258 ARENES AND AROMATICITY 11.16 Resonance structures are generated for cyclopentadienide anion by moving the unshared electron pair from the carbon to which it is attached to a position where it becomes a shared electron pair in a丌bond H H 11. 17 The process is an acid-base reaction in which cyclopentadiene transfers a proton to amide ion( the base)to give the aromatic cyclopentadienide anion. The sodium ion(Nat) has been omitted from the equation. H H, H NH3 ,3-Cyclopentadiene Amide ion Cyclopentadienide Ammonia 11.18 (b) Cyclononatetraenide anion has 10 T electrons; it is aromatic. The 10 T electrons are most easily seen by writing a Lewis structure for the anion: there are 2 T electrons for each of four double bonds, and the negatively charged carbon contributes 2 11.19 Indole is more stable than isoindole. Although the bonding patterns in both five-membered rings are the same, the six-membered ring in indole has a pattern of bonds identical to benzene and so is highly stabilized. The six-membered ring in isoindole is not of the benzene type Six-membered Six-membered ring does nh not have same pattern of to benzene bonds as benzene Indole Isoindole more stable less stable 11.20 The prefix benz- in benzimidazole(structure given in text) signifies that a benzene ring is fused to n imidazole ring. By analogy, benzoxazole has a benzene ring fused to oxazole Benzimidazole Benzoxazole Similarly, benzothiazole has a benzene ring fused to thiazole Benzothiazole Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
11.16 Resonance structures are generated for cyclopentadienide anion by moving the unshared electron pair from the carbon to which it is attached to a position where it becomes a shared electron pair in a bond. 11.17 The process is an acid–base reaction in which cyclopentadiene transfers a proton to amide ion (the base) to give the aromatic cyclopentadienide anion. The sodium ion (Na) has been omitted from the equation. 11.18 (b) Cyclononatetraenide anion has 10 electrons; it is aromatic. The 10 electrons are most easily seen by writing a Lewis structure for the anion: there are 2 electrons for each of four double bonds, and the negatively charged carbon contributes 2. 11.19 Indole is more stable than isoindole. Although the bonding patterns in both five-membered rings are the same, the six-membered ring in indole has a pattern of bonds identical to benzene and so is highly stabilized. The six-membered ring in isoindole is not of the benzene type. 11.20 The prefix benz- in benzimidazole (structure given in text) signifies that a benzene ring is fused to an imidazole ring. By analogy, benzoxazole has a benzene ring fused to oxazole. Similarly, benzothiazole has a benzene ring fused to thiazole. S N Benzothiazole N H N Benzimidazole O N Benzoxazole N H Indole more stable NH Isoindole less stable Six-membered ring corresponds to benzene. Six-membered ring does not have same pattern of bonds as benzene. H H NH2 H NH3 1,3-Cyclopentadiene Amide ion Cyclopentadienide anion Ammonia H H H H H H H H H H H H H H H H H H H H H H H H H 258 ARENES AND AROMATICITY Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ARENES AND ARO 259 11.21 Write structural formulas for the species formed when a proton is transferred to either of the two nitrogens of imidazole Protonation ofn-1 The species formed on protonation of N-1 is not aromatic. The electron pair of N-l that contributes to the aromatic 6 T-electron system of imidazole is no longer available for this purpose because it used to form a covalent bond to the proton in the conjugate acid. Protonation of N-3 H The species formed on protonation of N-3 is aromatic. Electron delocalization represented by the resonance forms shown allows the 6 T-electron aromatic system of imidazole to be retained in its conjugate acid. The positive charge is shared equally by both nitrogens H 11.22 Since the problem requires that the benzene ring be monosubstituted, all that needs to be examined are the various isomeric forms of the C4H, substituent. CH,CH,,CH3 CHCHCH (l-phenylbutane) (2-phenylbutane) C CH,CH(CH3) Isobutylbenzene tert-Butylbenzene (2-methyl-l-phenylpropane) (2-methyl-2-phenylpropane) These are the four constitutional isomers. sec-Butylbenzene is chiral and so exists in enantiomeric R and s forms 11.23 (a) An allyl substituent is-CH, CH=CH2 CHCHECH Allylbenzene Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
11.21 Write structural formulas for the species formed when a proton is transferred to either of the two nitrogens of imidazole. Protonation of N-1: The species formed on protonation of N-1 is not aromatic. The electron pair of N-1 that contributes to the aromatic 6 -electron system of imidazole is no longer available for this purpose because it is used to form a covalent bond to the proton in the conjugate acid. Protonation of N-3: The species formed on protonation of N-3 is aromatic. Electron delocalization represented by the resonance forms shown allows the 6 -electron aromatic system of imidazole to be retained in its conjugate acid. The positive charge is shared equally by both nitrogens. 11.22 Since the problem requires that the benzene ring be monosubstituted, all that needs to be examined are the various isomeric forms of the C4H9 substituent. These are the four constitutional isomers. sec-Butylbenzene is chiral and so exists in enantiomeric R and S forms. 11.23 (a) An allyl substituent is @CH2CH?CH2. Allylbenzene CH2CH CH2 Butylbenzene (1-phenylbutane) CH2CH2CH2CH3 sec-Butylbenzene (2-phenylbutane) CHCH2CH3 CH3 Isobutylbenzene (2-methyl-1-phenylpropane) CH2CH(CH3)2 tert-Butylbenzene (2-methyl-2-phenylpropane) C(CH3)3 H H N N N N H H H3O N N H 3 1 H H N N 3 1 H3O N N H 1 N N H H 3 3 1 ARENES AND AROMATICITY 259 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
