Lecture 10 More Aromatics February 15, 2018
ückel's Rule for Aromaticity To be Aromatic a compound must : 1. be Cyclic 2. have one P orbital on each atom in the ring 3. be planar or nearly so to give orbital overlap 4. have a closed loop of 4n+2 pi electrons in the cyclic arrangement of p orbitals
ückel's Rule Among planar, monocyclic, completely conjugated polyenes, only those with 4n + 2p electrons possess special stability (are aromatic) n 4n+2 0 2 1 6 benzene! 2 10 3 14 4 18 Magic Numbers
ückel's Rule Actually and inadvertently defines a condition for cyclic molecules in which the bonding molecular orbitals are filled and there are no electrons in non-bonding or antibonding orbitals. roughly analogous to the rare gas condition for atomic orbitals
p-mos of Benzene Antibonding Benzene 6 p electrons fill all of the bonding orbitals all p antibonding orbitals are empty Bonding
p-mos of Cyclooctatetraene Antibonding Nonbonding Cyclooctatetraene Bonding
Aromatic Ions Radical Anion p e s Radical Cation Cation Anion
Cyclopentadiene Let s fill these for each case, radical, anion and cation p e s Radical Cation Anion
Cycloheptatriene Radical Anion p e s Radical Cation Anion Cation
Cycloheptatriene Let s fill these for each case, radical, anion and cation p e s Radical Cation Anion
Compare Acidities of Cyclopentadiene and Cycloheptatriene pk a = 16 pk a = 36 K a = 10-16 K a = 10-36
Cyclopropenyl Cation + also written as + n = 0 4n +2 = 2 p electrons!!
n = 0 (4n+2 = 2) fills a bonding MO
Discovery of Cyclopropylium Cation Ron Breslow 1931 - While still in his twenties, Breslow made two groundbreaking contributions to mechanistic organic chemistry. is synthesis of a cyclopropenyl cation generalized the concept of aromaticity to cyclic systems with only 2 π-electrons. This work was bolstered by showing that cyclopropenyl anions and cyclopentadienyl cations, each with 4n π- electrons in a cyclic array, are antiaromatic (a term Breslow coined).
Cyclooctatetraene Dianion also 2 written as n = 2 4n +2 = 10 p electrons
eterocyclic Aromatic Compounds
eterocyclic Aromatic Compounds N N O S Pyridine Pyrrole Furan Thiophene
eterocyclic Aromatic Compounds and ückel's Rule N Pyridine 6 p electrons in ring lone pair on nitrogen is in an sp 2 hybridized orbital; not part of p system of ring
ückel and Pyridine
N Pyrrole lone pair on nitrogen must be part of ring p system if ring is to have 6 p electrons lone pair must be in a p orbital in order to overlap with ring p system
ückel and Pyrrole
Furan O two lone pairs on oxygen one pair is in a p orbital and is part of ring p system; other is in an sp 2 hybridized orbital and is not part of ring p system
uckel and Furan
Recognizing Aromatic Compounds Be careful with uckel s Rule
Some Nomenclature http://www.word-origins.com/definition/benzene.html A B A A Ortho or 1,2 Meta or 1,3 B B Para or 1,4 Please read about naming in Chapter 21!
Some Nomenclature Please read about naming in Chapter 21.3. Many substituted benzenes are so old and so common that they have been given nick names! O C 3 C 3 C 3 C 3 C 3 C 3 C 3
IR
Benzene rings--substitution patterns R 750,700 R 750 R 850,780, 700 R R 820 R R Unreliable with NO 2, CO 2 subs From Crewes, Rodriguez and Jaspars, ch 8 Out-of-plane bending combinations, quite small, but in a normally clean region of IR. Reliable even with nitro or carboxyl substitution
If the region between 1667-2000 cm -1 (w) is free of interference (C=O stretching frequency) a weak grouping of peaks is observed for aromatic systems. Analysis of this region can lead to a determination of the substitution pattern on the aromatic ring G G G G G G G Monosubstituted 1,2 disubstituted (ortho or o-) 1,2 disubstituted (meta or m-) 1,4 disubstituted (para or p-)
C 3 C 3 795 cm -1 C 3 C 3 768 cm -1 691 cm -1 C 3 C 3 742 cm -
Ring Current in Benzene Circulating p electrons Deshielded B o Secondary magnetic field generated by circulating p electrons deshields aromatic protons
NMR Spectrum of Toluene 5 C 3 3
[18]Annulene 18 p electrons satisfies ückel's rule resonance energy = 418 kj/mol bond distances range between 137-143 pm
nmr Spectroscopy Ring Current effect is massive in the larger annulenes, for example for [18]annulene the six hydrogens on the inside of the ring resonate at d - 3.00!! Up field of TMS!! the twelve hydrogens on the outside of the ring resonate at d 9.3
Coupling Constants - Aromatics Ortho s = 9 z Meta s = 3 z Para s < 1 z
NMR Spectrum of 1-iodo-4-methoxybenzene 3 I OC 3 2 2 CCl 3 impurity
NMR Spectrum of 1-bromo-4-ethoxybenzene 4 Br OC 2 C 3 3 2
TE p-disubstituted PATTERN CANGES AS TE TWO GROUPS BECOME MORE AND MORE SIMILAR All peaks move closer. Outer peaks get smaller.. and finally disappear. Inner peaks get taller. and finally merge. X X X Y X' X all equivalent X = Y X ~ X X = X
Phenol Joseph Lister A British surgeon and a pioneer of antiseptic surgery, who successfully introduced carbolic acid (now known as phenol) to sterilize surgical instruments and to clean wounds, which led to reducing post-operative infections and made surgery safer for patients. 1827-1909
Carbolic Acid Antiseptic
Phenols The functional group of a phenol is an -O group bonded to a benzene ring O O Phenol C 3 3-Methylphenol (m-cresol)