Amine equilibrium, acid

According to Mika and Tanaka (12), an acid-amine equilibrium is first established, equation (2a). This would require two equilibria to be present for the reaction of primary amine, one for the primary and the other with the formed secondary, reactions (11) and (12). 

An example of enhanced ion production. The chemical equilibrium exists in a solution of an amine (RNH2). With little or no acid present, the equilibrium lies well to the left, and there are few preformed protonated amine molecules (ions, RNH3+) the FAB mass spectrum (a) is typical. With more or stronger acid, the equilibrium shifts to the right, producing more protonated amine molecules. Thus, addition of acid to a solution of an amine subjected to FAB usually causes a large increase in the number of protonated amine species recorded (spectrum b). 

Griess (1864a) had already observed that the diazo compounds obtained from primary aromatic amines in acid solution are converted by alkalis into salts of alkalis. The reaction is reversible. The compounds which Hantzsch (1894) termed sjw-diazotates exhibit apparently the same reactions as the diazonium ions into which they are instantaneously transformed by excess of acid. Clearly the reaction depends on an acid-base equilibrium. 

Uncatalyzed amidations of acids have been realized under solvent-free conditions and with a very important microwave effect [67 a]. The best results were obtained by use of a slight excess of either amine or acid (1.5 equiv.). The reaction involves thermolysis of the previously formed ammonium salt (acid-base equilibrium) and is promoted by nucleophilic attack of the amine on the carbonyl moiety of the acid and removal of water at high temperature. The large difference in yields (MW > A) might be a consequence of interaction of the polar TS with the electric field (Eq. (15 a) and Tab. 3.6). 

Amine acid-base equilibrium mixing entropy... 

For the acid-base equilibrium of the allyl-amines and thiol groups, the equilibrium constants are given by ... 

Because primary aromatic amines are weaker bases/nucleophiles than aliphatic (due to interaction of the electron pair on N with the n orbital system of the aromatic nucleus), a fairly powerful nitrosating agent is required, and the reaction is thus carried out at relatively high acidity. Sufficient equilibrium concentration of unprotonated... 

To probe the thermodynamics of amine encapsulation, the binding affinities for different protonated amines for 1 were investigated. By studying the stabilization of the protonated form of encapsulated amines, the feasibility of stabilizing protonated intermediates in chemical reactions could be assessed. The thermodynamic cycle for encapsulation of a hypothetical substrate (S) is shown in Scheme 7.5. The acid-base equilibrium of the substrate is defined by Ki and the binding constant of the protonated substrate in 1 is defined by K2. Previous work has shown that neutral substrates can enter 1 [94] however, the magnitude of this affinity (K4) remains unexplored. Although neutral encapsulated amines were not observable in the study of protonated substrates, the thermodynamic cycle can be completed with K3, which is essentially the acid-base equilibrium inside 1. 

In reactions with amine nucleophiles the reaction leads to a zwitterionic adduct that is in rapid acid-base equilibrium with its anionic form, Equation (45). In some cases, the zwitterion is strongly... 

Crystal structures have been reported for the octahedral phenolate [Tc(dppo)3] (307), thiolate [Tc(dppbt)3] (307, 308), and propionate [Tc(dppp)3]-2dmso (306). In each case the three P atoms occupy mer positions. For the amine ligand (25) (R = NH2) an acid-base equilibrium is established and either the triply deprotonated [Tc(dppba)3] or salts of the doubly deprotonated [Tc(dppba)2(dppbaH)]+ may be isolated depending on the pH. The crystal structure of [Tc(dppba)2(dppbaH)]-... 

NMR spectra of HFA adducts with compounds containing active hydrogen atoms like alcohols 93a, amines 95, and thiols 126 have been reported (173). HFA also forms unstable ketal esters with a series of carboxylic acids. The equilibrium has been investigated by means of F NMR spectroscopy (206). 

Sections 3.3.1 and 4.2.1 dealt with Bronsted acid/base equilibria in which the solvent itself is involved in the chemical reaction as either an acid or a base. This Section describes some examples of solvent effects on proton-transfer (PT) reactions in which the solvent does not intervene directly as a reaction partner. New interest in the investigation of such acid/base equilibria in non-aqueous solvents has been generated by the pioneering work of Barrow et al. [164]. He studied the acid/base reactions between carboxylic acids and amines in tetra- and trichloromethane. A more recent compilation of Bronsted acid/base equilibrium constants, determined in up to twelve dipolar aprotic solvents, demonstrates the appreciable solvent influence on acid ionization constants [264]. For example, the p.Ka value of benzoic acid varies from 4.2 in water, 11.0 in dimethyl sulfoxide, 12.3 in A,A-dimethylformamide, up to 20.7 in acetonitrile, that is by about 16 powers of ten [264]. 

What acids can be used to protonate an amine Equilibrium favors the products of an acid-base reaction when the weaker acid and base are formed. Because the pA a of many protonated amines is 10-11, the pKi, of the starting acid must be less than 10 for equilibrium to favor the products. Amines are thus readily protonated by strong inorganic acids like HCl and H2SO4, and by carboxylic acids as well. 

The amine radical cation of triethylamine formed in these reactions can exist in an acid-base equilibrium, as shown in Scheme 6. 

Thus, for every [H ], seven equations have to be solved simultaneously in order to compute the relative concentrations of each species present. Five mass laws (four stability expressions for the four different amine complexes and the acid-base equilibrium of NH4 -NH3) and two concentration conditions make up the seven equations. As concentration conditions one can formulate equations defining Cut- and NH3/. 

The effect of a solvent on the rate of the two-step aromatic nucleophilic substitution reactions of primary or secondary amines are sometimes complicated by the acid-base equilibrium (30) and by the fact that the transition state,... 

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