Hydrogen halides, acid strengths

For an opposing view, see Devarajan, D., Gustafson, S. J., Bickelhaupt, F. M., Ess, D. H. (2015). Is There a Need to Discuss Atomic Orbital Overlap When Teaching Hydrogen-Halide Bond Strength and Acidity Trends in Organic Chemistry Journal of Chemical Education, 92(2), 286-290. 

Bond Strength The effect of bond strength is easy to see by comparing the acidities of the hydrogen halides... 

Acid Strength of the Hydrogen Halides in Aqueous Solution ... 

As an additional probe of metal activity, we monitored benzene hydrogenation activity. As seen in Figure 9, Pt-containing rare earth catalysts have lower hydrogenation activity than chlorided alumina catalysts this result reflects inhibition of metal activity on these supports relative to conventional transitional alumina supports. Whereas the acid strength can be adjusted close to that of chlorided and flourided aluminas, metal activity is somewhat inhibited on these catalysts relative to halided aluminas. This inhibition is not due to dispersion, and perhaps indicates a SMSI interaction between Pt and the dispersed Nd203 phase. 

When dissolved in water, the hydrogen halides behave as acids, and all except hydrogen fluoride are completely dissociated. Because water is a much stronger base than Cl-, Br-, or I- ion, the acid strengths of HCl, HBr, and HI cannot be differentiated in water. However, in a less basic solvent, such as glacial (pure) acetic acid, the acids show different strengths ... 

Thus, to explain the various acid strengths of the hydrogen halides, we must focus on the factors that determine AH0 and A5° for the acid dissociation reaction. 

Experimental measurements confirm many of the conclusions reached by the calculations. Combination of a hydrogen halide with an amine produces an ion pair in the gas phase for only the strongest acid/base pairs. In concert with spectral information obtained in solid matrix, the transition from neutral to ion pair appears to be a gradual one as the strengths of the acid and base reach threshold values. There are a number of systems examined which do not fall neatly into either the pure neutral or ion pair category, but are better described as having a broad minimum in their proton transfer potential the bottom of this well occurs... 

J. G. McGoubrey. Trans. Faraday Soc. 51, 743-7 (1955). Acid strength of hydrogen halides in aqueous solutions. 

It is this latter requirement that makes the halogen acids a poor choice despite their high acid strength, since the halide ions are good nucleophiles and will also add to the double bond at the same time as hydrogen ion does. Some halogen acids can serve as efficient co-catalysts, however, in combination with a Lewis acid. 

Aqueous solutions of hydrogen sulfide, selenide, and telluride are acidic acid strength increases as the group is descended H2S < H2Se < H2Te. The same trend was observed for increasing acidity of the hydrogen halides. The acid ionization constants are... 

F>Cl>Br> I as expected from the strength of van der Waals forces. In the hydrogen halides HF is exceptional because of strong hydrogen bonding (see Topic CIO). HF is a weak acid in water, the other HX compounds being strong acids (see Topic E2). 

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