The Halogens

The Halogens Carh/s).—The method of Carius, which is usually emplo ed, consists in oxidising the substance with fuming nitric acid under piessure in presence of silver nitrate. The silver halide which is formed is then separated by filtration and w eighed. 

A narro-dj weighing-ttibc 8—10 cm. (3—4 in.) long and sealed at one end, which will slip easily into the thick-wallcd tube. 

A Tube Furnace.—Various forms of furnace are used. Those which are heated on the principle of the Lothar Meyer hot-air furnace by a number of pin-hole gas jets are easily regulated, and can be raised to a high temperature. The Gattermann furnace, shown in the diagram (Fig. 20), is a very convenient form. 

I iunplr. Ill oinai etanilide ave the follow 1UJ4 1 esult - 

The halogens (Group Vllb) directly precede the rare gases in the Periodic Table. They are fiourine (Z — 9), chlorine (Z = 17), bromine (Z 35), iodine (Z = 53), and the recently discovered astatine (Z 86). 

Elemental fluorine is the most powerful oxidizing agent known it may therefore not be prepared by chemical oxidations under ordinary conditions. Commercial quantities of fluorine are prepared today by electrolysis of mixtures of potassium fluoride and hydrogen fluoride (such mixtures containing the F—H—F ion). Electrodes may be of carbon or of a metal (for example, Cu or Ni) that will form a protective fluoride coating. 

For laboratory preparations of small quantities of the halogens (except fluorine), oxidation of acidic solutions containing the appropriate halide, using permanganate or manganese dioxide, is convenient. 

As noted IN the preceding chapter, almost all elemental chlorine is made as a by-product of caustic soda production, although the obsolete Deacon process of 1868 has been revived (with improvements) for recycling CI2 onsite in plants where chlorination of hydrocarbons forms gaseous HC1, an objectionable waste product  

The reaction is exothermic (see Exercise 12.1), but, since it is very slow, a catalyst is necessary. Nitric oxide, once again, can serve as an oxygen carrier, as in the lead chamber process (Section 10.2) and in reaction 10.8, where (CH3)2S generated in the kraft process is converted to DMSO. Even so, at the elevated temperatures required, reaction 12.1 needs to be forced to completion by absorption of the steam in concentrated sulfuric acid or some other desiccant. In variants of the Deacon process, copper chloride acts as the catalyst or as an intermediate for chlorine regeneration. 

Two-thirds of the chlorine produced in North America is consumed by the organic chemicals industry (25% goes to ethylene dichloride production alone). Pulp and paper mills account for another 15%, while 5% of the total is used in water treatment. All of these applications, however, have environmental implications that led to demands from activist groups that the production of chlorine derivatives be reduced (even, in the extreme view, phased out entirely). While these concerns are being taken seriously in all quarters, and some chlorinated products have already been banned, the fact is that chlorine-based technologies make, and will continue to make, important positive contributions to human health and prosperity. Indeed, at the time of writing in 1996, chlorine consumption continues to rise, al- 

In addition, chlorine derivatives are important as intermediates in the chemical industry, and there are numerous chlorine-containing pharmaceuticals for which no substitutes are presently available. Furthermore, organochlorine compounds, some very toxic, do occur naturally on a large scale. Clearly, however, we must endeavor to avoid adding unnecessarily to the natural load of toxins as the old adage goes, it is the dose that makes the poison.4 Certainly, problems exist that require an intelligent and chemically informed resolution, but the total ban advocated by some on the use of chlorine and chlorinated compounds is neither necessary nor acceptable. 

Chlorine, when dissolved in water, undergoes rapid disproportionation (i.e., it oxidizes and reduces itself simultaneously) to hypochlorous acid and hydrochloric acid, with an equilibrium constant of 5 x 10 4 mol2 L 2  

This is an example of the middle row anomaly observed among the p block groups. 

