Introduction to Alkyl Halides, Alcohols, Ethers, Thiols, and ..., Schemes and Mind Maps of Construction

Download Introduction to Alkyl Halides, Alcohols, Ethers, Thiols, and ... and more Schemes and Mind Maps Construction in PDF only on Docsity! 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. In an alcohol, a hydroxy group, LOH, is bonded to the carbon of an alkyl group. Alcohols, too, are classified as methyl, primary, secondary, or tertiary. 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.L LCH2 L CH2HO OH ethylene glycol (1,2-ethanediol) "LH3C OH LH3C CH2 L OH LH3C CH2CH3 CH L OH " "LH3C CH(CH3)2 CH2CH3 C L OH methyl alcohol a primary alcohol a secondary alcohol a tertiary alcohol "LH3C Br LH3C CH2 L Br LH3C CH2CH3 CH L Br " "LH3C CH(CH3)2 CH2CH3 C L Br methyl bromide a primary alkyl bromide a secondary alkyl bromide a tertiary alkyl bromide 323 Introduction to Alkyl Halides, Alcohols, Ethers, Thiols, and Sulfides 8 8 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 323 Thiols, sometimes called mercaptans, are the sulfur analogs of alcohols. In a thiol, a sulfhydryl group, LSH, also called a mercapto group, is bonded to an alkyl group. An ex- ample of a thiol is ethanethiol (ethyl mercaptan), CH3CH2LSH. In an ether, an oxygen is bonded to two carbon groups, which may or may not be the same. A thioether, or sulfide, is the sulfur analog of an ether. 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. The common names of the following compounds should be learned. (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 H2CACHLCH2L group. This should not be confused with the vinyl group, H2CACHL , which lacks the ad- ditional LCH2L . Similarly, the benzyl group, PhLCH2L , should not be confused with the phenyl group. H2C CH L CH2 L Cl allyl chloride Ph L CH2 L Br benzyl bromide A H2C CH L Cl vinyl chloride A CCl4 carbon tetrachloride CH2Cl2LCH3CH2 Cl ethyl chloride methylene chloride (L CH2 L group = methylene group) LCH3CH2CH2CH2 Br butyl bromide L(CH3)2CH I isopropyl iodide L LCH3CH2 CH2CH3O diethyl ether L LCH3CH2 CH(CH3)2O ethyl isopropyl ether L LH3C CH2CH3S ethyl methyl sulfide 324 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES STUDY GUIDE LINK 8.1 Common Nomenclature 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 324 8.1 NOMENCLATURE 327 Only certain groups are cited as principal groups. The LOH and LSH groups are the only ones in the compound classes considered so far, but others will be added 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 LOH group receives precedence over the LSH group: Priority as principal group: LOH > LSH (8.1) 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. the lowest numbers for the principal groups; b. the lowest numbers for multiple bonds, with double bonds having priority over triple bonds in case of ambiguity; c. the lowest numbers for other substituents; d. the lowest number for the substituent cited first in the name. 4. Begin construction of the name with the name of the hydrocarbon corresponding to the principal chain. a. Cite the principal group by its suffix and number; its number is the last one cited in the name. (See the examples in Study Problem 8.1.) b. If there is no principal group, name the compound as a substituted hydrocarbon. (See Secs. 2.4C and 4.2A.) c. Cite the names and numbers of the other substituents in alphabetical order at the be- ginning of the name. CH3CH2 L SH ethane + thiol = ethanethiol 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 327 328 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES Study Problem 8.1 Provide an IUPAC substitutive name for each of the following compounds. (a) (b) Solution (a) From rule 1, the principal group is the LOH group. Because there is only one possibility for the principal chain, rule 2 does not enter the picture. By applying rule 3a, we decide that the principal group is located at carbon-2. From rule 4a, the name is based on the four-carbon hydrocarbon, butane. After dropping the final e and adding the suffix ol, the name is obtained: 2-butanol. (b) From rule 1, the principal group is again the LOH group, because —OH has precedence overLSH. From rules 2a–2c, the principal chain is the longest one containing both the LOH group and the double bond, and therefore it has seven carbons. Numbering the principal chain in accord with rule 3a gives the LOH group the lowest number at carbon-2 and a double bond at carbon-3: By applying rule 4a, we decide that the parent hydrocarbon is 3-heptene, from which we drop the final e and add the suffix ol, to give 3-hepten-2-ol as the final part of the name. (Notice that be- cause we have to cite the number of the double bond, the number for the LOH principal group is located before the final suffix ol.) Rule 4c requires that the methyl group at carbon-5 and theLSH group at carbon-7 be cited as ordinary substituents. (The substituent name of the LSH group is the mercapto group.) The name is To name an alcohol containing more than one LOH group, the suffixes diol, triol, and so on are added to the name of the appropriate alkane without dropping the final e. H3C CH OH " 1 L CHL CH2L CH3L2 3 4 5 2,3-pentanediol OH " 7-mercapto-5-methyl-3-hepten-2-ol number of the principal group number of the double bond substituent numbers; note the alphabetical citation of substituents CH3CHCH CHCHCH3 OHprincipal group CH2CH2SH " "A 1 2 3 4 6 5 7 principal chain numbering CH3CH2CHCH3 OH " 2-butanol 4 3 2 1 CH3CHCH CHCHCH3 OH CH2CH2SH " "ACH3CH2CHCH3 OH " 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 328 8.1 NOMENCLATURE 329 Study Problem 8.2 Name the following compound. Solution From rule 1, the principal groups are the LOH groups. By rule 3a, these groups are given numerical precedence; thus, they receive the numbers 1 and 3. Because two numbering schemes give these groups the numbers 1 and 3, we choose the scheme that gives the double bond the lower number, by rule 3b. From rule 4a, the parent hydrocarbon is cyclohexene, and because the suffix is diol, the final e is retained to give the partial name 4-cyclohexene-1,3-diol. Finally, notice that because the LSH group has been eliminated from consideration as the principal group, it is treated as an ordinary substituent group by rule 4c. The completed name is thus In a sidebar on p. 134 we introduced the 1993 IUPAC nomenclature recommendations. Although we are continuing to use the 1979 recommendations for the reasons given there, conversion of most names to the 1993 recommendations is not difficult. The handling of double bonds, triple bonds, and principal groups is the major change introduced by the 1993 recommendations. In the 1993 system, the number of the double bond, triple bond, or principal group immediately precedes the citation of the group in the name. Thus, the name of the compound in Study Problem 8.1(a) would be butan-2- ol rather than 2-butanol. The name of the compound in Study Problem 8.1(b) would be 7-mercapto- 5-methylhept-3-en-2-ol. The name 2,3-butanediol would be changed to butane-2,3-diol, and the name of the compound in Study Problem 8.2 would become 6-mercaptocyclohex-4-ene-1,3-diol. As in the 1979 nomenclature, the final e of the hydrocarbon name is dropped when the suffix begins in a vowel. Common and substitutive nomenclature should not be mixed. This rule is frequently violated in naming the following compounds: OH CH3 CH3 H3C LCL" " CH3 OH H3C LCHL" common: substitutive: incorrect: tert-butyl alcohol 2-methyl-2-propanol t-butanol or tert-butanol common: substitutive: incorrect: isopropyl alcohol 2-propanol isopropanol OH OH SH " " e%6 4 3 5 2 1 6-mercapto-4-cyclohexene-1,3-diol yM % % OH OH SH 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 329