Reactions of Halides

7 Key excerpts on "Reactions of Halides"

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  eBook - PDF

  Comparative Inorganic Chemistry

  • 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

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  Analytical Chemistry of Organic Halogen Compounds

  International Series in Analytical Chemistry

  • L. Mázor, R. Belcher, H. Freiser(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  The halogen atom containing an unpaired electron combines with the hydrocarbon molecule in a radical substitution reaction involving the splitting off of a hydrogen atom. Hydrogen halide and a hydrocarbon radical are formed and the latter will react further with another halogen molecule yielding a halogen atom capable of attacking another hydrocarbon molecule. Thus a characteristic chain reaction is started, which terminates by dimerization or dispropor-tionation of the residual free radicals. This process has been used mainly for the preparation of chlorine and bromine derivatives, because fluorine reacts too vigorously and the chain will be broken. On the other hand, the reaction of iodine is too slow; it usually takes place only on heating, and in this instance, reductive dehalo-genation of the alkyl iodide by the halogen iodide formed also occurs. Thus, the reaction is reversible and the equilibrium is shifted in the direction of dehalogenation. This can be reversed only by removing hydrogen iodide from the system. The above substitution reaction of chlorine and bromine does not yield a homogeneous product with alkanes, as the process can hardly be controlled to give the single product desired. This otherwise inexpensive method has been employed only when the formation of mixed chlorinated hydro-carbons is not disadvantageous. A more favourable method of alkyl halide synthesis yielding products of well-defined composition employs alcohols as starting materials. The hydroxyl groups can be replaced by a halogen atom in a nucleophilic substitution reaction: R -O H + Η φ . . . Cl© - ROH® + Cl© ^ R -C l + H 2 0 Bromo and iodo alkyl compounds are usually prepared by reacting the alcohols with phosphorus halides generated by addition of iodine to a suspension of red phosphorus in the hot alcohol. Hydrogen iodide reacts particularly readily.

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  Organic Chemistry

  • David R. Klein(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  Alkyl halide Aryl halide Vinyl halide X X X sp 3 sp 2 sp 2 (Chapter 23) (Chapter 23) X = Cl, Br, or Alkyl halides commonly undergo two general types of reactions. When treated with a nucle- ophile, an alkyl halide can undergo a substitution reaction, in which the nucleophile replaces the halogen. When treated with a base, an alkyl halide can undergo an elimination reaction, in which a π bond (an alkene) is formed: X Nuc X X H Base X Substitution Elimination + + + Nuc Base - - - - Since many reagents, such as hydroxide (HO − ), can function either as a nucleophile or as a base, substitution and elimination reactions will often compete with each other, as seen in the example below. In this chapter, we will explore substitution and elimination reactions, as well as the factors that govern the competition between them: Elimination product Cl NaOH OH Substitution product NaCl H 2 O (major product) (minor product) + + + During our coverage of substitution and elimination reactions, we will use the term substrate to refer to the alkyl halide. Substitution and elimination reactions occur for a variety of substrates, not just alkyl halides, as we will see in Section 7.10. In an alkyl halide, the halogen serves two critical functions that render the alkyl halide reactive towards substitution and elimination processes: 1. The halogen withdraws electron density via induction, rendering the adjacent carbon atom electrophilic, and therefore subject to attack by a nucleophile. This can be visualized with elec- trostatic potential maps of various methyl halides (Figure 7.1). In each image, the blue color indicates a region of low electron density. FIGURE 7.1 Electrostatic potential maps of methyl halides. CH 3 F CH 3 Cl CH 3 Br C X H H H δ– δ+ CH 3 282 CHAPTER 7 Alkyl Halides: Nucleophilic Substitution and Elimination Reactions 2. The halogen can serve as a leaving group, and substitution/elimination processes can only occur when a leaving group is present.

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  Principles of Organic Chemistry

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  7

  Nucleophilic Substitution and Elimination Reactions

  7.1 Reaction Mechanisms and Haloalkanes

  We introduced the concept of functional groups and their role in the organization of the structures of organic molecules in Section 1.9 . We described the importance of reaction mechanisms as an organizational device to classify chemical reactions in Section 2.9 . The details of the electrophilic addition reactions of alkenes (Section 4.9 ) and electrophilic substitution reactions of aromatic compounds (Section 5.5 ) are examples of two important reaction mechanisms. In this chapter we examine two more types of reactions mechanisms—nucleophilic substitution and elimination reactions. These mechanisms often occur in competition with one another and describe the reactions of several classes of compounds, such as haloalkanes (also called alkyl halides) and alcohols. In this chapter we focus on the substitution and elimination reactions of haloalkanes. These reactions illustrate the role of structure in determining the degree to which a given reaction mechanisms occurs.

