Organic Chemistry, 5th ed. Marc Loudon Chapter 16 The Chemistry of Benzene Deriva4ves Eric J. Kantorowski California Polytechnic State University San Luis Obispo, CA
Chapter 16 Overview 16.1 Nomenclature of Benzene Deriva6ves 16.2 Physical Proper6es of Benzene Deriva6ves 16.3 Spectroscopy of Benzene Deriva6ves 16.4 Electrophilic Aroma6c Subs6tu6on Reac6ons of Benzene 16.5 Electrophilic Aroma6c Subs6tu6on Reac6ons of Subs6tuted Benzenes 16.6 Hydrogena6on of Benzene Deriva6ves 16.7 Source and Industrial Use of Aroma6c Hydrocarbons 2
Nomenclature Subs6tu6ve nomenclature Note the resonance hybrids for the NO 2 group 16.1 Nomenclature of Benzene Deriva4ves 3
Common names Nomenclature Ortho, meta, and para prefixes 16.1 Nomenclature of Benzene Deriva4ves 4
Principle groups Nomenclature Other common names 16.1 Nomenclature of Benzene Deriva4ves 5
Nomenclature Numbers are required when more than two subs6tuents are present 16.1 Nomenclature of Benzene Deriva4ves 6
Nomenclature The benzene ring may also be a subs6tuent The phenyl group = C 6 H 5 = Ph The benzyl group = C 6 H 5 CH 2 16.1 Nomenclature of Benzene Deriva4ves 7
Physical Proper<es Benzene deriva6ves and hydrocarbons have similar boiling points Note how the mel6ng points of the highly symmetrical compounds are unusually high 16.2 Physical Proper4es of Benzene Deriva4ves 8
Physical Proper<es Para disubs6tuted deriva6ves typically have mp s higher than those of ortho and meta Benzene and other aroma6c hydrocarbon deriva6ves are insoluble in water If H bonding is possible, then water solubility is improved (e.g., phenol, PhOH) 16.2 Physical Proper4es of Benzene Deriva4ves 9
IR Spectroscopy 16.3 Spectroscopy of Benzene Deriva4ves 10
Benzene: δ 7.4 1 H NMR Spectroscopy Most benzene deriva6ves: δ 7.0 8.0 If an unknown compound has an unsatura6on number of 4, immediately check this region Note: Vinyl protons of alkenes are at δ 5.0 5.7 Why do benzene deriva6ves appear more downfield? 16.3 Spectroscopy of Benzene Deriva4ves 11
Ring Current in NMR Spectroscopy 16.3 Spectroscopy of Benzene Deriva4ves 12
Ring Current in NMR Spectroscopy Ring current is believed to be the best experimental evidence of aroma6c character Outer protons: 9.28 ppm Inner protons: 2.99 ppm (!) 16.3 Spectroscopy of Benzene Deriva4ves 13
1 H NMR Spectroscopy Benzylic protons also experience a downfield shid rela6ve to ordinary alkyl absorp6ons The OH absorp6on in phenols is typically higher (δ5 6) than that of alcohols (δ2 3) 16.3 Spectroscopy of Benzene Deriva4ves 14
Coupling Constants of Aroma<c Protons 16.3 Spectroscopy of Benzene Deriva4ves 15
Para Subs<tuted Benzenes A pair of leaning doublets with a large ortho coupling is characteris6c 16.3 Spectroscopy of Benzene Deriva4ves 16
13 C NMR Spectroscopy Benzene: δ 128.5 Most benzene deriva6ves: δ 110 160 Benzylic carbons: δ 18 30 Recall that carbons that do not bear Hs are considerably smaller in size 16.3 Spectroscopy of Benzene Deriva4ves 17
UV Vis Spectroscopy Aroma6c hydrocarbons typically have two bands: 210 nm (strong), 260 (weak) Subs6tuents can alter both the λ max values and the intensi6es This is more pronounced when the subs6tuent can extend conjuga6on of the ring 16.3 Spectroscopy of Benzene Deriva4ves 18
UV Vis Spectroscopy 16.3 Spectroscopy of Benzene Deriva4ves 19
Electrophilic Aroma<c Subs<tu<on A hydrogen of an aroma6c ring is replaced by an electrophile All electrophilic aroma4c subs4tu4on reac4ons occur by similar mechanisms 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 20
Halogena<on of Benzene Compare with the addi4on product for that of an alkene 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 21
Mechanism of Halogena<on 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 22
Mechanism of Halogena<on 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 23
Three Mechanis<c Steps in EAS 1. Genera6on of an electrophile 2. Nucleophilic reac6on of the π electrons of the aroma6c ring with the electrophile 3. Loss of a proton from the carboca6on to form a subs6tuted aroma6c compound 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 24
Nitra<on of Benzene Mechanism: 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 25
Nitra<on of Benzene 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 26
Sulfona<on of Benzene Mechanism: 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 27
Friedel CraMs Alkyla<on of Benzene Mechanism: 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 28
Friedel CraMs Alkyla<on of Benzene Compare the role of AlCl 3 with that of FeBr 3 in the bromina6on reac6on 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 29
Rearrangements in Friedel CraMs Alkyla<ons Stable carboca6ons do not rearrange 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 30
