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Introduction to Alkyl Halides, Alcohols, Ethers, Thiols, and Sulfides

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Introduction to Alkyl Halides, Alcohols, Ethers, Thiols, and Sulfides 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 323 8 Introduction to Alkyl 8 Halides, Alcohols, Ethers, Thiols, and Sulfides This chapter covers the nomenclature and properties of several classes of compounds. They are considered together because their chemical reactions are closely related. In an alkyl halide, a halogen atom is bonded to the carbon of an alkyl group. Alkyl halides are classified as methyl, primary, secondary, or tertiary, depending on the number of alkyl groups (red in the following structures) attached to the carbon bearing the halogen. A methyl halide has no alkyl groups, a primary halide has one, a secondary halide has two, and a tertiary halide has three. CH2CH3 H3C Br H3C CH2 Br H3C CH Br H3C "C Br L L L L L L L methyl bromide a primary "CH CH "CH(CH ) alkyl bromide 2 3 3 2 a secondary a tertiary alkyl bromide alkyl bromide In an alcohol, a hydroxy group, OH, is bonded to the carbon of an alkyl group. Alcohols, too, are classified as methyl, primary,L secondary, or tertiary. CH2CH3 H3C OH H3C CH2 OH H3C CH OH H3C "C OH L L L L L L L methyl alcohol a primary alcohol "CH2CH3 "CH(CH3)2 a secondary alcohol a tertiary alcohol Compounds that contain two or more hydroxy groups on adjacent carbons are called gly- cols. The simplest glycol is ethylene glycol, the main component of automotive anifreeze. HOCH2 CH2 OH LLL ethylene glycol (1,2-ethanediol) 323 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 324 324 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES Thiols, sometimes called mercaptans, are the sulfur analogs of alcohols. In a thiol, a sulfhydryl group, SH, also called a mercapto group, is bonded to an alkyl group. An ex- ample of a thiol is ethanethiolL (ethyl mercaptan), CH3CH2 SH. In an ether, an oxygen is bonded to two carbon groups, whichL may or may not be the same. A thioether, or sulfide, is the sulfur analog of an ether. CH3CH2 O CH2CH3 CH3CH2 O CH(CH3)2 H3CS CH2CH3 LL LL LL diethyl ether ethyl isopropyl ether ethyl methyl sulfide The introduction to the functional groups in this chapter is followed by chapters that de- scribe, in turn, the chemistry of each group. 8.1 NOMENCLATURE Several systems are recognized by the IUPAC for the nomenclature of organic compounds. Substitutive nomenclature, the most broadly applicable system, was introduced in the nomenclature of both alkanes (Sec. 2.4C) and alkenes (Sec. 4.2A), and will be applied to the compound classes in this chapter as well. Another widely used system that will be introduced in this chapter is called radicofunctional nomenclature by the IUPAC; for simplicity, this system will be called common nomenclature. Common nomenclature is generally used only for the simplest and most common compounds. Although the adoption of a single nomencla- ture system might seem desirable, historical usage and other factors have dictated the use of both common and substitutive names. A. Nomenclature of Alkyl Halides Common Nomenclature The common name of an alkyl halide is constructed from the name of the alkyl group (see Table 2.2) followed by the name of the halide as a separate word. STUDY GUIDE LINK 8.1 Common Nomenclature CH3CH2 Cl CH2Cl2 L ethyl chloride methylene chloride ( CH2 group methylene group) L L = CH3CH2CH2CH2 Br (CH3)2CH I L L butyl bromide isopropyl iodide The common names of the following compounds should be learned. H2CA CH CH2 Cl Ph CH2 Br H2CA CH Cl CCl4 L L L L L allyl chloride benzyl bromide vinyl chloride carbon tetrachloride (Compounds with halogens attached to alkene carbons, such as vinyl chloride, are not alkyl halides, but it is convenient to discuss their nomenclature here.) The allyl group, as the structure of allyl chloride implies, is the H2CACH CH2 group. This should not be confused with the vinyl group, H2CACH , which lacksL the ad-L ditional CH2 . Similarly, the benzyl group, Ph CH2 , shouldL not be confused with the phenylL group.L L L 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 325 8.1 NOMENCLATURE 325 or Ph CH2 or Ph CH2 cL L cL L L L phenyl group benzyl group The haloforms are the methyl trihalides. HCCl3 HCBr3 HCI3 chloroform bromoform iodoform Substitutive Nomenclature The IUPAC substitutive name of an alkyl halide is con- structed by applying the rules of alkane and alkene nomenclature (Secs. 2.4C and 4.2A). Halo- gens are always treated as substituents; the halogen substituents are named fluoro, chloro, bromo, or