Chemistry
Aryl Halide
Aryl halides are organic compounds that contain a halogen atom attached to an aromatic ring. They are also known as haloarenes or aryl halides. These compounds are important intermediates in organic synthesis and are used in a variety of applications, including pharmaceuticals, agrochemicals, and materials science.
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4 Key excerpts on "Aryl Halide"
- Axel Griesbeck, Michael Oelgemöller, Francesco Ghetti(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
369 16.1 Introduction Aryl Halides have an important role in photochemistry since a long time. Historically, studies of the phosphorescence of aromatics and of their halogenated derivatives have been instrumental in defin-ing the modern concept of triplet state, and both the intersystem crossing [1] and the fragmentation of phenyl halides have been long known [2]. In the early years of these studies, attention was given more often to photophysics than to photochemistry. This is because the introduction of electron-withdrawing substituents, such as halogen atoms, in the ring further increases the already high stability of aromatic compounds, both in the ground and excited states. Indeed, many aromatic compounds are quite photo-stable and the absorption of a photon leads to reemission rather than to reaction, since excitation causes only a partial decrease of the bonding character in the multiple π -electron array of such compounds and the rigid σ -bond skeleton disfavors geometric modifications. Indeed, stability to chemical agents and to irradiation is characteristic of many Aryl Halides, and this family includes some notoriously persis-tent environmental pollutants, such as DDT and polychlorobiphenyls. However, photochemistry, albeit often inefficient, is one of the few available paths for the degradation of such compounds and thus this is 16 Photochemistry of Aryl Halides 16.1 Introduction ...................................................................................... 369 16.2 Effect of Halide Substituents on the Photophysical Parameters of Aromatics ................................................................. 370 16.3 General Scheme of Photoreactivity ................................................ 372 16.4 Photochemical Reactions Not Involving theuni00A0Carbon–Halogen Bond .............................................................
eBook - PDF- Stanley E. Manahan(Author)
- 2009(Publication Date)
- CRC Press(Publisher)
These compounds consist of halogen-substituted hydrocarbon molecules, properties. These compounds consist of halogen-substituted hydrocarbon molecules, each of which contains at least one atom of F, Cl, Br, or I. They may be saturated each of which contains at least one atom of F, Cl, Br, or I. They may be saturated ( alkyl halides alkyl halides ), unsaturated ( ), unsaturated ( alkenyl halides alkenyl halides ), or aromatic ( ), or aromatic ( aromatic halides aromatic halides ). The ). The most widely manufactured organohalide compounds are chlorinated hydrocarbons, most widely manufactured organohalide compounds are chlorinated hydrocarbons, many of which are regarded as environmental pollutants or as hazardous wastes. many of which are regarded as environmental pollutants or as hazardous wastes. Alkyl Halides Alkyl Halides Substitution of halogen atoms for one or more hydrogen atoms on alkanes gives Substitution of halogen atoms for one or more hydrogen atoms on alkanes gives alkyl halides alkyl halides , example structural formulas of which are given in Figure 9.11. Most , example structural formulas of which are given in Figure 9.11. Most of the commercially important alkyl halides are derivatives of alkanes of low of the commercially important alkyl halides are derivatives of alkanes of low molecular mass. A brief discussion of the uses of the compounds listed in Figure 9.11 molecular mass. A brief discussion of the uses of the compounds listed in Figure 9.11 is given here to provide an idea of the versatility of the alkyl halides. is given here to provide an idea of the versatility of the alkyl halides. Dichloromethane Dichloromethane is a volatile liquid with excellent solvent properties for is a volatile liquid with excellent solvent properties for nonpolar organic solutes. It has been used as a solvent for the decaffeination of nonpolar organic solutes.
eBook - PDF- Andy Parsons(Author)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
5. ALKYL HALIDES Key point. Alkyl halides are composed of an alkyl group bonded to a halogen atom (X = F, Cl, Br, I). As halogen atoms are more elec- tronegative than carbon, the CX bond is polar and nucleophiles can attack the slightly positive carbon atom. This leads to the halogen atom being replaced by the nucleophile in a nucleophilic substitution reaction, and this can occur by either an S N 1 (two-step) mechanism or an S N 2 (concerted or one-step) mechanism. In competition with substitution is elimination, which results in the loss of HX from alkyl halides to form alkenes. This can occur by either an E1 (two-step) mechanism or an E2 (concerted ) mechanism. The mechanism of the substitution or elimination reaction depends on the alkyl halide, the solvent and the nucleophile/base. 5.1 Structure Alkyl halides have an alkyl group joined to a halogen atom by a single bond. The larger the size of the halogen atom (X = I > Br > Cl), the weaker the CX bond (Appendix 1). The CX bond is polar, and the carbon atom bears a slight positive charge and the electronegative halogen atom bears a slight negative charge. Alkyl halides have an sp 3 carbon atom and hence have a tetrahedral shape. • An aliphatic alkyl halide has the carbon atoms in a chain and not a closed ring, e.g. 1-bromohexane, CH 3 (CH 2 ) 5 Br. • An alicyclic alkyl halide has the carbon atoms in a closed ring, but the ring is not aromatic, e.g. bromocyclohexane, C 6 H 11 Br. • An aromatic alkyl halide has the carbon atoms in a closed ring, and the ring is aromatic, e.g. bromobenzene, C 6 H 5 Br. 5.2 Preparation 5.2.1 Halogenation of alkanes Alkyl chlorides or bromides can be obtained from alkanes by reaction with chlorine or bromine gas, respectively, in the presence of UV light. The reaction involves a radical chain mechanism. R 3 C R C R R X X = F, Cl, Br, I δ+ δ– R = alkyl or H sp 3 hybridisation (tetrahedral) X
eBook - ePubOrganic Chemistry Study Guide
Key Concepts, Problems, and Solutions
- Robert J. Ouellette, J. David Rawn(Authors)
- 2014(Publication Date)
- Elsevier(Publisher)
24 Aryl Halides, Phenols, and Anilines Keys to the Chapter This chapter is organized around the chemistry of halogens, hydroxy! groups, and amino groups when they are bonded to an aromatic ring. Most of the chapter focuses on the chemistry unique to those functional groups precisely because they are bonded to an aromatic ring. A comparison with the chemical reactions that we have encountered before for these functional groups illustrates the decreased reactivity that is a consequence of the aromatic ring. 24.1 Properties of Aromatic Compounds Based on our discussions of electrophilic aromatic substitution in Chapter 13, we know that electronegative atoms with lone-pair electrons affect the electron density of the aromatic ring and therefore the reactivity of the ring toward electrophilic aromatic substitution. The electron withdrawing inductive effect of the electronegative atom, and its ability to donate its lone pair electrons by resonance are both important. We recall that chlorine inductively withdraws electron density, and is a poor donor of electrons by resonance. Both oxygen and nitrogen inductively withdraw electron density, with oxygen being the strongest because it is more electronegative. Both oxygen and nitrogen are electron donors by resonance, but nitrogen is the more effective because it is less electronegative. The net effect is that both oxygen and nitrogen are net donors of electron density with nitrogen being more effective. It follows that the properties of the chlorine, oxygen, and nitrogen atoms bonded to the aromatic ring are themselves changed. The C—Cl bond in aryl chlorides is shorter than in alkyl chlorides because the sp 2 -hybridized bond of the aromatic ring carbon atom has a larger % s character. In addition, the bond energy is larger. The C—O and C—N bonds are also shorter in aromatic compounds than in saturated compounds for the same reason
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