Molecular Structure and the Strength of Acids

Which of the diagrams shown here represents a solution of sulfuric acid Water molecules have been omitted for clarity. 

Example 15.11 shows that for diprotic acids, if K, then we can assume that the concentration of ions is the product of only the first stage of ionization. Furthermore, the concentration of the conjugate base for the second-stage ionization is numerically equal to 

Phosphoric acid (H3PO4) is a polyprotic acid with three ionizable hydrogen 

We see that phosphoric acid is a weak polyprotic acid and that its ionization constants decrease markedly for the second and third stages. Thus, we can predict that, in a solution containing phosphoric acid, the concentration of the nonionized acid is the highest, and the only other species present in significant concentrations are and H2PO4 ions. 

The strength of an acid depends on a number of factors, such as the properties of the solvent, the temperature, and, of course, the molecular structure of the acid. When we compare the strengths of two acids, we can eliminate some variables by considering their properties in the same solvent and at the same temperature and concentration. Then we can focus on the structure of the acids. 

Let us consider a certain acid HX. The strength of the acid is measured by its tendency to ionize  

Two factors influence the extent to which the acid undergoes ionization. One is the strength of the H—X bond—the stronger the bond, the more difficult it is for the HX molecule to break up and hence the weaker the acid. The other factor is the polarity of the H—X bond. The difference in the electronegativities between H and X results in a polar bond like 

Bond Enthalpies for Hydrogen Halides and Acid Strength for Hydrohalic Acids 

Lewis structures of some common oxoacids. For simplicity, the formal charges have been omitted. 

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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). 

MOLECULAR STRUCTURE AND THE STRENGTH OF ACIDS Review Questions... 

These questions involve relative acid strengths. In this section, we will examine the relationship between molecular structure and the strengths of acids and bases. 

Bassam Z. Shakhashiri, "Effect of Molecular Structure on the Strength of Organic Acids and Bases in Aqueous Solutions," Chemical Demonstrations A Handbook for Teachers of Chemistry, Vol. 3 (The University of Wisconsin Press, Madison, 1989) pp. 158-161. 

The template, the functional monomers and the cross-linking monomers are dissolved in a non-polar solvent. The functional monomers and the template form complexes and the strength of these are reflected in the selectivity of the imprinted polymer. The choice of functional monomer is based on the template structure. Functional monomers are chosen for their ability to interact non-covalently with the template molecule. The most frequently used functional monomer so far is methacrylic acid (MAA). Also vinylpyridines have been frequently used. As cross-linking monomers, ethyleneglycol dimethacrylate (EDMA) or trimethylolpropane trimethacrylate (TRIM) are widely used. Several other types of functional and cross-linking monomers have been used in molecular imprinting experiments using the non-covalent approach. The choice of monomers is of course important to the... 

Insights were sought at Pfizer by which molecular structure and shape, electronic properties, acid strength, chelating ability, and lipo-philicity might be related to potency or range of antibacterial activity. These studies were complicated by the fact that large differences observed in vitro were often reduced or completely nullified in vivo. Such observations focused attention upon the interplay of structural and pharmacokinetic properties of tetracyclines. 

In order to probe these differences further, we investigated a series of S AP05 molecular -sieves synthesized with different templates and samples of the same structure but containing Mg (MgAP05), Co (CoAP05) and Zn (ZnAPOS). The conversion of n-butane was used as test reaction and t.p.d. of pyridine to assess the strength and the concentration of acid sites. 

In the field of heterogeneous catalysis, pyridine is frequently employed as a molecule probe of the surface acid sites of zeolites and other oxide catalysts, with which the amount and the strength of the acid sites are determined, for instance by infrared measurements of the intensity and the peah position of pyridine adsorbed on zeolite surfaces [9]. Despite its importance in this respect, details of the structure and the molecule orientation within the adsorbed pyridine layer on these nonconductive substrates are not known, due to the lack of techniques to yield such data before the advent of AFM, The series of our present work constitutes the first report on the determination, by means of. A.FM. of the array structure of the pyridine base adlayers on zeolite surfaces, and the estimation of the molecular orientation within the adsorbed layer. 

The strength of acids and bases is affected by molecular structure, steric effects, resonance, inductive effects, hydrogen bonding, and solvation. 

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