Basic strength method

A similar failure in the case of heterocyclic compounds is shown by the basicity of even heteroaromatic bases such as pyridine or quinoline. These all have similar basic strengths, as the simple PMO method predicts (Section IX). However the charge densities on the nitrogen atoms calculated51 by the Hiickel method are not the same (Table IV) application of Eq. (101) would then imply that the bases should differ considerably in strength. 

The effects of substituents on the acidic and basic strengths of imidazoles have been discussed by Hofmann1 and Pozharskii et al.3 in previous reviews. Precise data have been determined175 for the basicity of imidazole in D20-H20 mixtures. Perrin176 has developed methods based on the Hammett equation for predicting the pKa values in water of substituted imidazoles (and other heterocycles), while applications of the Hammett equation have also been made to 2-substituted imidazoles.177 Calculations of j>K values for 2-amino-imidazolium ions suggest that 2-aminoimidazoles are better regarded... 

The Hydrogen-Bonding Method The strength of the acid-base interaction depends on the acid strength of the acid as well as on the basic strength of the base. When the acid or the base is too weak to allow proton transfer, the interaction results in hydrogen bonding. In intermediate cases, the proton transfer may be only partial... 

F. Paradejordi, Cahiers Phys., 17, 393 (1963). On the Pariser and Parr Semierapirical Method for Computing Molecular Wave Functions. The Basic Strength of N-Heteroatomic Compounds and Their Monoamines. 

The dissociation constant of an acid is not the proper criterion for judging its strength in various solvents since the value of the constant in a given medium depends partially on the basic strength of the solvent. Bronsted therefore has proposed another method of expressing the strength of an acid. 

The position of the equilibrium in these acid-base reactions wUl depend on the relative acidity of the carbon acid and of the species BH (or, conversely, the basic strengths of and the carbanion). Some approximate pK values for typical carbon acids and the conjugate acids of some species commonly employed as bases are shown in Table 1.1. The numerical values recorded are approximate, since there is no method of accurately establishing absolute acidity in a single solvent medium for... 

Lewis [5] was the first to describe acids and bases in terms of their electron accepting and electron donating properties. Mulliken [6] further refined the understanding of the acid base interactions for which he was awarded the Nobel Prize for Chemistry. His quantum mechanical approach introduced the concept of two contributions, an electrostatic and a covalent, to the total acid-base interaction. Pearson [7] introduced the concept of hard and soft acids and bases, the HSAB principle, based on the relative contributions from the covalent (soft) interaction and the electrostatic (hard) interaction. In his mathematical treatment he defined the absolute hardness of any acid or base in terms of its ionisation potential and electron affinity. Pearson s is probably the most robust approach, but the approaches in most common use are those developed by Gutmann [8] and Drago [9], who separately developed equations and methods to quantify the acid or basic strength of compounds, from which their heats of interaction could be calculated. 

The corabination of an inductively coupled plasma ion source and a magnetic sector-based mass spectrometer equipped with a multi-collector (MC) array [multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)] offers precise and reliable isotope ratio data for many solid elements. In fact, MC-ICP-MS provides data, the trueness (accuracy) and precision of which is similar to, or, in some cases, even superior to, that achieved by thermal ionization mass spectrometry (TIMS), considered the benchmark technique for isotope ratio measurements of most solid elements [1], The basic strength of ICP-MS lies in the ion source, which achieves extremely high ionization efficiency for almost all elements [2, 3]. Consequently, MC-ICP-MS is likely to become the method of choice for many geochemists, because it is a versatile, user-friendly, and efficient method for the isotopic analysis of trace elements [4-8], The ICP ion source also accepts dry sample aerosols generated by laser ablation [9-16], The combination of laser ablation (LA) with ICP-MS is now widely accepted as a sensitive analytical tool for the elemental and isotopic analysis of solid samples. 

Another method of analyzing a mixture of bases is to utilize the difference in the basicity of its components. As an example, let us take a mixture of aromatic and aliphatic amines. Since aliphatic amines are more strongly basic, one would expect to get a titration curve with two breaks, one for the aliphatic amine and one for the aromatic amine. However, you must not use glacial acetic acid for this titration because you will get a curve similar to curve B in Fig. 1. In other words, you get one potentiometric end point for the sum of the two. The reason is that glacial acetic acid reacts with aliphatic amines to form the acetate ion, which has about the same basic strength as the aromatic amine. Glacial acetic acid levels these two amines to the same strength. What you have to do is employ a nonaqueous solvent like acetonitrile and titrate with perchloric acid dissolved in dioxane. If you do this. 

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