Acid-base strength and molecular structure

Many detailed discussions have been published on the effects of substituents on the strengths of organic acids and bases/ We have seen that caution is necessary in interpreting small differences in dissociation constants in terms of molecular models, and this chapter will deal only with a few of the more striking effects, with special reference to some which are of interest in reaction kinetics. We shall consider first the strengths of hydrides and oxyacids of different elements. 

The corresponding value pi C(PH3) = 27 was derived from kinetic measurements of deuterium exchange. These gave values for the velocity constant of the reaction PH3-hOH - PH2 +H2O, and the equilibrium constant was estimated by assuming that the reverse reaction took place on every collision. 

In the series of hydrogen halides pX(HF) = 3 can be measured directly, though some correction is necessary for the formation of bifluoride ion. The acid strength of HCl has been estimated in a number of ways, all of which involve the measured vapour pressure of HCl over concentrated aqueous solutions. This can be used to calculate the 

There is less abundant information about HBr and HI, but if we apply Robinson s treatment to the vapour pressure data we find piC(HBr)= -8, pK(Hl)= -9, compared with pR (HCl) = -6, suggesting that the strengths of the three acids are approximately in the ratio 1 10 10. Further, investigations in non-aqueous solvents often show that HBr is a considerably stronger acid than HCl. Thus conductivity measurements in anhydrous acetic acid indicate a ratio of about 20 between the dissociation constants of these two acids, while in acetonitrile we have piC(HBr) = 5.5 and pK(HCl) = 8.9. It is also frequently found that HBr is a much more effective acid catalyst than HCl under conditions where both acids are undissociated. It is of interest that studies 

Consider first the reaction HX H +X in the gas phase. This can be split up into a number of steps as follows  

---

Acid-Base Strength and Molecular Structure which becomes... 

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. 

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. 

Skill 25.2 Recognize the relationship between acid and base strength, pH, and molecular structure. 

The number of cases where adsorption microcalorimetry has been successfully applied to this end has increased in recent years, especially in the field of determination of the acidic function of molecular sieves, and extensive reviews of the systems investigated using this methodology have been published [2,5-18]. In particular, a review has been written [19] summarizing some of the most recently pubUshed results concerning the appUcations of microcalorimetry to the study of the acid/base sites of zeoUtes and meso-porous materials. The efficiency of thermal analysis techniques for the characterization of the acid/base strength of zeoUte materials is discussed, as weU as their ability to provide information consistent with catalytic data [19]. The reader is referred to the Atlas of Zeolite Structures [20] for nomenclature used throughout the text. 

In this part of the chapter, we develop a quantitative measure of the strengths ol weak acids and bases. We then use this information to explore how acid strength is related to molecular structure. 

Sections 10.9 and 10.10 discuss the relationship between molecular structure and the strengths of acids. The same ideas can be applied to bases, (a) Explain the relative strengths of the bases OH, NH,, and CH. (see Table 10.3). 

Why is NaOH a strong base, HNO3 a strong acid, and CH3 OH neither Each of these compounds contains O-H bonds, yet their proton transfer properties are strikingly different. In this section we examine the effect of molecular structure on acid strength. 

Table 8.1 outlines properties of acids and bases that you have examined in previous courses. In this section, you will review two theories that help to explain these and other properties. As well, you will use your understanding of molecular structure to help you understand why acids and bases differ in strength. 

---