THE HYDROGEN HALIDES

All the halogens form stable diatomic molecules with hydrogen. Aqueous solutions of HCl, HBr, and HI are strong acids. The hydrogen hahdes can be formed by direct reaction of the elements. The most important means of preparing HF and HCl, however, is by reacting a salt of the halide with a strong nonvolatile acid, as in the reaction 

Are these reactions acid-base reactions or oxidation-reduction reactions  

Write a balanced equation for the formation of hydrogen bromide gas from the reaction of solid sodium bromide with phosphoric acid. 

Analyze We are asked to write a balanced equation for the reaction between NaBr and H3PO4 to form HBr and another product. 

Plan As in Equation 22.20, a metathesis reaction takes place, aao (Section 4.2) Let s assume that only one H in H3PO4 reacts. (The actual number depends on the reaction conditions.) The H2P04 and Na will form NaH2P04 as one product. 

Write the balanced equation for the preparation of HI from Nal and H3PO4. Answer Nal(s) + H3P04(/)- NaH2P04(s) + HI(g) 

Recently studies of reactions in liquid BrF3 have been made. Although it is very difficult to obtain this interhalogen compound in very pure state, 

Spectral studies indicate that the compounds IF and IF7 have almost unique structures, the former a square-base pyramid the latter a pentagonal bipyramid (Fig. 14-1). 

It will be recalled that hydrogen fluoride and hydrogen chloride may be easily prepared by treatment of their readily available salts (for example, NaCl and CaF2) with sulfuric acid when heated, the volatile hydrides are driven from the reaction mixtures. A similar reaction is not the method of choice for preparing HBr or HI, for both of these halides are oxidized to the elementary halogens by concentrated H2SO4. Hydrogen bromide may be prepared either directly from the elements or by hydrolysis of 

In dilute aqueous solutions, the three halides, HC1, HBr, and HI, are essentially fully ionized, but in the poorer ionizing solvent, methanol, ionization is partial. In the latter solvent, HC1 is the most weakly ionized of the three whereas HI is the most strongly ionized this order of acidities has already been discussed (p. 90 see also Exercise 14). The ionization constant for hydrofluoric acid in dilute solutions is 7.2 X 10 4, but in very concentrated solutions the degree of ionization appears to rise sharply, a behavior opposite to that of almost all other weak electrolytes. This increase in acid strength is almost certainly due to the increase in concen- 

HBr and HI are all colourless gases at room temperatures liquid HF has a boiling temperature of 20 °C. 

This is a general way of making volatile acids that are robust enough to resist attack by H2SO4. However, it does not work for HBr and HI because they are oxidized by the acid to bromine and iodine, respectively. These two gases can be made by the hydrolysis of phosphorus trihalides for example 

A simple way of making hydrogen chloride in the laboratory. The gas is dried by bubbling it through concentrated sulfuric acid, which is extremely soluble in water, and mops up water vapour. 

Alternative preparative methods for HCl, HBr and HI include the direct combination of hydrogen and halogen, the reaction being facilitated by a platinum catalyst in the case of HBr and HI. 

The weakness of HF as an acid has much to do with the fact that because oxygen and fluorine are highly electronegative, the H30 and F ions can form ion pairs held together by hydrogen bonds (Structure 6.3). This reduces the concentration of free H30+(aq) in dilute aqueous solutions and makes HF appear a weak acid. 

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For the formation of the hydrogen halides by the direct combination of the elements, the enthalpies of formation are ... 

The dipole moments of the hydrogen halides decrease with increasing atomic number of the hydrogen, the largest difference occurring between HF and HCl, and association of molecules is not an important factor in the properties of FICl, HBr and HI. This change in dipole moment is reflected in the diminishing permittivity (dielectric constant) values from HF to HI. 

All the hydrogen halides are freely soluble in water and react according to the general equation... 

Anhydrous halides, however, are obtained when the metal is heated with the dry hydrogen halide or the halogen. In the case of elements with more than one oxidation state, the hydrogen halide produces a lower halide and the halogen a higher halide, for example... 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

Table 1 3 lists the dipole moments of various bond types For H—F H—Cl H—Br and H—I these bond dipoles are really molecular dipole moments A polar molecule has a dipole moment a nonpolar one does not Thus all of the hydrogen halides are polar molecules To be polar a molecule must have polar bonds but can t have a shape that causes all the individual bond dipoles to cancel We will have more to say about this m Section 1 11 after we have developed a feeling for the three dimensional shapes of molecules... 

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

The order of reactivity of the hydrogen halides parallels their acidity HI > HBr > HCl >> HF Hydrogen iodide is used infrequently however and the reaction of alco hols with hydrogen fluoride is not a useful method for the preparation of alkyl fluorides Among the various classes of alcohols tertiary alcohols are observed to be the most reactive and primary alcohols the least reactive... 

The first step of this new mechanism is exactly the same as that seen earlier for the reaction of tert butyl alcohol with hydrogen chloride—formation of an alkyloxonmm ion by proton transfer from the hydrogen halide to the alcohol Like the earlier exam pie this IS a rapid reversible Brpnsted acid-base reaction... 

The reachvity of the hydrogen halides reflects their ability to donate a proton Hydrogen iodide IS the strongest acid of the hydrogen halides and reacts with alkenes at the fastest rate... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

The hydrogen halide adds to the initially formed alkenyl halide m accordance with Markovmkov s rule Overall both protons become bonded to the same carbon and both halogens to the adjacent carbon... 

The deposition of metals directly from these halides would require high temperatures to be efficient, but because of the thermodynamic stabilities of the hydrogen halides, direct reduction can readily be carried out with hydrogen at lower temperamres. The general reaction... 

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

The order of reactivity of the hydrogen halides is HI > HBr > HCl, and reactions of simple alkenes with HCl are quite slow. The studies that have been applied to determining mechanistic details of hydrogen halide addition to alkenes have focused on the kinetics and stereochemistry of the reaction and on the effect of added nucleophiles. The kinetic studies often reveal complex rate expressions which demonstrate that more than one process contributes to the overall reaction rate. For addition of hydrogen bromide or Itydrogen... 

Oxidations usually proceed in the dark at or below room temperature in a variety of solvents ranging from aqueous bicarbonate to anhydrous benzene-pyridine. Base is quite commonly used to consume the hydrogen halide produced in the reaction, as this prevents the formation of high concentrations of bromine (or chlorine) by a secondary process. The reaction time varies from a few minutes to 24 hours or more depending on the nature of the reagent and the substrate. Thus one finds that NBS or NBA when used in aqueous acetone or dioxane are very mild, selective reagents. The rate of these oxidations is noticeably enhanced when Fbutyl alcohol is used as a solvent. In general, saturated, primary alcohols are inert and methanol is often used as a solvent. 

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