Chapter 15: Benzene and Aromaticity (Electrophilic Aromatic Substitution) Flashcards
Benzene
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
Arene
generic term for a substituted benzene
Aryl Group
arene as a substituent
Phenyl
parent aryl groups (C6H5)
Polycyclic Aromatic Hydrocarbons
several benzene rings fused together to give more extended π systems with more extensive delocalization of e-
Huckel’s Rule
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
Charge Cyclic Molecules and Huckel’s Rule
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
Aromatic Compounds
- cyclic
- planar
- fully conjugated
- 4n + 2 e- in π/conjugated system (Huckel’s Rule)
Antiaromatic Compounds
- cyclic
- planar
- fully conjugated
- 4n e-
higher in energy than nonaromatic
ex: cyclobutadiene
Electrophilic Aromatic Substitution
benzene is attacked by electrophiles resulting in substitution of hydrogen atoms
Electrophilic Aromatic Substitution Mechanism
- electrophile attacks benzene nucleus (step not favored thermodynamically)
- resonance stabilized cationic nonaromatic intermediate loses a proton to regenerate aromatic ring
Halogenation of Benzene
halogens must be activated by a Lewis Acid Catalyst (FeX3 or AlX3) to become a better electrophile
Lewis Acid Catalyst
FeX3 or AlX3
is attacked by X2 to form +X - X - -FeBr3 (+X is a good electrophile)
Nitration of Benzene
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
Sulfonation of Benzene
fuming sulfuric acid (SO3) is more electrophilically reactive than concentrated sulfuric acid due to e- withdrawing effects
readily reversible
Friedel-Crafts Alkylation
haloalkane reacts with benzene in the presence of an aluminum halide
Friedel-Crafts Alkylation Mechanism
- haloalkane activation (RCH2-X attacks ALX3, makes X a good LG and C a good electrophile)
- electrophilic attack (benzene attacks C, ALX4- is pushed off)
- proton loss to restore aromaticity (H next to carbocation on benzene is abstracted by AlX4-)
Polyalkylation
occurs when alkylated benzene becomes more e- rich than benzene and more susceptible to electrophilic attack
products have multiple electrophilic substituents
Carbocation Rearrangement
starting haloalkanes rearrange by hydride shift to more thermodynamically favored carbocations in the presence of a Lewis acid
Friedel-Crafts Aceylation (Alkanoylation)
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:+
Friedel-Crafts Aceylation Reagents
- acyl chloride, AlCL3
2. H2O, H+
Frieden-Crafts Alkylation Reagents
RX and ALX3
Acyl Chlorides
reactive derivatives of carboxylic acids
RC=OOH + SOCl2 => RC=OCl + SO2 + HCl
Acylium Cations
key reactive intermediates in Frieden-Crafts Aceylation
R-C+=O
