Chemistry
Reactions of Halogens
The reactions of halogens involve their ability to react with other elements and compounds. They can undergo redox reactions, displacing less reactive halogens from their compounds. Halogens can also form compounds with metals, creating salts known as halides. These reactions are important in various industrial processes and in the synthesis of organic compounds.
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8 Key excerpts on "Reactions of Halogens"
eBook - PDF- Bernard Moody(Author)
- 2013(Publication Date)
- Arnold(Publisher)
The hydrolysis of chlorides is studied in detail on p. 420. 3 Combination with metallic elements Most metals unite with halogens. Reaction may bring incandescence or mere surface corrosion. Chlorine is more reactive than bromine while iodine is the least reactive and requires heating to higher temperatures for reactions to occur. The halide of the higher valency state, if a number of halides exist under the conditions of the reaction, is formed. The higher valency state represents a higher state of oxidation. In the case of mercury, the relative proportions of mercury and iodine ground together with a little alcohol determines whether mercury(n) or dimercury(i) di-iodide is formed: Hg + I 2 -Hgl 2 mercury(n) iodide (red) 2Hg + I 2 -Hg 2 I 2 dimercury(i) di-iodide (green) Selecting examples from different groups: union of the elements yields the chlorides, bromides and iodides of potassium (e.g. KC1), magnesium (e.g. MgBr 2 ), aluminium (e.g. AII3), tin(iv) (e.g. SnCU) and bismuth (e.g. BiB^). The reactions will vary in intensity according to the electropositive nature of the metal. Potassium will act with violence. Alu-minium iodide is synthesized in an atmosphere of hydrogen as it inflames on formation in air. Iron reacts with chlorine to give iron(m) chloride (FeCb) with incandescence, with bromine vapour to give first iron(n) bromide (FeBr 2 ) and then the rather unstable iron(m) bromide (FeBr3), while iron(n) iodide (Fel 2 ), there being no iron(m) iodide, is the final product with iodine. 4 Order of displacement Chlorine displaces bromine from bromides and iodine from iodides while bromine displaces iodine from iodides. This order is reversed with displace-ment from some of the oxosalts. Bromine appears as a reddish-brown coloration. Iodine forms the brown complex ion, 13, and then a dark precipi-tate of the element, when there is insufficient iodide
eBook - PDFAnalytical Chemistry of Organic Halogen Compounds
International Series in Analytical Chemistry
- L. Mázor, R. Belcher, H. Freiser(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
The halogens exhibit the strongest oxidative character of any periodic group of elements; this weakens gradually from fluorine to iodine. Fluorine and chlorine combine with most elements; fluorine is capable of oxidizing water, with release of oxygen. Neither fluorine nor chlorine reacts with oxygen directly. An interesting characteristic of halogens is their ability to combine with each other, forming interhalogen compounds. Compounds of fluorine with chlorine, bromine or iodine (C1F 3 , C1F, BrF, BrF 3 , IF, I F 5 , IF 7 ) are known, as well as those of chlorine with bromine or iodine (BrCl, ICI, I 2 C1 6 ); bromine can also form such compounds with iodine (IBr). These compounds are of importance in the halogenation of organic compounds. Fluorine can form stable, crystalline compounds with the heavier noble gases (XeF 3 ). Halogens of stronger oxidizing power are able to oxidize halogens of weaker oxidizing power: 21-+ Cl 2 I 2 + 2C1-I 2 + 5C1 2 + 6 H 2 0 -> 2IO3-+ 10C1- + 12H + Chlorine and bromine also react with water: Cl 2 + H 2 0 ^ CI- + OC1- + 2H+ Br 2 + H 2 0 i± B r - + OBr + 2H+ This reaction takes place only to a very limited extent with iodine. The hypochlorous and hypobromous acids thus produced are decomposed by light. 2H+ + 2 0 C 1 - ^ 2C1- + 2H+ + 0 2 2H+ + 2 0 B r Î ± 2Br~ + 2H+ + 0 2 PROPERTIES, PREPARATION AND REACTIONS 2 1 In alkaline solutions, the three halogens form hypohalites: Cl 2 + 2 0 H - = CI- + OC1- + H 2 0 Perfectly dry halogens are less reactive, and do not attack massive samples of certain metals (Fe, Cu). In these instances a compact, continuous layer of metal halide protects the surface against further attack. Thus, for example, dry chlorine gas can be stored in iron containers. Some important properties of halogen elements are given in Table I.
