Hydrogen halides nonaqueous solvent properties

It is common practice to refer to the molecular species HX and also the pure (anhydrous) compounds as hydrogen halides, and to call their aqueous solutions hydrohalic acids. Both the anhydrous compounds and their aqueous solutions will be considered in this section. HCl and hydrochloric acid are major industrial chemicals and there is also a substantial production of HF and hydrofluoric acid. HBr and hydrobromic acid are made on a much smaller scale and there seems to be little industrial demand for HI and hydriodic acid. It will be convenient to discuss first the preparation and industrial uses of the compounds and then to consider their molecular and bulk physical properties. The chemical reactivity of the anhydrous compounds and their acidic aqueous solutions will then be reviewed, and the section concludes with a discussion of the anhydrous compounds as nonaqueous solvents. 

The properties of HF reflect the strong hydrogen bonding that persists even in the vapor state. As a result of its high polarity and dielectric constant, liquid HF dissolves many ionic compounds. Some of the chemistry of HF as a nonaqueous solvent has been presented in Chapter 10. Properties of the hydrogen halides are summarized in Table 15.9. 

The properties of all other hydrogen halides are far removed from those of AHF and, therefore, their use as nonaqueous solvents is rather limited. Nevertheless, they are used for physical studies of solutions and for some synthetic purposes, such as in the formation of salts with HCl2 or BCl4 and related anions. Anhydrous HX can be considered to undergo self-ionization (equation 90) and can therefore be used to perform reactions of an acid-base nature, and solvolytic and redox reactions (equations 91-93). 

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