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OC307-6-11-Ch6.1 - Organic Chemistry Nucleophilic...

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Organic Chemistry Nucleophilic Substitution H. D. Roth 1 ORGANIC CHEMISTRY 307 Chapter 6 Alkyl Halides, Nucleophilic Substitution, Elimination In the past five chapters you (we) have reviewed some basic concepts, have studied one reaction type, and have learned to expect that reactions at stereocenters will pose special challenges in any reaction that you will learn about. In this chapter we begin a survey of chemical reactions that can occur at specific functional groups. Our first topic involves the class of haloalkanes, compounds that you have learned to prepare by free- radical halogenation of hydrocarbons. I. Haloalkanes – Nomenclature and Properties A. Nomenclature The halogen functional group is treated by the IUPAC system on a par with alkyl groups substituted on the main chain. The pertinent rules concerning longest chain, lowest number, alphabetical ranking stay in effect: any halogens are alphabetized along with the alkyls. Some simple haloalkanes also can be named as alkyl halides, such as isobutyl bromide, CH 3 CH(CH 3 )CH 2 Br, sec -butyl bromide, CH 3 CHBrCH 2 CH 3 , or tert - butyl bromide, (CH 3 ) 3 CBr. Haloalkanes that have more than one halogen often have common names such as CHCl 2 (methylene chloride), CHCl 3 (chloroform), CHI 3 (iodoform, an antiseptic) and CCl 4 (carbon tetrachloride). Most of these have characteristic odors. Halogen-containing alkanes in which all hydrogens have been replaced by halogens are called perhaloalkanes, for example, perchloroethane. Some of these are used as dry cleaning solvents. Freons (chlorofluorocarbons) have been used as refrigerants;
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Organic Chemistry Nucleophilic Substitution H. D. Roth 2 their use has been limited, because they are implicated in destruction of the ozone layer in the stratosphere. Halogenated alkanes as well as cyclic and bi- and tricyclic alkanes have been used as pesticides B. Bonding and Properties Halogens are highly electronegative; carbon–halogen bonds are polar covalent bonds, involving an sp 3 hybridized carbon. Important trends in the halogens (Column VIIA of the periodic table, top to bottom, F –> I ): increasing size (F is very small) decreasing electronegativity decreasing bond polarity increasing polarizability increasing bond length decreasing bond strength increasing boiling points Size: Polarizability:
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Organic Chemistry Nucleophilic Substitution H. D. Roth 3 Polarity: The boiling points of the haloalkanes reflect a) the polarity of the C-X bond and b) the polarizability of the larger halogens. These factors favor dipolar interactions; as a result, the boiling points of haloalkanes are higher than the corresponding alkanes (Table 6.2). The densities of the haloalkanes, especially bromo- and iodoalkanes, are high compared to the corresponding alkane. You can understand this because the volume of even the larger halogens is smaller than or comparable to that of a methyl group, but their mass is much greater. The C-F bond is stronger than C-H bonds, all other C-X bonds are weaker; therefore haloalkanes are much more reactive than alkanes.
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