eBook - PDFInorganic Chemistry
Butterworths Intermediate Chemistry
- C. Chambers, A. K. Holliday(Authors)
- 2016(Publication Date)
- Butterworth-Heinemann(Publisher)
Br 2 + H 2 0 ^ HBr + HBrO lies further to the left. If 'chlorine water' is boiled the chloric(I) acid decomposes as above, but a little may break down into steam and the acid anhydride, dichlorine monoxide: 2HC10 ^ C1 2 0 + H 2 0 The smell of chlorine water, somewhat different from that of gaseous chlorine, may be due to minute amounts of evolved dichlorine monoxide. The reactions with a/kalis Oxygen difluoride, OF 2 , is obtained when gaseous fluorine is passed through very dilute (2 per cent) sodium hydroxide solution: 2F 2 + 2NaOH -2NaF + OF 2 + H 2 0 but with more concentrated alkali, oxygen is formed: 2F 2 + 4NaOH - 4NaF + 2H 2 0 + 0 2 The reactions of the other halogens can be summarized in the two equations: (1) X 2 + 2 0 H - - X -+ XCT + H 2 0 (2) 3XO - 2X- + XO3 284 Group VII: the halogens Reaction (1) is favoured by using dilute alkali and low temperature; with more alkali or higher temperatures the disproportionation reaction (2) occurs and the overall reaction becomes 3X 2 + 6 0 H - - 5X- + X0 3 - + 3H 2 0 The stability of the halate(I) anion, XO~, decreases from chlorine to iodine and the iodate(I) ion disproportionates very rapidly even at room temperature. The formation of halate(V) and halide ions by reaction (2) is favoured by the use of hot concentrated solutions of alkali and an excess of the halogen. When chlorine is passed over molten sodium or potassium hydroxide, oxygen is evolved, the high temperature causing the chlorate(V) ion to decompose: 11.3.4 Other displacement and oxidation reactions Many of the Reactions of Halogens can be considered as either oxidation or displacement reactions; the redox potentials (Table 11.2) give a clear indication of their relative oxidizing power in aqueous solution. Fluorine, chlorine and bromine have the ability to displace hydrogen from hydrocarbons, but in addition each halogen is able to displace other elements which are less electronegative than itself.
eBook - PDFExperimental Inorganic/Physical Chemistry
An Investigative, Integrated Approach to Practical Project Work
- M A Malati(Author)
- 1999(Publication Date)
- Woodhead Publishing(Publisher)
8 The Halogens 8.1 INTRODUCTION The halogens (F, CI, Br, I and At) occupy group 17 of the Periodic Table. The atoms have 7 outer electrons and hence share a pair of electrons between two atoms forming a σ bond in the molecules Xj. The bonds become longer down the group. The bond dissociation enthalpy decreases from chlorine down the group. However, fluorine has a value close to that of iodine. As the Van der Waals forces increase with the size of the molecules, the boiling points of the elements increase. Thus fluorine and chlorine are gases whereas bromine is a liquid and iodine is a solid. These facts and the trend in the dissociation enthalpy explain the high reactivity of fluorine and the decrease in reactivity down the group. The halogen atom requires one electron to complete the stable configuration of the next noble gas. Hence the anions X' represent the most stable oxidation state of -I. The electron attachment energy becomes less negative from CI to I as expected from their atomic radii. However, F has a less negative value than CI. This behaviour and the anomalous dissociation enthalpy of F2 are ascribed to interelectronic repulsions in the compact 2p sub-shell of F. The halogens tend to form σ bonds with other non-metals as well as between themselves. E-X bonds (where Ε is a non-metal and X is a halogen) increase in length but decrease in strength from F to I. Because F cannot expand its valence shell beyond 8, it forms one E-F bond whereas the lower halogens have available d orbitals. Hence they can reach a maximum covalence of seven. Fluorine is also unique because of its high electronegativity (the highest in the Periodic Table) and its small radius. Thus it forms strong hydrogen bonds: F-H-E where E=F, Ο or N. The positive oxidation numbers of the halogens from +1 to +7 increase by two units.
