1825 Faraday discovers benzene by pyrolysis
of whale oil:
Colorless liquid bp ~80°C. Very unreactive.
Analysis: :C H :1 1
Synthesis: Mitscherlich by chemical degradation of
Benzene and Aromaticity
Many common names because many benzene
derivatives occur in nature. Functional groups
have priority, but others are
alkylbenzene, halobenzene, nitrobenzene.
ortho meta para
Functional groups take over:
General term for benzene-containing compounds:
C6H5- is phenyl; general aryl.
C6H5CH2- is phenylmethyl or benzyl.
Because many benzene derivatives exhibit a nice smell, compounds
containing the benzene ring were called historically “aromatic”.
Structure: Regular Hexagon
Both the π and ζ frame symmetrize the structure
Benzene is unusually unreactive. Does this mean
that it is also especially stable
thermodynamically? Look at ΔH hydrogenation:
Special stability is now called aromaticity. All cyclic 6e arrangements
are aromatic, including transition states.
Most fused benzenoids are aromatic: Resonance
forms with full benzenoid e-sextets. The number of
resonance forms of isomers often indicative of relative
stability: Compare anthracene with phenanthrene.
More resonance forms: More stable (by 5.7 kcal mol-1).
Hückel’s rule: Cyclically delocalized polyenes
with [4n +2] π electrons are aromatic
(stabilized relative to acyclic
reference), those with [4n] π electrons are
antiaromatic (destabilized). Interruption of
cyclic conjugation: Nonaromatic. Easiest
detection by 1H NMR: Aromatic systems show
deshielded outer Hs (shielded inner Hs).
Antiaromatic systems show the reverse:
shielded outer, deshielded inner Hs.
Examples of cyclic, delocalized polyenes:
Unstable, very reactive (Diels-Alder
dimerization), rectangular (not square).
Stabilized by bulky substituents.
1H NMR of tri(tert-butyl)cyclobutadiene: δ = 5.38 ppm: High field!
Tetra(tert-butyl)cyclobutadiene X-ray structure:
Nonplanar, which inhibits delocalization:
Nonaromatic (like a polyene).
Barrier to ring flip 11 kcal mol-1
Planar structure too crowded by the
inside hydrogens. Other isomers
suffer bond angle strain: Undergo
electrocyclic ring closures!trans,trans Isomer
- 0.50 ppm
Solution to this problem:
Shielded by ring
5.57 ppm Corr. for
charge: ~7.5 ppm
But, the cyclopentadienyl cation is antiaromatic, very
unstable, much worse than allyl cation: 4 electrons!
Bromocycloheptatriene spontaneously dissociates: Six e. But, the
cycloheptatrienyl anion is unstable: pKa of cycloheptatriene ~
39!, 8 electrons.
(propene = 40)
Hydride shift in carbocations: 2 e TS
[4n +2], n = 0.
Hückel’s Rule in TSs
What are these?
Aromaticity is an important concept, includes heterocycles
(Chapter 25). 300,000 papers since 1981!
The Intermediate Cation in
X-ray structure of
D. Stasko, C. A. Reed,
JACS 2002, 124, 1148
1. Halogenation: F2 violent; Cl2, Br2 need catalyst;
-10 kcal mol-1
Other Lewis acids: BF3, AlCl3, etc.
Stops. Br is e-withdrawing, deactivates ring
2. Nitration with nitric acid HNO3 = HO-NO2
Unified Mechanistic Concept of Electrophilic Aromatic Nitration: Convergence of
Computational Results and Experimental Data, Pierre M. Esteves, George A. Olah, G. K.
Surya Prakash et al., J. Am. Chem. Soc. 2003, 125, 4836.
3. Sulfonation: Reversible. Reagent is fuming
sulfuric acid: H2SO4 + SO3
Driven by large heat
of hydration of SO3
(will become important in Chapter 16)
Applications of sulfonic acids:
Sulfa drugs: Antibacterial (urinary, malaria, leprosy)
Hoffmann La Roche
4. Friedel-Crafts reactions: Alkylation and alkanoylation
Often low yields. Problems: 1. Product contains an e-pushing alkyl
group, making the benzene more reactive, causing overalkylation; 2.
Lewis acid activation makes carbocationic alkyl, which is prone to
rearrangements and polymerization.
Lewis acid needed in equimolar
amounts, because it binds to product:
EAS: Immediate synthetic take home lessons
1. Halobenzenes make Grignards, lithium reagents
2. Alkanoylbenzenes have carbonyl function
We shall see next (Chapter 16) how to use the
nitro and sulfonyl functions.