chapter 17: benzene and aromaticity
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8-methyl-N-vanillyl-6-nonenamide ( Capsaicin). Chapter 17: Benzene and Aromaticity. Buckminsterfullerene. TNT. Key points & objectives:. Aromatic molecules are cyclic, conjugated, flat, and unusually stable 4n + 2 electrons ( n = 0, 1, 2, …. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 17: Benzene and Aromaticity
TNT
8-methyl-N-vanillyl-6-nonenamide (Capsaicin)
Buckminsterfullerene
Key points & objectives:
• Aromatic molecules are cyclic, conjugated, flat, and unusually stable
• 4n + 2 electrons (n = 0, 1, 2, ….• Hydrocarbon aromatics: benzene, naphthalene,
anthracene, toluene, xylylene• Heterocyclics: pyridine, pyrimidine, imidazole, pyrrole,
thiophene, furan, indole….• Molecular orbitals using Frost diagrams (inscribed
circles)• Ring current deshields NMR signals – downfield
Benzene1st isolated by Michael Faraday in 1825
From “Benzoin,” corrupt form of the Arabic "luban jawi” for the “frankincense of Java”
Frankincense
Boswellia sacra
triterpene
Cancer drug
anti-inflammatoryhepatotoxicity
Not “aromatic” in the technical sense
Aromatic-fragrant
myrth
Commiphora myrrha tree
Antiseptic, embalming agent, incense
Cinnamon
DiabetesAntimicrobialantioxidant
(2E)-3-phenylprop-2-enal
cinnamaldehyde
8-methyl-N-vanillyl-6-nonenamide (Capsaicin)
Capsaicin
16,000,000 Scovilles
psoriasis relieve the pain of peripheral neuropathytrigger apoptosis in human colon and lung cancer
Vanilla
Tincture (ethanol extract) of vanilla
aphrodisiac and a remedy for feverscatecholamines (including adrenaline)addictive
Aromatic molecules
• Flat• Conjugated• (4n +2) pi electrons• Unusually stable• Ring current (deshielding protons)
Anesthetics & analgesics
Advil, and Motrin
Sunscreens
Only complete UVA block
12
• Very high energy radiation (UVC) is currently blocked by the ozone layer (ozone hole issue)
• High energy radiation (UVB) does the most immediate damage (sunburns)
• But lower energy radiation (UVA) can penetrate deeper into the skin, leading to long term damage
Source: N.A. Shaath. The Chemistry of Sunscreens. In: Lowe NJ, Shaath NA, Pathak MA, editors. Sunscreens, development, evaluation, and regulatory aspects. New York: Marcel Dekker; 1997. p. 263-283.
Skin Damage
Sources and Names of Aromatic Hydrocarbons
From high temperature distillation of coal tar Heating petroleum at high temperature and pressure over
a catalyst
Aromatics are less reactive than Alkenes
Aromatics Nomeclature
Aromatics Nomeclature
Agent orange
PolychlorinatedbiphenylsPCB’s
Thermally stable, electrically insulating heat transfer liquid
Casting wax for lost wax process for making metal things
Mueller 1948 Nobel Prize in Medicine
dichlorodiphenyltrichloroethane
Malaria mosquito
Thermodynamic stability of benzene: Heats of Hydrogenation
Monosubstituted Benzenes• Most monosubstituted aromatics are named using
-benzene as the parent name preceded by the substituent name (as a prefix; all one word):
F NO2 CH2CH3
fluorobenzene
fluoro nitro ethyl
nitrobenzene ethylbenzene
Alkyl-substituted Benzenes• Alkyl substituted benzenes are named according to the
length of the carbon chain of the alkyl group.• With six carbons or fewer in the alkyl chain, they are named
as ‘alkylbenzene.’• e.g., propylbenzene:
CH2CH2CH3
Alkyl-substituted Benzenes• With more than six carbons in the alkyl chain, they are
named as a ‘phenylalkane,’ where the benzene ring is named as a substituent (phenyl) on the alkane chain• e.g., 4-phenylnonane
CHCH 2CH2CH2CH2CH3
CH3CH2CH2CHCH2CH2CH2CH2CH3
CH2CH2CH3
CH3CH2CH2CHCH2CH2CH2CH2CH3=
4-phenylnonane
The Benzyl Group• The benzyl group is a common name for a methyl
substituted benzene (toluene) having substitution for one of the hydrogens on the methyl group.
CH2Br
the benzyl group benzyl bromide benzyl alcohol
CH2BrCH2 CH2OH
Common Names of Subs. Benzenes• There are a number of nonsystematic (common)
names commonly used for certain monosubstituted benzenes (see next slide)
• These ten common names should be memorized.• These common names are used as base names when
naming more their more highly substituted derivatives. Examples of these will be given later.
Mono-substituted Benzene Nomenclature: Common Names
Disubstituted Benzenes• Disubstituted benzenes can be named in one of two ways.
Each method describes the relative positions of the two groups on the benzene ring.• Systematic numbering of the aromatic ring.• Using the prefixes ortho-, meta-, or para-.
• When numbering the ring carbons, carbon # 1 is always a substituted carbon.
• The substituents are listed alphabetically.
Disubstituted Benzenesortho- (abbreviated o- ) = 1,2-disubstituted
(two groups on adjacent carbons on the ring)
o-difluorobenzene or1,2-difluorobenzene
FF
FF
o-bromoethylbenzene or1-bromo-2-ethylbenzene
CH2CH3
Br
Disubstituted Benzenesmeta- (abbreviated m- ) = 1,3-disubstituted
(two groups having one unsubstituted carbon between them)
m-dibromobenzene or1,3-dibromobenzene
BrBr
m-bromonitrobenzene or1-bromo-3-nitrobenzene
NO2Br
Disubstituted Benzenespara- (abbreviated p- ) = 1,4-disubstituted
(two groups on opposite sides of the ring)
p-dichlorobenzene or1,4-dichlorobenzene
Cl
Cl
p-bromochlorobenzene or1-bromo-4-chlorobenzene
Br
Cl
NH2
Br OHCl
Disubstituted Benzenes• When one of the substituents changes the base name,
either o-, m-, and p- or numbers may be used to indicate the position of the other substituent.