eBook - PDFThe Chemistry of Nonaqueous Solvents VB
Acid and Aprotic Solvents
- J.J. Lagowski(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Substitution occurs when iodine trichloride reacts with aromatic compounds; but with aryltin or arylmercury compounds, the diaryliodonium derivative is obtained. C. The Most Reactive Interhalogens: C1F 3 , BrF 3 , BrF 5 , and IF 7 Their reaction with oxides is generally vigorous. For chlorine trifluoride, ignition occurs. For bromine trifluoride and pentafluoride (and their Lewis salts), oxygen is evolved and a whole range of oxides and oxysalts of the central halogen atom are formed. 2 Characteristic of this is the reaction with glass or quartz which is probably initiated by hydrogen fluoride. 4HF + Si0 2 -> SiF 4 + 2H 2 0 (23) The water thus produced may promote a cycle of reactions such as is shown below for IF 7 : IF 7 + H 2 0 -> IOF 5 + 2HF (24) On the other hand, IF 7 may directly attack Si0 2 : 2IF 7 + Si0 2 -> 2IOF 5 + SiF 4 (2j) But if the reactive halogen fluorides, in fact, used are very pure and are manipulated under vacuum conditions, they all can be handled in glass, because they only etch it very slowly. With water, if no special conditions are used to slow the reaction (like dilution or low temperature), bromine trifluoride and chlorine trifluoride react violently to form a whole range of compounds including, for example, hydrogen fluoride, hydrogen chloride, and oxygen. Only for iodine pentafluoride, where HI0 3 is formed, is the central atom not reduced. Tantot and Bougon 45 have prepared KBr0 2 F 2 using bromine pentafluoride to replace one oxygen in KBr0 3 . This process can also lead to KBrOF 4 . 45a These halogen fluorides fluorinate the metal halides. At 250°C 2 chlorine 176 D. MARTIN, R. ROUSSON, AND J.-M. WEULERSSE trifluoride reacts with the chlorides NiCl 2 , AgCl, and CoCl 2 to give the fluorides NiF 2 , AgF 2 , and CoF 3 ; at 180°C SF 4 is also fluorinated to SF 6 . For bromine trifluoride nearly all the metallic halides are fluorinated and even oxidized to the highest valence state of the metal.
eBook - PDFThe Chemistry of Chlorine, Bromine, Iodine and Astatine
Pergamon Texts in Inorganic Chemistry, Volume 7
- A. J. Downs, C. J. Adams(Authors)
- 2016(Publication Date)
- Pergamon(Publisher)
Such catalytic effects stress once again the importance of kinetic and mechanistic factors in the aqueous chemistry of the halogens. In fact, nearly all the positive oxidation states would be denied existence in aqueous solution but for the slowness of decomposition into the molecular halogen (or halide ion) and oxygen. We are still far from having a clear picture of the mechanisms of many reactions of the halogens in aqueous media. However, as a general rule, the acceptor character of the halogen molecule with respect to nucleophilic reagents like H 2 0, OH - or X - leads to labile addition compounds, which may well function as essential reaction intermediates. The formation and characteristics of such compounds provide the theme of the next subsection. !94 A. I. Vogel, A Text-book of Quantitative Inorganic Analysis, 3rd edn., p. 944. Longmans, London (1961). 195 C. L. Berthollet, Mém. Acad. (1785) 276. 1196 CHLORINE, BROMINE, IODINE AND ASTATINE: A. J. DOWNS AND C. J. ADAMS Acceptor Functions: Charge-Transfer Interactions 32 » 33 · 168 ' 196 ~ 204 In common with molecules like sulphur dioxide, oxygen, the hydrogen halides, quinones and polynitroaromatic systems, halogen molecules of the types X 2 and XY exhibit a primary function as electron-acceptors forming complexes with a wide range of donor species. Not only is this capacity of the halogens at the root of their behaviour as solutes and, in many circumstances, as chemical reagents, its most direct expression, that is in complex-formation, has influenced profoundly the development of our understanding, in particular, of so-called charge-transfer interactions and, in general, of donor-acceptor functions. It is the vacant anti-bonding a u orbital of the halogen molecule (see Fig. 11), inevitably more diffuse than the bonding a g orbital, which furnishes the acceptor capacity.
