Highlights of Halogen Chemistry

Let s examine the compounds the halogens form with hydrogen and with each other, as well as their oxides, oxoanions, and oxoacids. 

The Hydrpgen Halides The halogens form gaseous hydrogen halides (HX) through direct combination with H2 or through the action of a concentrated acid on the metal halide. Commercially, HCl forms as a by-product during the chlorination of hydrocarbons to form useful materials, such as poly(vinyl chloride). 

In water, gaseous HX molecules form hydrohalic acids. Only HF, with its relatively short, strong bond, forms a weak acid  

The others dissociate completely to form the stoichiometric amount of H3O ions  

Halogen Oxides, Oxoacids, and Oxoanions The Group 7A(17) elements form many oxides that are powerful oxidizing agents. Dichlorine monoxide (CI2O), chlorine dioxide (CIO2, with an unpaired electron and Cl in the unusual -t-4 oxidation state), and dichlorine heptaoxide (CLOy) are important examples. 

The halogen oxoacids and oxoanions are produced by reaction of the halogens and their oxides with water. Most of the oxoacids are stable only in solution. Table 14.3 (next page) shows ball-and-stick models of the acids in which each atom has its lowest formal charge note that H is bonded to O. (We ll discuss factors that determine the relative strengths of the halogen oxoacids in Chapter 18.) 

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How the Halogens and Alkali Metals Contrast Physically Why the Halogens Are So Reactive Highlights of Halogen Chemistry... 

The aim of this chapter is to summarize some of the more recent developments in the application of the lithium-halogen exchange reaction and to highlight the practical considerations relevant to that chemistry. The choice of topics and examples is necessarily subjective and incomplete. However, the vast primary literature, detailing the synthetic utility and mechanistic vagaries of the exchange reaction, has been extensively reviewed. In this connection, it should be noted that some of what follows has been taken from two Lithium Link publications one on the use of... 

Figure 8.4 Schematic showing the chemical cycling of the gas-phase halogens (iodine highlighted in dark gray) in the atmosphere, and the links to aerosol formation and chemistry (adapted from Saiz-Lopez and Plane, 2004a).

In thematic volumes of The Alkaloids seminal developments in areas of exceptional interest are summarized and highlighted. The previous thematic volume of this series. Volume 71, focused on halogenated alkaloids. In Volume 76, aU four chapters are devoted to indole alkaloids, one of the supreme fields in alkaloid chemistry. 

Another synthetically useful reaction is catalytic allylic alkylation (Eq. 12.81). With this reaction, the X group can be a halogen, as well as other commonly available substrates such as acetate or carbamate. The synthetic utility derives mostly from stereochemical control, which is briefly introduced in the Connections highlight below. This reaction is widespread in organometallic chemistry and has been found to be catalyzed by a variety of metals, including nickel, palladium, platinum, rhodium, iron, ruthenium, molybdenum, and tungsten. 

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