260 ARENES AND AROMATICITY (b) The constitution of l-phenyl-1-butene is C6HS CH=CHCH, CH3. The E stereoisomer is H CHCH The two higher ranked substituents, phenyl and ethyl, are on opposite sides of the double bond (c) The constitution of 2-phenyl-2-butene is CH, C=CHCH, The Z stereoisomer is C6H5 H,C (Z)-2-Phenyl-2-butene The two higher ranked substituents, phenyl and methyl, are on the same side of the double bond (d) 1-Phenylethanol is chiral and has the constitution CH_ CHC6Hs. Among the substituents attached to the stereogenic center, the order of decreasing precedence is HO >CHs>CH3>H In the R enantiomer the three highest ranked substituents must appear in a clockwise sense in proceeding from higher ranked to next lower ranked when the lowest ranked substituent is di- rected away from you (R)-1-Phenylethanol (e) A benzyl group is CHs CH2. Benzyl alcohol is therefore CBH_ CH,OH and o-chlorobenzyl alcohol is -CH-OH (f) In p-chlorophenol the benzene ring bears a chlorine and a hydroxyl substituent in a 1, 4 (g) Benzenecarboxylic acid is an alternative IuPAC name for benzoic acid CO,H 2-Nitrobenzenecarboxylic acid Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(b) The constitution of 1-phenyl-1-butene is C6H5CH?CHCH2CH3. The E stereoisomer is The two higher ranked substituents, phenyl and ethyl, are on opposite sides of the double bond. (c) The constitution of 2-phenyl-2-butene is . The Z stereoisomer is The two higher ranked substituents, phenyl and methyl, are on the same side of the double bond. (d) 1-Phenylethanol is chiral and has the constitution . Among the substituents attached to the stereogenic center, the order of decreasing precedence is HO C6H5 CH3 H In the R enantiomer the three highest ranked substituents must appear in a clockwise sense in proceeding from higher ranked to next lower ranked when the lowest ranked substituent is directed away from you. (e) A benzyl group is C6H5CH2@. Benzyl alcohol is therefore C6H5CH2OH and o-chlorobenzyl alcohol is ( f ) In p-chlorophenol the benzene ring bears a chlorine and a hydroxyl substituent in a 1,4- substitution pattern. (g) Benzenecarboxylic acid is an alternative IUPAC name for benzoic acid. CO2H NO2 2-Nitrobenzenecarboxylic acid OH Cl p-Chlorophenol CH2OH Cl H3C C OH H (R)-1-Phenylethanol OH CH3CHC6H5 (Z)-2-Phenyl-2-butene H C C CH3 H3C CH3C C6H5 CHCH3 (E)-1-Phenyl-1-butene CH2CH3 C C H H 260 ARENES AND AROMATICITY Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ARENES AND ARO 261 (h) Two isopropyl groups are in a 1, 4 relationship in p-disopropylbenzene CH(CH3)2 CH(CH3)2 (i) Aniline is CSHSNH, Therefore NH 2.4.6-Tribromoaniline G Acetophenone(from text Table Il. 1)is C6H-CCH3 Therefore C () Styrene is CH CH=CH, and numbering of the ring begins at the carbon that bears the side chain CHCH 4-Bromo-3-ethylstyrene 11.24 (a) Anisole is the name for CH_OCH,, and allyl is an acceptable name for the group HIC=CHCH2-. Number the ring beginning with the carbon that bears the methoxy group (b) Phenol is the name for CHsOH. The ring is numbered beginning at the carbon that bears the hydroxyl group, and the substituents are listed in alphabetical order (c) Aniline is the name given to CHSNH,. This compound is named as a dimethyl derivative of aniline Number the ring sequentially beginning with the carbon that bears the amino group OCH NH C CH CH,CH=CH, Estragole 4-Allvlanisole 2,6-D.Diosphenol m-Xylidine Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(h) Two isopropyl groups are in a 1,4 relationship in p-diisopropylbenzene. (i) Aniline is C6H5NH2. Therefore (j) Acetophenone (from text Table 11.1) is . Therefore (k) Styrene is and numbering of the ring begins at the carbon that bears the side chain. 11.24 (a) Anisole is the name for C6H5OCH3, and allyl is an acceptable name for the group . Number the ring beginning with the carbon that bears the methoxy group. (b) Phenol is the name for C6H5OH. The ring is numbered beginning at the carbon that bears the hydroxyl group, and the substituents are listed in alphabetical order. (c) Aniline is the name given to C6H5NH2. This compound is named as a dimethyl derivative of aniline. Number the ring sequentially beginning with the carbon that bears the amino group. OCH3 CH2CH CH2 1 6 2 5 3 4 Estragole 4-Allylanisole OH I I NO2 1 6 2 5 3 4 Diosphenol 2,6-Diiodo-4-nitrophenol NH2 H3C CH3 1 6 2 5 3 4 m-Xylidine 2,6-Dimethylaniline H2C CHCH2 CH3CH2 Br CH CH2 3 4 1 5 6 2 4-Bromo-3-ethylstyrene C6H5CH CH2 C H3C NO2 O m-Nitroacetophenone C6H5CCH3 O NH2 Br Br Br 2,4,6-Tribromoaniline p-Diisopropylbenzene CH(CH3)2 CH(CH3)2 ARENES AND AROMATICITY 261 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website