- R.G. Compton, C.H. Bamford, C.F.H. Tipper†(Authors)
- 1971(Publication Date)
- Elsevier Science(Publisher)
Chapter 2 The Decomposition of Halogen Compounds E. S. SWINBOURNE 1. Introduction Halogen compounds exhibit a pattern of decomposition behaviour which may be generally related to the strengths of the carbon-halogen bonds. With alkyl halides, for example, the bond dissociation enthalpie~~~' are of the order of 106 kcal.mole-' for C-F, 81 kcal.mole-' for C-Cl, 69 kcal.mole-' for C-Br and 54 kcal.mole-' for C-I. The dominant decomposition process for fluorine com- pounds is understandably, one involving initial rupture of carbon-carbon bonds, while, with other halogen compounds, rupture of the carbon-halogen bond is much more likely. Many decompositions are therefore free-radical in nature. Chlorine and bromine atoms are attacking species and readily abstract hydrogen from organic molecules to form hydrogen halide. Iodine atoms, on the other hand, do not behave in this way and usually combine to form molecular iodine. Apart from the free-radical mode of decomposition, many halogen compounds decompose by unimolecular mechanisms, the most common of these being the direct unimolecular elimination of hydrogen halide. There is evidence that these types of unimolecular reactions involve charge separation of the carbon-halogen bond in the transition state, and they have received considerable attention in recent years. In this chapter the emphasis has been placed on the kinetic aspects of decomposi- tion behaviour and their bearing upon possible mechanisms for chemical change. Information based on product formation without rate data has been treated only briefly or omitted from the review. The author is grateful to Mr. J. D. Rock for assistance in the preparation of the section on the radiolysis of halogen compounds. 2. Thermally induced decompositions 2.1 FLUORINE COMPOUNDS The relatively high thermal stability of fluorine compounds is largely attributable to the carbon-fluorine bond strength which is much greater than that of carbon- hydrogen or of carbon4arbon.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
Radical Reactions 11.1 Radicals 11.2 Common Patterns in Radical Mechanisms 11.3 Chlorination of Methane 11.4 Thermodynamic Considerations for Halogenation Reactions 11.5 Selectivity of Halogenation 11.6 Stereochemistry of Halogenation 11.7 Allylic Bromination 11.8 Atmospheric Chemistry and the Ozone Layer 11.9 Autooxidation and Antioxidants 11.10 Radical Addition of HBr: Anti-Markovnikov Addition 11.11 Radical Polymerization 11.12 Radical Processes in the Petrochemical Industry 11.13 Halogenation as a Synthetic Technique 11 DID YOU EVER WONDER . . . how certain chemicals are able to extinguish fires more successfully than water? F ire is a chemical reaction called combustion, in which organic compounds are converted into CO 2 and water together with the liberation of heat and light. The combus- tion process is a chain (self-perpetuating) process that occurs via free radical intermediates. Understanding the nature of these free radicals is the key to understanding how fires can be extinguished most effectively. This chapter will focus on radicals. We will learn about their structure and reactivity, and we will explore some of the important roles that radicals play in the food and chemical industries and in our overall health. We will also return to the topic of fire to explain how specially designed chemicals are able to destroy the radical intermediates in a fire, thereby stopping the combustion process and extinguishing the fire. 482 CHAPTER 11 Radical Reactions 11.1 RADICALS Introduction to Radicals In Section 6.1, we mentioned that a bond can be broken in two different ways: heterolytic bond cleavage forms ions, while homolytic bond cleavage forms radicals (Figure 11.1). FIGURE 11.1 An illustration of the difference between homolytic and heterolytic bond cleavage.
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