  Reactivity of Haloalkanes

  Haloalkanes have a halogen atom bonded to an sp3 -hybridized carbon atom. As a result of the greater electronegativity of the halogens, the carbon atom of the carbon-halogen bond bears a partial positive charge and the halogen atom has a partial negative charge.

  where X = F, Cl, Br, I

  Since a carbon-halogen bond is polar, a haloalkane has two sites of reactivity. One is at the carbon atom bonded to the halogen atom. This carbon atom is electropositive and reacts with nucleophiles. The second site of reactivity in a haloalkane is the hydrogen atom bonded to the carbon atom adjacent to the carbon atom bonded to the halogen atom. This hydrogen atom is more acidic than the hydrogen atoms in alkanes because the halogen atom on the adjacent carbon atom withdraws electron density by an inductive effect.

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  Inorganic 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.

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  Sodium Dithionite, Rongalite And Thiourea Oxides: Chemistry And Application

  Chemistry and Application

  • Sergei V Makarov, Attila K Horv??th;Radu Silaghi-Dumitrescu;Qingyu Gao;(Authors)
  • 2016(Publication Date)
  • WSPC (EUROPE)

    (Publisher)

  Chapter 6

  Organic Reactions

  6.1Synthesis of Organofluorine Compounds

  Sulfur-containing reductants are used in organofluorine chemistry as sulfina-todehalogenation agents. Sulfinatodehalogenation is one of the most important methods of introducing fluorine atoms in organic molecules [283 –285 ]. It is a simple and efficient reaction for synthesizing of polyfluoroalkanesulfinates and sulfonates. The reaction can also be applied for the polyfluoroalkylation of organic compounds [284 ]. Sulfinatodehalogenation can directly convert a perfluoroalkyl halide into the corresponding perfluoroalkanesulfinate, without using harsh reaction conditions. Among numerous fluoroalkyl halides, iodides are more reactive than bromides, but chlorides are inert in common conditions. Substrates with long fluoroalkyl chains are usually more reactive than those with short ones [283 ]. The sulfinatodehalogenation reaction can easily be scaled up to an industrial level. Perfluoroalkanesulfinates can also be transformed into perfluoroalkanesulfonic acids and their derivatives, which are excellent surfactants and can as well be used to prepare ion exchange membranes [283 ]. Low price of reagents, mild reaction conditions, good yields and applicability to wide range of substrates all make sulfinatodehalogenation very popular.

  The sulfinatodehalogenation reaction was discovered by Huang and his coworkers, in 1981, when the sulfinate salt −O2 SCF2 CF2 OCF2 CF2 (sodium or potassium salt) was formed if the difluoroiodomethyl-containing compound ICF2 CF2 OCF2 CF2 SO2 F was treated with sodium sulfite in aqueous 1,4-dioxane (that is, the CFJ group was transformed into a CF2 SO2 Na(K) group) [286 ,287 ]. Interestingly, this reaction does not proceed in aqueous solution. It was found that the presence of a small amount of dioxane hydroperoxide in the dioxane used was responsible for the reaction, i.e. sulfite should be oxidized to sulfite anion radical [284 ]. A single electron transfer (SET) process was proposed to explain the course of the process [284

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  BIOS Instant Notes in Organic Chemistry

  • Graham Patrick(Author)
  • 2004(Publication Date)
  • Taylor & Francis

    (Publisher)

  The polar C–X bond means that alkyl halides have a substantial dipole moment. Alkyl halides are poorly soluble in water, but are soluble in organic solvents. They have boiling points which are similar to alkanes of comparable molecular weight. The polarity also means that the carbon is an electrophilic center and the halogen is a nucleophilic center. Halogens are extremely weak nucleophilic centers and therefore, alkyl halides are more likely to react as electrophiles at the carbon center.

  Reactions

  The major reactions undergone by alkyl halides are (a) nucleophilic substitution where an attacking nucleophile replaces the halogen (Figure 1a ), and (b) elimination where the alkyl halide loses HX and is converted to an alkene (Figure 1b ).

  Figure 1. Reactions of alkyl halides.

  Spectroscopic analysis

  The IR spectra of alkyl halides usually show strong C–X stretching absorptions. The position of these absorptions depends on the halogen involved i.e. the absorptions for C–F, C–Cl, C–Br and C–I occur in the regions 1400–1000, 800–600, 750–500 and 500 cm−1 respectively.

  The presence of a halogen can sometimes be implicated by the chemical shifts of neighboring groups in the nmr spectra. For example the chemical shifts in the 1 H nmr spectrum for CH 2 I, CH 2 Br and CH 2 Cl are 3.2, 3.5 and 3.6 respectively.

  Good evidence for the presence of a halogen comes from elemental analysis and mass spectrometry. In the latter, there are characteristic peak patterns associated with particular halogens as a result of the natural abundance of various isotopes. For example, bromine has two naturally occurring isotopes of 79 and 81 that occur in a ratio of 1:1. This means that two peaks of equal intensity will be present for any organic compound containing bromine. For example, the mass spectrum for ethyl bromide has two peaks of equal intensity at m/e 108 and 110 for the molecular ions 12 C2 1 H5 79 Br and 12 C2 1 H5 81 Br respectively.

  In contrast, chlorine occurs naturally as two isotopes (35 Cl and 37

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