Among the +S anions, the most useful oxidant is the iodate, whose reactions proceed 

Taube and Bray noted in 1940 that the standard potential for the F(g)/F couple is 4.04 V (310). Berdnikov and Bazhin obtained a solution-phase potential by use of a thermochemical cycle that involves estimating the free energy of hydration of the fluorine atom (45) this led to a calculated E° of 3.6 V for the F/F couple and a corresponding AfG° of 68 kJ/mol for F. The chemistry of F2 and HOF has been discussed in Thompson s review (315), but there is no evidence of fluorine-containing radicals in the reactions of these species. There does not seem to be any reported evidence for the existence of F2 in aqueous solution, although the species has been detected in irradiated crystals. In the reversible reaction of H with F to give e q and HF, it has been argued that HF does not exist as an intermediate but only as a transition state (16). 

There have been several reports on the potential of the Cl/Cl couple. The first of these was by Taube and Bray (310), in which it was estimated that the potential of the (H+, 0H)/H20 couple exceeded that of the Cl/Cr couple by 0.12 V. Subsequent estimates gave absolute values for the couple these are collected in Table I. With the exception of Pearson s estimate (240), the various thermochemical estimates were based on the standard free energy of formation of the chlorine atom in 

These equilibrium constants have been determined only once (173), but they are estimated to lead to an uncertainty in E° of +0.03 V. The results of Schwarz and Dodson (279) also lead to a standard free energy of formation of the aqueous chlorine atom of 101 kJ/mol. Thus the hydration energy of Cl is substantially greater than that of Ar. 

As in bromine and iodine systems, chlorine atoms bind chloride as in reaction 15. 

The equilibrium constant (1.9 x 10s Mrx) was measured by use of pulse radiolysis (173). A more recent determination (335) of this constant (18 M-1) is clearly in error because of the internal incompatibility of the data, as pointed out in a very recent study of the decay of Cl2 (324). The analogous values for Br2 and I2 are discussed below however, the disagreement in the published results for these systems suggests that the reader should be cautious in accepting the above result for Cl2. Some degree of confirmation of the original Cl2 result may be found in a recent measurement (223) of the rate constant of association of Cl with Cl- (k = 8 x 109 M 1 sec-1) that is close to the value reported by Jayson, Parsons and Swallow (2.8 x 1010 M-1 sec-1) (173). As reported by Schwarz and Dodson, combining E° for Cl/Cl- with the above equilibrium constant leads to E° = 2.09 V for the C12 /2C1- couple (279). These results also lead to AfG° = — 61 kJ/mol for Cl2- and E° = 0.70 V for the C12/C12- couple. 

Balard did not discover bromine, rather bromine discovered Balard. 

Comment by Justus von Liebig about Antonie Jerome Balard 

Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main-Group Elements, First Edition. Abhik Ghosh and Steffen Berg. 

Feel free to have a quick look at Table 1.5 for a sense of the relative X2/X reduction potentials. The data will clearly show F2 to be one of the strongest oxidants. Chlorine, much more moderate by comparison, arguably offers more bang for the buck in the sense that it s cheap, much easier to handle, and remarkably versatile. 

Halides are nucleophilic. The Swain-Scott nucleophilicity increases down the group (see, e.g.. Table 1.1)  

A description of the elementary halogens, fluorine, chlorine, bromine, and iodine, was given in Chapter 9. The hydrogen halides, their salts, and some other halogen compounds were also described in that chapter. 

The oxygen compounds of the halogens (other than fluorine) are very important substances. A few of them, such as potassium chlorate, have been mentioned in earlier chapters. The chemistry of these substances is complex, but it can be systematized and clarified by correlation with the electronic theory of valence. This is the reason that the treatment of these compounds was not included in Chapter 9 but was postponed to this place. Some aspects of the chemistry of the halogens themselves and of the hydrogen halides are also presented in this chapter. 

The great electronegativity of fluorine causes it to occur only in oxidation states 0 and —1 there are no oxygen acids of fluorine. The only oxygen compound of fluorine is OF2, with the structure 

This compound is considered to contain fluorine with oxidation number —1, because fluorine is more electronegative than oxygen it is called oxygen fluoride rather than fluorine oxide. 

