neand Be Arom nze aticity
1825 Faraday discove be neby pyrolysis of whaleoil: rs nze C olorle liquid bp ~80C Ve unre ss . ry active . C: H 1 :1 S ynthe Mitsche sis: rlich by che ical de m gradation of be nzoic acid (1833): Analysis:
C OOH
Michae Faraday l 1791-1867
C H6 6
C yclic structure Ke : kul 1865
Nomenclature
Many com on nam s be m e causem be nede any nze rivative occur s in nature Functional groups havepriority, but othe are . rs substitue he alkylbe ne halobe ne nitrobe ne nts, nce nze , nze , nze .
Disubstitute d:
ortho
m ta e
para
Functional groups takeove r:
I UPAC acce d pte
Ge ral te for be necontaining com ne rm nze pounds: Are . ne C H5- is phe ; ge ral aryl. 6 nyl ne C H C - is phe e or be . H nylm thyl nzyl
Be causem be nede any nze rivative e s xhibit a nicesm ll, com e pounds containing the be nering we calle historically "arom . nze re d atic"
S tructure Re : gular He xagon
Both the and fram sym e e m trizethestructure
Be neis unusually unre nze active Doe this m an that it is also . s e e cially stablethe odynam spe rm ically? Look at H of hydroge nation:
RE = (2x28.6) 54.9 = 2.3
Product is thesam for all: cyclohe e xane
Calculated H(H ) = (3x28.6) 6.9 = 78.9
S cial stability is now calle arom pe d aticity. All cyclic 6earrange e arearom m nts atic, including transition state s.
Aromatic Transition States
All 6 eTS s
Molecular Orbitals
.
p
Spectra
UV-Visible C ple se of pe at 250290 nm : om x t aks , which shift to visiblerangewith re sonating substitue dye nts: s.
max ( ) = 289 (18,600)
nm suntan lotions. :
I R:
Mass spe ctrum :
Re lative stableM ly
1
H NMR
Be ne = 7.27 (s) ppm Low fie ("to theleft") nze : : ld!
"Arom re atic gion" in 1H NMR: ~ 6.5 8.5 ppm .
Why? Ring curre nt!
We ns H ake
S nghte H tre ns
H
Substitued Benzenes Can Show Complex NMR Patterns
S trong push-pull e cts e cially on ortho-H: ffe spe
Pull
Jortho = 9 Hz
Push
Alde s hyde de lde shie d
13CNMR: Alke -like ne
Be ne = 128.7 ppm nze
Polycyclic Benzenoid Hydrocarbons
Fusion: Line and angular ar
Naphthale : Arom ne atic
C lar's rule Full e ctron se ts are"good" : le xte
UV
Fullerenes: "Curved Graphite"
Harry Kroto, Robe C Richard S alle rt url, m y NP 1996
Conjugated Cyclopolyenes: Annulenes
Hcke rule l's : C yclically de localize polye s ("annule s") with [4n +2] d ne ne e ctrons arearom (stabilize re le atic d lativeto acyclic re re ), fe nce thosewith [4n] e ctrons areantiarom (de le atic stabilize ); n = d 1, 2, 3, e tc. I nte rruption of cyclic conjugation: Nonarom . atic
Erich Hcke l 1896-1980
De ction by 1H NMR: Arom syste s show de lde oute te atic m shie d r Hs (shie d inne Hs). Antiarom syste s show there rse lde r atic m ve : shie d oute , de lde inne Hs. lde r shie d r
Re call:
S lding hie zone
De l-ding shie zone S lding hie zone
De l-ding shie zone
Exam s of cyclic, de ple localize polye s: d ne
Cyclobutadiene ([4]Annulene)
Antiaromatic
1
Unstable ve re , ry active(Die ls-Alde dim rization at r e 35K), re ctangular (not square S ). tabilize by bulky d substitue nts.
H NMR of tri(te -butyl)cyclobutadie : = 5.38 ppm High fie Re rt ne : ld! lativeto arom syte s and 1,3-butadie ( = 6.27 ppm atic m ne ).
Te te -butyl)cyclobutadie X-ray structure tra( rt ne :
Cyclooctatetraene ([8]Annulene)
Nonplanar, which inhibits de localization: Nonarom (likea polye ). atic ne
Barrie to ring flip 11 kcal m -1 r ol
[10]Annulene: Aromatic?
