Faraday discovers benzene and its aromatic properties
1. 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
benzoic acid:
C6H6
Benzene and Aromaticity
COOH
Cyclic structure:
Kekulé 1865
Michael Faraday
1791-1867
2.
3. Nomenclature
Many common names because many benzene
derivatives occur in nature. Functional groups
have priority, but others are
substituents, hence
alkylbenzene, halobenzene, nitrobenzene.
Disubstituted:
ortho meta para
4. Functional groups take over:
General term for benzene-containing compounds:
Arene.
C6H5- is phenyl; general aryl.
C6H5CH2- is phenylmethyl or benzyl.
5. Because many benzene derivatives exhibit a nice smell, compounds
containing the benzene ring were called historically “aromatic”.
7. 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.
Cyclohexane
10. Spectra
UV-Visible: Complex set of peaks at 250–
290 nm, which shift to visible range with
resonating substituents: Dyes.
λmax (ε ) = 289
(18,600) nm: suntan
lotions.
20. 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).
22. Conjugated Cyclopolyenes:
Annulenes
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.
Erich Hückel
1896-1980
23. Recall:
Examples of cyclic, delocalized polyenes:
Deshiel-
ding zone
Deshiel-
ding zone
Shielding
zone
Shielding
zone
24. Cyclobutadiene
([4]Annulene)
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:
26. [10]Annulene
H
H
Planar structure too crowded by the
inside hydrogens. Other isomers
suffer bond angle strain: Undergo
electrocyclic ring closures!trans,trans Isomer
all-cis Isomer
154º
con
dis
27. 7.10 ppm
- 0.50 ppm
Solution to this problem:
Bridged annulenes.
Shielded by ring
current
31. Charged Annulenes
5.57 ppm Corr. for
charge: ~7.5 ppm
But, the cyclopentadienyl cation is antiaromatic, very
unstable, much worse than allyl cation: 4 electrons!
Six e
Bromocycloheptatriene spontaneously dissociates: Six e. But, the
cycloheptatrienyl anion is unstable: pKa of cycloheptatriene ~
39!, 8 electrons.
(propene = 40)
Six e
9.17 ppm
32. Hydride shift in carbocations: 2 e TS
[4n +2], n = 0.
Hückel’s Rule in TSs
34. Antiaromatic
But:
Base
10 e
What are these?
Aromatic
Aromaticity is an important concept, includes heterocycles
(Chapter 25). 300,000 papers since 1981!
36. The Intermediate Cation in
EAS
X-ray structure of
[C6H7][HCB11Me5Br6]
D. Stasko, C. A. Reed,
JACS 2002, 124, 1148
.
.
37.
38. 1. Halogenation: F2 violent; Cl2, Br2 need catalyst;
I2 endothermic
-10 kcal mol-1
Electron poor
Other Lewis acids: BF3, AlCl3, etc.
Stops. Br is e-withdrawing, deactivates ring
39. 2. Nitration with nitric acid HNO3 = HO-NO2
Mechanism:
HNO3, H2SO4
N+
O–
O
40. 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.
Recent stuff!!
42. Driven by large heat
of hydration of SO3
(will become important in Chapter 16)
Applications of sulfonic acids:
Benzenesulfonyl
chloride
43. Sulfa drugs: Antibacterial (urinary, malaria, leprosy)
S
Cl
O O
ROH
S
OR
O O
RNH2
S
NHR
O O
Recall: Nu
Sulfonamides
Sulfonic esters
Hoffmann La Roche
Kidney Model
44. 4. Friedel-Crafts reactions: Alkylation and alkanoylation
A. Alkylation
Mechanism:
Charles Friedel
1832-1899
James Craft
1839-1917
45. 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.
49. 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.
