Uses of the Halogens

Applications of the halogens and their compounds are widespread in industry, health care, and other areas. One such application is fluoridation, the practice of adding small quantities of fluorides (about 1 ppm by mass) such as NaF to drinking water to reduce dental caries. 

One of the most important inorganic fluorides is uranium hexafluoride (UFe), which is essentia] to the gaseous diffusion process for separating isotopes of uranium (U-235 and U-238). Industrially, fluorine is used to produce polytetrafluoroethylene, a polymer better known as Teflon  

Chlorine plays an important biological role in the human body, where the chloride ion is the principal anion in intracellular and extracellular fluids. Chlorine is widely used as an industrial bleaching agent for paper and textiles. Ordinary household laundry bleach contains the active ingredient sodium hypochlorite (about 5 percent by mass), which is prepared by combining chlorine gas with a cold solution of sodium hydroxide  

Chlorine is also used to purify water and disinfect swinuning pools. When chlorine dissolves in water, it undergoes the following reaction  

It is thought that the C10 ions destroy bacteria by oxidizing life-sustaining compounds within them. 

Fluorine is used to prepare fluorocarbons— very stable carbon—fluorine compounds used as refrigerants, lubricants, and plastics. Teflon ( FIGURE 22.9) is a polymeric fluorocarbon noted for its high thermal stability and lack of chemical reactivity. 

What is the oxidation state of Ci in each Ci species in Equation 22.19  

Sodium hypochlorite (NaClO) is the active ingredient in many liquid bleaches. Chlorine is also used in water treatment to oxidize and thereby destroy bacteria. 3- (Section 18.4) 

A common use of iodine is as KI in table salt. Iodized salt provides the small amount of iodine necessary in our diets it is essential for the formation of thyroxin, a hormone secreted by the thyroid gland. Lack of iodine in the diet results in an enlarged thyroid gland, a condition called goiter. 

When CI2 dissolves in cold dilute base, it converts into Cl and hypochlorite, CIO  

---

The reaction is less exothermic as the halogen becomes heavier. In general, the reactivity, and also the danger associated with use of the halogens in reactions, decreases as one descends the periodic table. 

The most common preparative procedures involve use of the halogen, usually bromine, in acetic acid. Other suitable halogenating agents include IV-bromosuccinimide, tetrabromocyclohexadienone, and sulfuryl chloride. 

Recently, the arylation of several specific primary amines have been studied because of the potential biological relevance of the products or products further downstream in a synthetic sequence. For example, cyclopropylamine was shown to be a viable substrate for the coupling under standard conditions [203]. Reactions of 7-azabicyclo[2.2.1]heptane have also been conducted [204] under relatively standard conditions, but with bis(imidazol-2-ylidene) as ligand. Complexes of this ligand and DPPF showed similar catalytic activities, which proved to be superior to those of most bis(phosphine)s. ortfio-Halo anilines were also studied, in this case to provide access to carbolines after use of the halogen as a means of effecting cycliza-tions by an electrophilic or reductive C-C bond formation with the other N-aryl group [205]. 

Substituted styrenes and vinylic compounds are versatile intermediates in organic synthesis, so various methods have been published in the literature [33, 34]. Among them, the Heck reaction is one of the best-studied methods for preparing these compounds [34]. However, for this reaction, the use of the halogen-carbon bond is essential for making C-C bonds. If direct addition of otherwise unreactive C-H bond to acetylenes takes place, this method will become one of the simplest methods for preparing substituted styrenes and vinylic compounds. In this section, we will describe the transition metal-catalyzed vinyla-tion of aromatic compounds by using acetylenes. 

B. Conformational effects B. Usefulness of the halogens thiocyanate or thiourea... 

This article describes strategies for the use of the halogen-dance reaction in the functionalization of aromatic heterocycles, and the synthesis of two pyridine-derived natural products, caerulomycin C and WS75624 B (Figure 1). The origins of this research lie in the... 

This work provides another illustration of the use of the halogen-dance reaction in synthesis, and further evidence that it can be a valuable tool for the functionalization of heterocycles. Perhaps the most useful feature of this reaction is that it allows for the introduction of halogens at remote sites in a that are difficult to access by other means. However, this work also illustrates that this reaction can exhibit certain limitations, some of which are rather unexpected and surprising. Ultimately, as the reaction sees more use in synthesis, its scope will be better defined and it will be easier to gauge the success of the reaction in a particular system. 

Chlorine is the most industrially useful of the halogens. In 2008, total production was 21 billion pounds, making it one of the top ten most produced chemicals in the United States —- (Section 1.1). Unlike fluorine, chlorine reacts slowly with water to form relatively stable aqueous solutions of HCl and HOCl (hypochlorous acid) ... 

In a study, which can be considered to be the first chemical application of chlorine, bromine and iodine NMR, Masuda and Kanda [280] made use of the halogen chemical shifts to obtain information on the dissociation of HCl, HBr and HI in aqueous solution. From the marked downfield shifts with increasing acid concentration the presence of undissociated molecules was inferred. However, the determination of the degrees of dissociation was made difficult by other... 

---