Chapter 15: Benzene and Aromaticity (Electrophilic Aromatic Substitution)

Question 1

Benzene

Answer 1

general formula C6H6
has 4 degrees of unsaturation
unusually unreactive due to cyclic 6 e- arrangement
all C-C bonds have length of 1.39 Å, are sp2 hybridized and each p orbital overlaps equally with two neighbors creating a delocalized circular π cloud above and below the ring
resonance energy = ~ 30 kcal/mol

Question 2

Arene

Answer 2

generic term for a substituted benzene

Question 3

Aryl Group

Answer 3

arene as a substituent

Question 4

Phenyl

Answer 4

parent aryl groups (C6H5)

Question 5

Polycyclic Aromatic Hydrocarbons

Answer 5

several benzene rings fused together to give more extended π systems with more extensive delocalization of e-

Question 6

Huckel’s Rule

Answer 6

cyclic conjugated polyenes are only aromatic if they contain (4n +2) π e-
4n π circuits can be destabilized by conjugation and are antiaromatic
nonplanar systems of cyclic overlap have alkene-like properties and are nonaromatic

Question 7

Charge Cyclic Molecules and Huckel’s Rule

Answer 7

applies as long as cyclic delocalization can occur
ex: cyclopentadienyl anion is aromatic
4n π cyclic systems can be converted to aromatic by two e- oxidation and reductions

Question 8

Aromatic Compounds

Answer 8

1. cyclic
2. planar
3. fully conjugated
4. 4n + 2 e- in π/conjugated system (Huckel’s Rule)

Question 9

Antiaromatic Compounds

Answer 9

1. cyclic
2. planar
3. fully conjugated
4. 4n e-
   higher in energy than nonaromatic
   ex: cyclobutadiene

Question 10

Electrophilic Aromatic Substitution

Answer 10

benzene is attacked by electrophiles resulting in substitution of hydrogen atoms

Question 11

Electrophilic Aromatic Substitution Mechanism

Answer 11

1. electrophile attacks benzene nucleus (step not favored thermodynamically)
2. resonance stabilized cationic nonaromatic intermediate loses a proton to regenerate aromatic ring

Question 12

Halogenation of Benzene

Answer 12

halogens must be activated by a Lewis Acid Catalyst (FeX3 or AlX3) to become a better electrophile

Question 13

Lewis Acid Catalyst

Answer 13

FeX3 or AlX3

is attacked by X2 to form +X - X - -FeBr3 (+X is a good electrophile)

Question 14

Nitration of Benzene

Answer 14

HNO3 is a poor electrophile, must be activated by concentrated sulfuric acid (H2SO4) to protonate, loses water to make the nitronium ion (NO2+)
NO2+ is attacked by benzene ring

Question 15

Sulfonation of Benzene

Answer 15

fuming sulfuric acid (SO3) is more electrophilically reactive than concentrated sulfuric acid due to e- withdrawing effects
readily reversible

Question 16

Friedel-Crafts Alkylation

Answer 16

haloalkane reacts with benzene in the presence of an aluminum halide

Question 17

Friedel-Crafts Alkylation Mechanism

Answer 17

1. haloalkane activation (RCH2-X attacks ALX3, makes X a good LG and C a good electrophile)
2. electrophilic attack (benzene attacks C, ALX4- is pushed off)
3. proton loss to restore aromaticity (H next to carbocation on benzene is abstracted by AlX4-)

Question 18

Polyalkylation

Answer 18

occurs when alkylated benzene becomes more e- rich than benzene and more susceptible to electrophilic attack
products have multiple electrophilic substituents

Question 19

Carbocation Rearrangement

Answer 19

starting haloalkanes rearrange by hydride shift to more thermodynamically favored carbocations in the presence of a Lewis acid

Question 20

Friedel-Crafts Aceylation (Alkanoylation)

Answer 20

benzene reacts with acyl halides in the presence of aluminum halides to give phenyl ketones
proceeds through an intermediacy of asylum cations with general structure RC—O:+

Question 21

Friedel-Crafts Aceylation Reagents

Answer 21

1. acyl chloride, AlCL3

2. H2O, H+

Question 22

Frieden-Crafts Alkylation Reagents

Answer 22

RX and ALX3

Question 23

Acyl Chlorides

Answer 23

reactive derivatives of carboxylic acids

RC=OOH + SOCl2 => RC=OCl + SO2 + HCl

Question 24

Acylium Cations

Answer 24

key reactive intermediates in Frieden-Crafts Aceylation

R-C+=O

Question 25

Electrophilic Acylation

Answer 25

benzene attacks acylium ion adding an acyl substituent

Question 26

Acyl Substituent

Answer 26

e- withdrawing and deactivates ring, prevents other substitution

Question 27

1. acyl chloride, AlCL3

2. H2O, H+

Answer 27

Friedel-Crafts Aceylation Reagents

Question 28

RX and ALX3

Answer 28

Frieden-Crafts Alkylation Reagents

Question 29

Halogenation Reagents

Answer 29

X2, FeX3

Question 30

X2, FeX3

Answer 30

Halogenation Reagents

Question 31

Nitration Reagents

Answer 31

HNO3, H2SO4

Question 32

HNO3, H2SO4

Answer 32

Nitration Reagents

Question 33

Sulfonation Reagents

Answer 33

SO3, H2SO4

Question 34

SO3, H2SO4

Answer 34

Sulfonation Reagents

Question 35

Desulfonation Reagents

Answer 35

H2O, cat. H2SO4

Question 36

H2O, cat. H2SO4

Answer 36

Desulfonation Reagents

Question 37

Zn, HCl

Answer 37

Reactions with Acyl Products

Ph-C=O-CH3 => Ph-CH2-CH3

Question 38

Reactions with Acyl Products

Ph-C=O-CH3 => Ph-CH2-CH3

Answer 38

Zn, HCl

Question 39

Reactions with Acyl Products

Ph-C=O-CH3 => Ph-CHOH-CH3

Answer 39

NaBH4

Question 40

NaBH4

Answer 40

Reactions with Acyl Products

Ph-C=O-CH3 => Ph-CHOH-CH3

Question 41

Reactions with Acyl Products

Ph-C=O-CH3 => Ph-CH2-CH3

Answer 41

Pd/C, H2

Question 42

Pd/C, H2

Answer 42

Reactions with Acyl Products

Ph-C=O-CH3 => Ph-CH2-CH3

Question 43

CrO3, H2SO4, H2O

Answer 43

Ph-CH2-CH3 => Ph-C=O-R