H H
trans,trans I som r e
Planar structuretoo crowde by theinside d hydroge . Othe isom rs suffe bond anglestrain: ns r e r Unde e ctrocyclic ring closure rgo ! le s
H H
154
H h con H
, 25 C
H dis
, 10 C
H
all-cis I som r e
H
H
S olution to this proble : Bridge m d annule s. ne
- 0.50 ppm
S lde by ring hie d curre nt
7.10 ppm
[18]Annulene
Franz S ondhe e im r 1926-1981
Aromatic
-2.99 ppm shie d lde 9.28 ppm de lde shie d
[16]Annulene: Antiaromatic
10.43 ppm
5.40 ppm
TheAnnule s ne
Cyclobut adiene: planar, ant iaromat ic
Benzene: planar, aromat ic
Cyclooct at et raene: nonplanar = nonaromat ic
[10]Annulene: nonplanar = nonaromat ic
[12]Annulene: planar = ant iaromat ic
[14]Annulene: planar = aromat ic
[16]Annulene: planar = ant iaromat ic
[18]Annulene: planar = aromat ic
1, 3- Cyclopent adiene: noncyclically delocalized = nonaromat ic
1, 4- Cyclohexaadiene: noncyclically delocalized = nonaromat ic
Charged Annulenes
5.57 ppm Whe corre d for . n cte charge ~7.5 ppm :
(prope = 40) ne
S e ix
But, thecyclope ntadie cation is antiarom nyl atic, ve unstable m worsethan allyl ry , uch cation: 4 e ctrons! le
9.17 ppm Whe corre d for . n cte charge ~7.5 ppm :
S e ix
Brom ocyclohe ptatrie spontane ne ously dissociate S e But, thecyclohe s: ix . ptatrie anion nyl is unstable pKa of cyclohe : ptatrie ~ 39!, 8 e ctrons. ne le
Hckel's Rule in Charged Transition States
Wehavealre se n ne ady e utral e xte TS but this is ge ral for othe [4n -se t s, ne r +2], including charge TS . d s
Hydrideshift in carbocations: 2 eTS [4n +2], n = 0.
Cycloprenium ion: planar, aromat ic
Hckel's Rule in Polycycles
Good approxim ation: Count pe riphe e ctrons ral le
Arom atic
I solate d
Pe riphe arom ry atic: 14 e
3 Be ne and pe nze s riphe 18 e ry
But:
Antiarom atic
Base
10 e
Arom atic
like What arethe ? se
Arom aticity is an im portant conce include he rocycle pt, s te s (C hapte 25). >300,000 pape since1981! r rs
Electrophilic Aromatic Substitution (EAS)
(Not addition)
Me chanism :
The Intermediate Cation in EAS
-
. .
E=H X-ray structureof [C H ] +
+
1. Haloge nation: F2 viole C 2, Br2 ne d catalyst ; nt; l e I2 e ndothe ic rm
Ele ctron poor
-HBr
3 Resonanceform s
S tops. Br is e -withdrawing, de activate ring s
-10 kcal m ol
Othe catalytic Le acids: BF3, AlCl3, e r wis tc.
2. Nitration with nitric acid
HNO = HO--NO2
O N+
HNO3 , H 2 SO4
O
Me chanism : C olor: Nu, E, L
3. S ulfonation: Re rsible Re nt is fum sulfuric acid: ve . age ing H2S 4 + S 3 O O
: :O: Valence :O S O or shell S O ex panded: O: :O:
:
:
:
:
SO3 , H 2 SO4
:
O S
O OH
Zwitte rion
S ulfonation
:
(will be eim com portant in Chapte 16) r
Drive by largehe of n at hydration of SO O3 3
Applications of sulfonic acids:
Be ne nze sulfonyl chloride
Re ROH RC with PC 5 (C call l l hapte 9) r
O S
O NHR
O S RNH 2
O Cl
O S ROH
O O R
S ulfonam s ide
Re call: Nu: S ulfonic e rs ste
S drugs: Antibacte (urinary, m ulfa rial alaria, le prosy)
Malaria occurre nce
4. Frie l-C de rafts re actions: Alkylation and alkanoylation
(acylation)
A. Alkylation
Le acid wis
Me chanism :
Charle Frie l s de 1832-1899
Jam s Craft e 1839-1917
Ofte low yie Proble s: 1. Product contains an e n lds. m -pushing alkyl group, m aking thebe ne nze m re ore active causing ove , ralkylation; 2. Le acid activation m s carbocationic alkyl, which wis ake is proneto re arrange e and polym rization. m nts e
B. Alkanoylation (acylation) - se ctive le
Me chanism :
Le acid ne de in e wis e d quim am olar ounts, be causeit binds to product:
EAS I m e : m diatesynthe takehom le tic e ssons 1. Halobe ne m Grignards, lithiumre nts nze s ake age 2. Alkanoylbe ne havecarbonyl function nze s Weshall se ne (C e xt hapte 16) how to usethenitro and sulfonyl r functions.