example and, because acetaldehyde is only very weakly acidic in aqueous solution (pKa = 16.7) [18], the proposed enolate carbanion intermediate can be present at only extremely low concentrations. 

AFM has been used in real time to image reacting surfaces in contact with a liquid phase [35]. A study ofthe self-passivating reactionbetween p-chloranil and an aromatic amine included monitoring the surface topography of the p-chloranil in contact with the amine in aqueous solution at 25-second intervals, Fig. 5.27. Under the conditions employed, passivation by the reaction product was shown to be complete in 5 minutes a concomitant hydroxide induced dissolution could also be monitored [7]. 

As an example, let us look at a method for acetylating aromatic amines in aqueous solution. This reaction has a special name—the Lumiere-Barbier method. We shall consider the acetylation of aniline PhNH2 (a basic aromatic amine) using acetic anhydride. The procedure for this reaction is as follows. 

Finally, stereochemical or simply chemical considerations have led several authors to postulate the formation of a p-lactone whenever they have had to deal with a reaction involving ammonia or an amine in aqueous or alooholic solution and a P-halo aoid. Some of the older literature on this subject has been discussed by Darapsky and others, > notably the formation of 3-phenylhydracrylamidu by t-hc reaction of ammoDium hydxosidc with 3-bromo-3-phenyl-propionic acid ... 

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