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Home , Alkyl, Allyl group, Methylene group, Methyl group, Phenyl group, Thiol

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8

Introduction to 8 Halides, , , , 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 atom is bonded to the 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 , a , 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 are called gly- cols. The simplest glycol is glycol, the main component of automotive anifreeze.

HOCH2 CH2 OH LLL ethylene glycol (1,2-ethanediol)

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324 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND

Thiols, sometimes called mercaptans, are the analogs of alcohols. In a , 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 , an 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

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 (Sec. 2.4C) and (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 chloride ( CH2 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 attached to carbons, such as vinyl chloride, are not alkyl halides, but it is convenient to discuss their nomenclature here.)

The , as the structure of allyl chloride implies, is the H2CACH CH2 group. This should not be confused with the , H2CACH , which lacksL the ad-L ditional CH2 . Similarly, the , 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

benzyl group

The haloforms are the methyl trihalides.

HCCl3 HCBr3 HCI3 chloroform bromoform

Substitutive Nomenclature The IUPAC substitutive name of an alkyl halide is con- structed by applying the rules of and alkene nomenclature (Secs. 2.4C and 4.2A). Halo- gens are always treated as ; 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 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 ,” 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 ol + = 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 + = 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 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. 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 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. 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 328

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) CH3CH2CHCH3 (b) CH3CHCHA CHCHCH3

"OH ""OH CH2CH2SH

Solution (a) From rule 1, the principal group is the OH group. Because there is only one possibility for the principal chain, rule 2 does not enterL 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, . After dropping the final e and adding the suffix ol, the name is obtained: 2-butanol.

4321 CH3CH2CHCH3 "OH 2-butanol

(b) From rule 1, the principal group is again the OH group, because —OH has precedence over SH. From rules 2a–2c, the principal chain Lis the longest one containing both the OH groupL and the double bond, and therefore it has seven carbons. Numbering the principalL chain in accord with rule 3a gives the OH group the lowest number at carbon-2 and a double bond at carbon-3: L

123 45 CH3CHCHA CHCHCH3

principal group ""OH CH2CH2SH 6 7 principal chain numbering

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 OH principal group is located before the final suffix ol.) Rule 4c requires that the Lat carbon-5 and the SH group at carbon-7 be cited as ordinary substituents. (The substituent name of the SH groupL is the mercapto group.) The name is L

substituent numbers; note the alphabetical citation of substituents

7-mercapto-5-methyl-3-hepten-2-ol

number of the principal group number of the double bond

To name an alcohol containing more than one OH group, the suffixes diol, triol, and so on are added to the name of the appropriate alkaneL without dropping the final e. 1 234 5 H3C CH CH CH2 CH3 L L L L "OH "OH 2,3-pentanediol 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 329

8.1 NOMENCLATURE 329

Study Problem 8.2 Name the following compound.

SH % OH M y % OH

Solution From rule 1, the principal groups are the OH groups. By rule 3a, these groups are given numerical precedence; thus, they receive the numbersL 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 SH group has been eliminated from consideration as the principal

group, it is treated as anL ordinary substituent group by rule 4c. The completed name is thus SH % 6 1 OH 5 "

4 e 2 3 "OH 6-mercapto-4-cyclohexene-1,3-diol

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:

CH3 common: tert-butyl alcohol H3C "C OH substitutive: 2-methyl-2-propanol L L incorrect: t-butanol or tert-butanol "CH3

H3C CH CH3 common: isopropyl alcohol L L substitutive: 2-propanol "OH incorrect: isopropanol 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 330

330 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES

PROBLEMS 8.4 Draw the structure of each of the following compounds. (a) sec-butyl alcohol (b) isobutyl alcohol (c) 3-ethylcyclopentanol (d) 3-methyl-2-pentanol (e) (E)-6-chloro-4-hepten-2-ol (f) 2-cyclohexenol 8.5 Give the substitutive name for each of the following compounds.

(a) CH3CHCH2CH2OH (b) CH3CH2CH2CH2OH "Br

(c) H3C CH3 (d) HO Cl (e) OH A $CC) ) ) H3CCH CH "CH CH3 ) H C LLL L H) $ CHCH2Cl 3 "OH "CH2 "OH "CH2 CH2 CH3 L L

(f) OH (g) CH3CH2CH2CHCH2SH (h) CH3

" "OH H3C"C CH3 L L y "SH

C. Nomenclature of Ethers and Sulfides Common Nomenclature The common name of an ether is constructed by citing as sepa- rate words the two groups attached to the ether oxygen in alphabetical order, followed by the word ether.

CH3CH2 O CH2CH3 H3CCO 2H5 LL LL diethyl ether ethyl methyl ether (also called ethyl ether or simply ether) A sulfide is named in a similar manner, using the word sulfide. (In older literature, the word thioether was also used.)