The physical and chemical properties of the halogens have been discussed in Chapter 9, in which also brief mention was made of methods of preparation. 

Fluorine can be acccumulated and metabolised by a number of plant species, in particular the African Dichapetalum species and the Australian Acacia georginae. These plants all contain fluoroacetate which may be up to 50 mg kg-1 dry weight. High concentrations of up to 1 % F in dry plant tissues have been found in plants growing on fluorite-rich spoil (Cooke et al., 1976) as compared with normal values of around 30 mg kg 1 (Kabata-Pendias and Pendias, 1984). 

While marine plants are often rich in Br and I the concentrations of these elements are generally low in terrestrial plants (Kabata-Pendias and Pendias, 1984 Fuge and Johnson, 1986). 

Our last chance to view very active elements occurs in Group 7A(17). The halogens begin with fluorine (F), the strongest electron grabber of all. Chlorine (Cl), bromine (Br), and iodine (I) also form compounds with most elements, and even rare astatine (At) is thought to be reactive [Group 7A(17) Family Portrait, p. 448]. 

Bisulfate salts, such as NaHS04, can be dehydrated to produce disulfates (pyrosulfates). 

Sulfuric acid has been used as a nonaqueous solvent, and some proton transfer may take place as a result of autoionization in 100% H2S04. 

The presence of ionic species is demonstrated by the conductivity of the solutions. It is a strongly acidic solvent that protonates alcohols, ethers, and acetic acid. These substances are not normally bases, but they have an unshared pair of electrons that can function as a proton acceptors. 

In the reaction of sulfuric acid with nitric acid, the nitronium ion, N02+, is generated, 

Sulfuric acid is manufactured on an enormous scale with an annual output of around 90 billion pounds. During the mid-1900s (when the production of sulfuric acid was less than half what it is now), about a third of the sulfuric acid produced was used in the production of fertilizer, but that use rose to about two-thirds in the later 1900s. During that time the world population grew from perhaps 3 billion to about 6 billion. 

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MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Taking into account the range of wavelength and the intensity of emission beams, certain elements cannot be determined by atomic absorption, such as C, H, 0, N, S, and the halogens. 

When Mendeleef devised his periodic table the noble gases were unknown. Strictly, their properties indicate that they form a group beyond the halogens. Mendeleef had already used Group VIIl to describe his transitional triads and the noble gases were therefore placed in a new Group O. 

Halogen derivatives of silanes can be obtained but direct halogena-tion often occurs with explosive violence the halogen derivatives are usually prepared by reacting the silane at low temperature with a carbon compound such as tetrachloromethane, in the presence of the corresponding aluminium halide which acts as a catalyst. 

Germanium forms divalent compounds with all the halogens. Germaniunil 1) chloride can be prepared by passing the vapour of germanium(IV) chloride (see below) over heated germanium. The reaction is reversible and disproportionation of germanium(II) chloride is complete at about 720 K at atmospheric pressure ... 

Nitrogen does form a number of binary compounds with the halogens but none of these can be prepared by the direct combination of the elements and they are dealt with below (p. 249). The other Group V elements all form halides by direct combination. 

Sulphur is less reactive than oxygen but still quite a reactive element and when heated it combines directly with the non-metallic elements, oxygen, hydrogen, the halogens (except iodine), carbon and phosphorus, and also with many metals to give sulphides. Selenium and tellurium are less reactive than sulphur but when heated combine directly with many metals and non-metals. 

It reduces the halogen elements in aqueous solution depositing sulphur ... 

The electronic configuration of each halogen is one electron less than that of a noble gas, and it is not surprising therefore, that all the halogens can accept electrons to form X" ions. Indeed, the reactions X(g) + e - X (g), are all exothermic and the values (see Table 11.1), though small relative to the ionisation energies, are all larger than the electron affinity of any other atom. 

Electron affinity and hydration energy decrease with increasing atomic number of the halogen and in spite of the slight fall in bond dissociation enthalpy from chlorine to iodine the enthalpy changes in the reactions... 

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