Basicity in aqueous solution

Photoelectron spectroscopic studies show that the first ionization potential (lone pair electrons) for cyclic amines falls in the order aziridine (9.85 eV) > azetidine (9.04) > pyrrolidine (8.77) >piperidine (8.64), reflecting a decrease in lone pair 5-character in the series. This correlates well with the relative vapour phase basicities determined by ion cyclotron resonance, but not with basicity in aqueous solution, where azetidine (p/iTa 11.29) appears more basic than pyrrolidine (11.27) or piperidine (11.22). Clearly, solvation effects influence basicity (74JA288). 

Also the value reported by Haney and Franklin of 186 1 kcal/mol is in agreement. The latter workers determined the proton affinity of ammonia as 207 kcal/mol which is approximately 21 kcal/mol larger than that for phosphine. The greater basicity of ammonia as compared to phosphine is showi by the difference of about 20 pH units in their relative basicities in aqueous solutions. The difference in the basicities of the aqueous solutions of 23-32 kcal/mol, which is comparable to that in the gas phase, leads to the somewhat surprising conclusion that solvent effects appear to play an unessential part in the relative basicities of PH3 and NH3 in aqueous solutions. The proton affinities of HjO and H2S, 164 and 170 kcal/mol, respectively, are in the reverse order. 

Correlation of nucleophilic rate data for phenyldimethylsulfonium ions with common nucleophiles, with pX e values shows that the slopes of the lines, jS[ e, correlate qualitatively with the Edwards hardness parameter for the nucleophile and not with the Swain-Scott n parameter.144 cw,cw-2,4,6-Trimethyl-l,3,5-triaminocyclohexane is weakly basic in aqueous solution, because of steric inhibition to solvation of the conjugate acid.145 The three NH2 groups are axial and the steric effect also results in reduced reactivity as a nucleophile in, S n2 reactions. Highly stereoselective syntheses of N-. and O-glycosides have been carried out by addition of anionic nucleophiles to glycosyl iodides.146 5 n2 reactions are involved, but some substrates are susceptible to E2 elimination when treated with highly basic anions. 

Phosphoric acid was the original of the three water molecule type ,- replacement of these molecules by basic oxides in typical phosphates, investigated by Clark, led Graham to the theory of basicity. In aqueous solutions phosphoric acid has played a no less important part as an example of successive dissociation of hydrogen ions. The theory of isomorphism, enunciated by Mitscherlich and others, was founded on the phosphates and arsenates. 

Early gas phase data represented an important contribution to the understanding of solvent effects on acidity/basicity. In aqueous solution the basicity order was known to be (CH3)3N <(CH3)2NH >CH3NH2, and it was recognised that this is not the anticipated inductive order. The advent of Munson s gas phase order, (CH3)3N >(CH3)2NH >CH3NH2, settled the case by pointing out that only differences in solvation energies could explain the irregular solution behaviour [24]. 

Eckert, F., Klamt, A. Accurate prediction of basicity in aqueous solution with COSMO-RS. J. Comput. Chem. 2006, 27(1), 11-9. 

Although the oxide dissolves in acids, it is only weakly basic. In aqueous solution, Hg(II) salts that are ionized (e.g. Hg(N03)2 and HgS04) are hydrolysed to a considerable extent and many basic salts are formed, e.g. HgO-HgCl2 and [0(HgCl)3]Cl (a substituted oxonium salt). 

The possibility of using 2,6-disubstituted pyridines and 2,6,7-trisubstituted quinuclidines, where the substituents feature remote atoms with lone pairs to stabilize the hydrogen upon protonation, are proposed snperbases that have been explored by computational approaches. There is interest in synthesizing macrocyclic proton chelaters as catalytically active organic snperbases,and a new strnctnral motif for snperbases featuring caged secondary amines has been reported. The alkali metal hydroxides, of eqnal basicity in aqueous solution, have proton affinities in the order LiOH (1000 kJ/mol) < NaOH < KOH < CsOH (1118 kJ/mol). This order matches the increasing ionic character of the alkah metal-hydroxide bonds. 

Oxide and Hydroxide Compounds Can Be Acidic or Basic in Aqueous Solution Depending on Their Composition 600... 

For example, in protic solvents such as water (H2O) and water-ethanol (ethyl alcohol, CH3CH2OH) mixtures, large ions that are more polarizable and may more readily provide an electron pair for the displacement process are also more poorly solvated than small ions. Thus, with the same substrate (e.g., bromoethane [ethyl bromide, CH3CH2Br]) as shown in Table 7.5, nucleophilicity increases with increasing atomic number in any one group (column) of the periodic table (i.e., T > Br > Cl > F") in such solvents. In this vein, it should be noted that this means that nucleophilicity does not parallel basicity since, as noted earlier, the order of basicity in aqueous solution is Cl > Br > F. 

It is therefore a great deal of interest to be able to predict which chemical species Y will act as nucleophiles and which will act as bases. Later in this text, we will discuss quantitative measures of basicity. These basicity scales are based on the ability of a substance to remove protons and refer to equilibria or are related to equilibrium measurements. The definition of basicity in aqueous solution is given in thermodynamic terms by the equilibrium constant, which indicates the ability of a substance to remove protons from water ... 

Like ammonia, phosphine can form phosphonium compounds such as PH4CI and PH4I. Unlike ammonia, phosphine is not basic in aqueous solution. 

Again, these can only be considered as operational values for an ill-defined species but since we have so many good data for other weak bases in acetic acid and since they parallel basicities in aqueous solution so well it is a partial answer to a question for which there are probably an infinite number of answers. 

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