CH3CH2 S CH3 (CH3)2CHS CH(CH3)2 LL LL ethyl methyl sulfide diisopropyl sulfide (also ethyl methyl thioether)

Substitutive Nomenclature In substitutive nomenclature, ethers and sulfides are never cited as principal groups. Alkoxy groups (RO ) and alkylthio groups (RS ) are always cited as substituents. L L

ethoxy substituent CH3CH2O CH3 methyl substituent

principal chain CH3"CHCH2CH2"CHCH3 2-ethoxy-5-methylhexane In this example, the principal chain is a six-carbon chain. Hence, the compound is named as a

hexane, and the C2H5O group and the methyl group are treated as substituents. The C2H5O group is namedL by dropping the final yl from the name of the alkyl group and adding the suffixL oxy. Thus, the C2H5O group is the (ethy΋΋l oxy) ethoxy group. The number- ing follows from nomenclature ruleL 3d on p. 327. + = 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 331

8.1 NOMENCLATURE 331

The nomenclature of sulfides is similar. An RS group is named by adding the suffix thio to the name of the R group; the final yl is not dropped.L

methylthio substituent SCH3

principal chain CH3"CHCH2CH2CH2CH3 1 23 4 5 6

2-(methylthio)hexane

The parentheses in the name are used to indicate that “thio” is associated with “methyl” rather than with “hexane.”

Study Problem 8.3 Name the following compound.

CH3CH2CH2CH2 O CH2CH2CH2 OH L L L Solution The OH group is cited as the principal group, and the principal chain is the L chain containing this group. Consequently, the CH3CH2CH2CH2O group is cited as a butoxy substituent (buty΋΋l oxy) at carbon-3 of the principal chain: L + 321 CH3CH2CH2CH2 O CH2CH2CH2 OH LL L principal chain (contains the principal group OH) L 3-butoxy-1-propanol

Heterocyclic Nomenclature A number of important ethers and sulfides contain an oxy- gen or sulfur atom within a ring. Cyclic compounds with rings that contain at least one atom other than carbon are called heterocyclic compounds. The names of some common hetero- cyclic ethers and sulfides should be learned.

O O O O S O furan tetrahydrofuran thiophene 1,4-dioxane oxirane (often called THF) (often called simply dioxane) (ethylene oxide)

(The IUPAC name for tetrahydrofuran is oxolane, but this name is not commonly used.) Oxirane is the parent compound of a special class of heterocyclic ethers, called , which are three-membered rings that contain an oxygen atom. A few epoxides are named tra- ditionally as oxides of the corresponding alkenes:

O O

H2C$ $ CH2 H2C A CH2 Ph CH$ $CH2 Ph CH A CH2 L L L ethylene oxide ethylene styrene oxide styrene 08_BRCLoudon_pgs5-1.qxd 12/8/08 11:05 AM Page 332

332 CHAPTER 8 • INTRODUCTION TO ALKYL HALIDES, ALCOHOLS, ETHERS, THIOLS, AND SULFIDES

However, most epoxides are named substitutively as derivatives of oxirane. The atoms of the ring are numbered consecutively, with the oxygen receiving the number 1 regardless of the substituents present.

1 O CH3 H2CC$ $ 3 2 L L CH3 2,2-dimethyloxirane

PROBLEMS 8.6 Draw the structure of each of the following compounds. (a) ethyl propyl ether (b) dicyclohexyl ether (c) dicyclopentyl sulfide (d) tert-butyl isopropyl sulfide (e) allyl benzyl ether (f) phenyl vinyl ether (g) (2R,3R)-2,3-dimethyloxirane (h) 5-(ethylthio)-2-methylheptane 8.7 Give a substitutive name for each of the following compounds.

(a) (CH3)3C O CH3 (b) CH3CH2 O CH2CH2 OH (c) L L (d)L L L CH3OCH2 H S $CCA )

H) $ CH2CH2 OH L 8.8 (a) A chemist used the name 3-butyl-1,4-dioxane in a paper. Although the name unambigu- ously describes a structure, what should the name have been? Explain. (b) Give the structure of 2-butoxyethanol, which is an ingredient in whiteboard cleaner and kitchen cleaning sprays.

8.2 STRUCTURES

In all of the compounds covered in this chapter, the bond angles at carbon are very nearly tetrahedral. For example, in the simple methyl derivatives (the methyl halides, , , dimethyl ether, and dimethyl sulfide) the H C H bond angle in the methyl group does not deviate more than a degree or so from 109.5L. InL an alcohol, thiol, ether, or sul- fide, the bond angle at oxygen or sulfur further defines the shape° of the . You learned in Sec. 1.3B that the shapes of such can be predicted by thinking of an unshared electron pair as a bond without an atom at the end. This means that the oxygen or sulfur has four “groups”: two electron pairs and two alkyl groups or . These molecules are therefore bent at oxygen and sulfur, as you can see from the structures in Fig. 8.1. The angle at sulfur is generally found to be closer to 90 than the angle at oxygen. One reason for this trend is that the unshared electron pairs on sulfur° occupy orbitals derived from energy level 3 that take up more space than those on oxygen, which are derived from level 2. The repulsion between these unshared pairs and the electrons in the chemical bonds forces the bonds closer together than they are on oxygen. The lengths of bonds between carbon and other atoms follow the trends discussed in Sec. 1.3B. Within a column of the periodic table, bonds to atoms of higher atomic number are longer. Thus, the C S bond of methanethiol is longer than the C O bond of methanol (see Fig. 8.1 and Table 8.1).L Within a row, bond lengths decrease toward Lhigher atomic number (that is, to the right). Thus, the C O bond in methanol is longer than the C F bond in methyl fluoride (see Table 8.1); similarly,L the C S bond in methanethiol is longerL than the C Cl bond in methyl chloride. L L