Overalkyla<on in Friedel CraMs Alkyla<ons The alkyl benzene products are typically more reac4ve than benzene itself A large excess of benzene can minimize this problem 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 31
Friedel CraMs Alkyla<ons Alkenes and alcohols can also be used as alkyla6ng reagents 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 32
Friedel CraMs Acyla<on of Benzene The acyl group is typically derived from an acid chloride 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 33
Friedel CraMs Acyla<on of Benzene Mechanism: 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 34
Friedel CraMs Acyla<on of Benzene In contrast to the alkyla6on version, one equivalent of AlCl 3 is required 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 35
Intramolecular Friedel CraMs Reac<ons Both the alkyla6on and acyla6ons can occur intramolecularly The reac6on occurs at an ortho posi6on and is best for five and six membered rings 16.4 Electrophilic Aroma4c Subs4tu4on Reac4ons of Benzene 36
Direc<ng Effects of Subs<tuents Regioselec6vity is observed in EAS reac6ons on monosubs6tuted benzene rings Ortho, para direc6ng: Meta direc6ng: 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 37
Direc<ng and Ac<va<ng Effects 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 38
Alkyl groups Ortho, Para Direc<ng Groups Groups that have unshared electron pairs on atoms directly akached to the benzene ring 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 39
Ortho, Para Direc<ng Groups The charge on the meta derived intermediate cannot be delocalized onto the OCH 3 group 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 40
Ortho, Para Direc<ng Groups 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 41
Ortho, Para Direc<ng Groups Alkyl subs6tuted benzene rings have a similar explana6on 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 42
The Ortho, Para Ra<o Rarely do o,p directors provide twice as much ortho product as para product Typically para predominates over ortho Some6mes this is due to spa6al demands, but many cases are less easily explained Fortunately, ortho and para products have different physical proper6es and can be separated 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 43
Meta Direc<ng Groups Groups that have posi6ve charges adjacent to the benzene ring Groups that have bond dipoles with a par6al posi6ve charge adjacent to the benzene ring 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 44
Meta Direc<ng Groups 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 45
Meta Direc<ng Groups 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 46
Ac<va<ng and Deac<va<ng Effects Ac<va<ng group: A group that allows the deriva6ve to react more rapidly than benzene Deac<va<ng group: A group that causes the deriva6ve to react more slowly than benzene 1. All meta directors are deac6va6ng 2. All ortho, para directors are ac6va6ng 3. Halogens are deac6va6ng 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 47
Ac<va<ng and Deac<va<ng Effects Controlled by two simultaneously opera4ng proper6es of subs6tuents Resonance effect: Polar effect: 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 48
Ac<va<ng and Deac<va<ng Effects 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 49
Ac<va<ng and Deac<va<ng Effects Orbital overlap may also affect the degree to which the resonance effect operates 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 50
Use of EAS in Organic Synthesis With two or more subs6tuents, the ac6va6ng and direc6ng effects are roughly the sum of the effects of the individual subs6tuents 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 51
Use of EAS in Organic Synthesis 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 52
Use of EAS in Organic Synthesis Some EAS reac6ons can be carried out under very mild condi6ons and without a catalyst 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 53
Use of EAS in Organic Synthesis 16.5 Electrophilic Aroma4c Subs4tu4on Reac4ons of Subs4tuted Benzenes 54
Hydrogena<on Aroma6c rings are resistant to hydrogena6on More extreme condi6ons are generally required (higher T and/or P) 16.6 Hydrogena4on of Benzene Deriva4ves 55
Hydrogena<on The reac6on cannot be selec6vely stopped 16.6 Hydrogena4on of Benzene Deriva4ves 56
Aroma<c Hydrocarbons The most common source of aroma6c hydrocarbons is petroleum Coal tar is another poten6ally important, but currently minor source Benzene is the principle source of styrene, ethylbenzene, and cumene 16.7 Source and Industrial Use of Aroma4c Hydrocarbons 57
Aroma<c Hydrocarbons Xylenes are also obtained from petroleum Para xylene is the most important of these Virtually the en6re produc6on of p xylene is used for prepara6on of terephthalic acid Terephthalic acid is used in polyester synthesis 16.7 Source and Industrial Use of Aroma4c Hydrocarbons 58