Acids hydrohalic

The polarity of the H—X bond actually decreases from H—F to H—I, largely because F is the most electronegative element. This would suggest that HF would be the strongest of the hydrohalic acids. Based on the data in Table 16.9. however, bond enthalpy is the more important factor in determining the strengths of these acids. 

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 

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Halides. Calcium haUdes are made by reaction of elemental calcium and the halogens directly or more conveniently by the reaction of the corresponding hydrohalic acid and CaCO, CaO, or Ca(OH)2. 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

The alkali metal halides are all high-melting, colourless crystalline solids which can be conveniently prepared by reaction of the appropriate hydroxide (MOH) or carbonate (M2CO3) with aqueous hydrohalic acid (HX), followed by recryslallization. Vast quantities of NaCl and KCl are available in nature and can be purihed if necessary by simple crystallization. The hydrides have already been discussed (p. 65). 

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 introduction of halogen into organic molecules can be carried out by a variety of addition or substitution reactions. The classical methods for the addition of halogen to double bonds or the substitution of halogen for hydroxyl by hydrohalic acids are too well known to bear repetition here. Discussed below, then, are methods that are of interest because of their stereospecific outcome or because they may be used on sensitive substrates. 

A halogenating system related to the preceding case is formed by the reaction of triphenylphosphine with molecular bromine or chlorine. The system is not as sensitive to moisture as the phosphine-carbon tetrahalide system (see preceding section), but it suffers from the disadvantage that hydrohalic acids are produced as the reaction proceeds. Nevertheless, sensitive compounds can be successfully halogenated by the system, as exemplified by the preparation of cinnamyl bromide from the alcohol. 

A mixture of cis- and /ra 5-l, 4-cycIohexanediols obtained from the hydrogenation of hydroquinone can be converted into the bridged 1,4-oxide by dehydration over alumina. The oxide, which is required to have a cis geometry, can then be cleaved by hydrohalic acids to give stereospecifically the trans disubstituted products. 

Halogenwasserstoff, m. hydrogen halide. -sSure, /. halogen hydracid, hydrohalic acid (general name for hydrochloric acid, hydro-bromic acid, etc.), -verbindung, /. hydrogen halide. 

Because all the hydrohalic acids except HF are strong acids, they arc leveled in water. Therefore, to determine their relative strengths, they must be studied in a solvent that is a poorer proton acceptor than water (such as pure acetic acid). 

Compounds of the type [PeX(R2dtc)2] have been obtained by treating [Fe(R2dtc)3] complexes with concentrated hydrohalic acids. [FeCl(Et2dtc)3] has been studied by Hoskins and White (264) it has a square pyramidal structure, with the chlorine atom at the apex, and with the Fe atom situated 62 pm above the basal plane of the four sulfur atoms. A similar structure is found (265) for the monoiodo derivative [FeI(Et2dtc)2]. The chloro complex has been synthesized (266) by the following reaction. 

Fig. 14. Equipment for the hydrothermal method, with hydrohalic acids as solvents.

Reaction of diazo ketones with hydrohalic acids... 

Rare-earth oxides dissolve in dilute hydrohalic acids to produce solutions of the trihalides which can be crystallized, giving six to nine waters of hydration depending upon the halide (cf. Table I). These hydrates cannot be thermally dehydrated, as oxohalides are formed ... 

In contrast to the other three hydrohalic acids, HF is a weakly dissociating acid. One consequence of this property is that ion exchange is superior to distillation for HF reprocessing [Da3]. When diluted in water HF dissociates into H+, F and various hydrofluoric species such as HF2- and (HF)2 according to the reactions ... 

Br, and I (but not F) for example, hydrochloric acid, HCl, and hydrobromic acid, HBr (HCl and HBr are hydrohalic acids acids that have hydrogen bonded to atoms of the halogen elements.)... 

Down a group, bond strength is the most important factor. Acid strength increases as bond strength decreases. A weaker bond means that the hydrogen atom is more easily pulled away from the atom to which it is attached. For example, hydrofluoric acid is a stronger acid than water, but HF is the weakest of the hydrohalic acids because the H-F bond is relatively strong. 

Hydrohalic acids can cause cleavage of the ether. Hydrofluoric acid, HF, doesn t work as well as the other acids in the group (HCl, HBr, or HI)-Secondary and tertiary ethers undergo reaction, while methyl and primary ethers undergo Sp 2 reaction. A generic example is in Figure 3-34, and the mechanism is in Figure 3-35. 

Mechanism forthe cleavage of an ether by a hydrohalic acid. 

Both acyl halides and anhydrides react with water (hydrolysis). Acyl halides react to form one mole of the carboxylic acid and one mole of the hydrohalic acid, HX. Anhydrides react to form two moles of carboxylic acid. 

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