Bases strength in aqueous solutions

Considering the following pK values,21 explain why dilute sodium methoxide (NaOCH3) and sodium ethoxide (NaOCH2CH3) are leveled to the same base strength in aqueous solution. Write the chemical reactions that occur when these bases are added to water. 

As with the relation between pK and K, we know that pA" , the negative logarithm of the base-dissociation constant, decreases with increasing K, (that is, increasing base strength). In aqueous solution, the two large classes of weak bases are (1) ammonia and the amines and (2) the anions of weak acids. 

Acid-Base Properties of Salts Saits That Produce Neutrai Soiutions Saits That Produce Basic Soiutions Base Strength in Aqueous Solutions Salts That Produce Acidic Solutions... 

Acetic acid in this system may be considered as slightly basic since it combines with hydrogen ions. Salts such as sodium acetate become strong bases. Acids may vary from their corresponding strengths in aqueous solutions in a manner dependent upon the relative tendencies of water and acetic acid to take up hydrogen ions. Measurements have... 

Some of the most important processes in chemical and biological SYSTEMS ARE ACID-BASE REACTIONS IN AQUEOUS SOLUTIONS. In THIS FIRST OF TWO CHAPTERS ON THE PROPERTIES OF ACIDS AND BASES, WE WILL STUDY THE DEFINITIONS OF ACIDS AND BASES, THE pH SCALE, THE IONIZATION OF WEAK ACIDS AND WEAK BASES, AND THE RELATIONSHIP BETWEEN ACID STRENGTH AND MOLECULAR STRUCTURE. WE WILL ALSO LOOK AT OXIDES THAT CAN ACT AS ACIDS OR BASES. 

When we dealt with acid-base behaviour in aqueous solution in Chapter 6, we saw that the strength of an acid HX (equation 8.5) depended upon the relative proton donor abiUties of HX and [H30]. ... 

Similarly, the strength of a base, B, in aqueous solution depends upon the relative proton accepting abilities of B and [OH] (equation 8.6). 

The titration of a weak base in an anhydrous medium has been recently reported by Tamele (9 ). The catalyst, suspended in benzene, was titrated with n-butylamine, using p-dimethylaminoazobenzene as indicator. This indicator does not color pure silica gel or pure alumina. A number of solid acids of known strength in aqueous solutions were titrated in the same system. It was found that this method detects acids whose dissociation constants are about 10 or more in aqueous... 

Despas et al. [63] described the use of high-frequency impedance measurements to study the reactivity of sifica gel and Stober silica with respect to molecular and ionic bases of various sizes and strengths in aqueous solution. The experimental setup has been previously described [64]. With this technique it is possible to distinguish between silanol and silanolate groups. Stober silica shows a different behavior compared to silica gel due to the microporosity present in the former. 

Table 7.2 Classification of acid-base pairs in aqueous solution according to their acidic strength.

Some of the most important processes in chemical and biological systems are acid-base reactions in aqueous solutions. In this first of two chapters on the properties of acids and bases, we will study the definitions of acids and bases, the pH scale, the ionization of weak acids and weak bases, and the relationship between acid strength and molecular structure. We will also look at oxides that can act as acids and bases. 

The Bronsted-Lowry Theory 10-5 The Autoionization of Water 10-6 Amphoterism 10-7 Strengths of Acids 10-8 Acid-Base Reactions in Aqueous Solutions... 

We recall from Chapter 18 that the relationship K = KJCi, describes the acid and base strengths of any conjugate acid-base pair in aqueous solution. For instance, we can use this relationship for the CH3NH3 /CH3NH2 pair. Appendix G gives Ab for methylamine, CH3NH2. 

Procedures to compute acidities are essentially similar to those for the basicities discussed in the previous section. The acidities in the gas phase and in solution can be calculated as the free energy changes AG and AG" upon proton release of the isolated and solvated molecules, respectively. To discuss the relative strengths of acidity in the gas and aqueous solution phases, we only need the magnitude of —AG and — AG" for haloacetic acids relative to those for acetic acids. Thus the free energy calculations for acetic acid, haloacetic acids, and each conjugate base are carried out in the gas phase and in aqueous solution. 

According to the Arrhenius definitions, an acid ionizes in water to produce protons (H" ) and a base produces hydroxide ions (HO ). The strength of an acid is given by its equilibrium constant for ionization in aqueous solution ... 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

Pyridine bases are well known as ligands in complexes of transition metals, and it might well be anticipated that the equilibrium constants for the formation of such complexes, which are likely to be closely related to the base strength, would follow the Hammett equation. Surprisingly, only very few quantitative studies of such equilibria seem to have been reported, and these only for very short series of compounds. Thus, Murmann and Basolo have reported the formation constants, in aqueous solution at 25°, of the silver(I) complexes... 

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