iodo. F CH3CH2 Cl Br H3C"CH CH2CH2CH3 L 0L LL chloroethane 2-fluoropentane bromocyclohexane H3CCH CH CH2CH2CH3 CH3CH2CHCH CH2CH3 LLL LL ""Cl CH3 CH3"CH2 "I 2-chloro-3-methylhexane 3-ethyl-4-iodohexane H3C CHA CH CH2 CH2 Cl LLL L 5-chloro-2-pentene PROBLEMS 8.1 Give the common name for each of the following compounds, and tell whether each is a pri- mary, secondary, or tertiary alkyl halide. (a) (CH3)2CHCH2 F (b) CH3CH2CH2CH2CH2CH2 I L L (c) Br (d) CH3 L H3CC"C l LL "CH3 8.2 Give the structure of each of the following compounds. (a) 2,2-dichloro-5-methylhexane (b) chlorocyclopropane (c) 6-bromo-1-chloro-3-methylcyclohexene (d) methylene iodide 8.3 Give the substitutive name for each of the following compounds. (a) CHL 3 (b) H3C Cl (c) H3CCH CH CCl3 LLL Cl $CCA ) ""Br F LL H) $ CH2CH3 Br 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 326 326 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES (d) chloroform (e) neopentyl bromide (see Table 2.2) (f) Br (g) Cl CH3 M ) ) V Br M ) ` CH(CH3)2 B. Nomenclature of Alcohols and Thiols Common Nomenclature The common name of an alcohol is derived by specifying the alkyl group to which the OH group is attached, followed by the separate word alcohol. L H3C OH (CH3)2CH OH OH CH3CH2CH2 OH L L 0L L methyl alcohol isopropyl alcohol propyl alcohol cyclohexyl alcohol H2CA CH CH2 OH Ph CH2 OH L L L L allyl alcohol benzyl alcohol A few glycols have important traditional names. HO CH2CH2 OH CH2 CH CH3 CH2 CH CH2 L L L L L L ethylene glycol "OH "OH "OH" OH "OH propylene glycol glycerol (glycerin) Thiols are named in the common system as mercaptans; this name, which means “captures mercury,” comes from the fact that thiols readily form heavy-metal derivatives (Sec. 8.6A). CH3CH2 SH L ethyl mercaptan Substitutive Nomenclature The substitutive nomenclature of alcohols and thiols in- volves an important concept of nomenclature called the principal group. The principal group is the chemical group on which the name is based, and it is always cited as a suffix in the name. For example, in a simple alcohol, the OH group is the principal group, and its suffix is ol. The name of an alcohol is constructedL by dropping the final e from the name of the parent alkane and adding this suffix. CH3CH2 OH L ethane ol ethanol + = The final e is generally dropped when the suffix begins with a vowel; otherwise, it is retained. For simple thiols, the SH group is the principal group, and its suffix is thiol. The name is constructed by adding thisL suffix to the name of the parent alkane. Note that because the suf- fix begins with a consonant, the final e of the alkane name is retained. 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 327 8.1 NOMENCLATURE 327 CH3CH2 SH L ethane thiol ethanethiol + = Only certain groups are cited as principal groups. The OH and SH groups are the only ones in the compound classes considered so far, but othersL will be addedL in later chapters. If a compound does not contain a principal group, it is named as a substituted hydrocarbon in the manner illustrated for the alkyl halides in Sec. 8.1A. The principal group and the principal chain are the key concepts defined and used in the construction of a substitutive name according to the general rules for substitutive nomencla- ture of organic compounds, which follow. The simplest way to learn these rules is to read through the rules briefly and then concentrate on the study problems and examples that follow, letting them guide you through the application of the rules to specific cases. 1. Identify the principal group. When a structure has several candidates for the principal group, the group chosen is the one given the highest priority by the IUPAC. The IUPAC specifies that the OH group receives precedence over the SH group: L L Priority as principal group: OH SH (8.1) L >L A complete list of principal groups and their relative priorities are summarized in Ap- pendix I. 2. Identify the principal carbon chain. The principal chain is the carbon chain on which the name is based (Sec. 2.4C). The principal chain is identified by applying the following criteria in order until a decision can be made: a. the chain with the greatest number of principal groups; b. the chain with the greatest number of double and triple bonds; c. the chain of greatest length; d. the chain with the greatest number of other substituents. These criteria cover most of the cases you’ll encounter. 3. Number the carbons of the principal chain consecutively from one end. In numbering the principal chain, apply the following criteria in order until there is no ambiguity: a.
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