• Carbon # 1 is always the carbon bearing the substituent that changes the base name.
p-bromoaniline or4-bromoaniline
o-chlorophenol or2-chlorophenol
12
34 1 2
Common Names of Disubs. Benzenes• There are a few nonsystematic (common) names for
disubstituted benzenes that you should be familiar with:
CH3
CH3
CH3
CH3
CH3
CH3o-xylene m-xylene p-xylene
CH3
OHCH3
OH
CH3
OHo-cresol m-cresol p-cresol
Disubstituted Benzenes Relative positions on a benzene ring
ortho- (o) on adjacent carbons (1,2) meta- (m) separated by one carbon (1,3) para- (p) separated by two carbons (1,4)
Describes reaction patterns (“occurs at the para position”)
Polysubstituted Benzenes• Polysubstituted benzenes must be named by numbering the
position of each substituent on the ring (with more than two substituents, o-, m-, and p-can NOT be used.)
• The numbering is carried out to give the substituents the lowest possible numbers. Carbon #1 always has a substituent.
• List the substituents alphabetically with their appropriate #s.
2-ethyl-1-fluoro-4-nitrobenzene
CH2CH3
NO2
F 1 234
Polysubstituted Aromatics having a Common base name
• Common names of the monosubstituted benzenes are used as parent names for polysubstituted aromatics when one of the substituents changes the base name.
• For such rings with common names, the carbon bearing the substituent responsible for the common name is always carbon #1.
• The substitutents are listed in alphabetical order.
5-bromo-2-chlorotoluene
CH3Cl
Br
1
2
34
5
toluene
chloro
bromo
Polysubstituted Benzenes
Br
CH2CH3
NO2
OH
Cl
Br12
34
5
12
34
4-bromo-2-ethyl-1-nitrobenzene 5-bromo-2-chlorophenol
Polysubstituted Benzenes
CH3
Cl
Br
O2N
CH2CH3
Cl
Br
O2N
13
45
6
6
54
3
221
2-bromo-6-chloro-4-nitrotoluene 1-bromo-3-chloro-2-ethyl-5-nitrobenzene
38
• A benzene substituent is called a phenyl group, and it can be abbreviated in a structure as “Ph-”.
• Therefore, benzene can be represented as PhH, and phenol would be PhOH.
Naming Benzene as a Substituent
Polycyclic Aromatic Hydrocarbons (PAH)
anthracenephenanthrene
naphthalene
pyrene benzo [a] pyreneMetabolic byproducts of benzo [a] pyrene react with DNA to form adducts, leading to carcinogenesis (cancer).
40
Naphthalene Orbitals Three resonance forms and delocalized electrons
41
Figure 17.2
13C NMR Absorptions of Dibromobenzenes
• The number of signals (lines) in the 13C NMR spectrum of a disubstituted benzene with two identical groups indicates whether they are ortho, meta, or para to each other.
42
Figure 17.5
Drugs that Contain a Benzene Ring
Heterocyclic Aromatics
Heterocyclic Aromatics
45
Pyridine A six-membered heterocycle with a nitrogen atom in its ring electron structure resembles benzene (6 electrons) The nitrogen lone pair electrons are not part of the aromatic system
(perpendicular orbital) Pyridine is a relatively weak base compared to normal amines but
protonation does not affect aromaticity
Protonation of Pyrroles and Pyridines
47
Pyrrole A five-membered heterocycle with one nitrogen electron system similar to that of cyclopentadienyl anion Four sp2-hybridized carbons with 4 p orbitals perpendicular to the ring
and 4 p electrons Nitrogen atom is sp2-hybridized, and lone pair of electrons occupies a p
orbital (6 electrons) Since lone pair electrons are in the aromatic ring, protonation destroys
aromaticity, making pyrrole a very weak base
Structure and Stability of Benzene: Molecular Orbital Theory
Benzene reacts slowly with Br2 to give bromobenzene (where Br replaces H)
This is substitution rather than the rapid addition reaction common to compounds with C=C, suggesting that in benzene there is a higher barrier
50
Heats of Hydrogenation as Indicators of Stability The addition of H2 to C=C normally gives off about 118 kJ/mol – 3 double
bonds would give off 356kJ/mol Two conjugated double bonds in cyclohexadiene add 2 H2 to give off 230
kJ/mol Benzene has 3 unsaturation sites but gives off only 206 kJ/mol on reacting
with 3 H2 molecules Therefore it has about 150 kJ more “stability” than an isolated set of three
double bonds
32 kcal/mole
52
Benzene’s Unusual Structure All its C-C bonds are the same length: 139 pm — between
single (154 pm) and double (134 pm) bonds Electron density in all six C-C bonds is identical Structure is planar, hexagonal C–C–C bond angles 120° Each C is sp2 and has a p orbital perpendicular to the
plane of the six-membered ring
53
Four structural criteria must be satisfied for a compound to be aromatic:1. A molecule must be cyclic. • To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms.
The Criteria for Aromaticity
54
2. A molecule must be planar.• All adjacent p orbitals must be aligned so that the electron density can
be delocalized.
• Since cyclooctatetraene is nonplanar and not aromatic, it undergoes addition reactions just like those of other alkenes.
The Criteria for Aromaticity
55
3. A molecule must be completely conjugated.
• Aromatic compounds must have a p orbital on every atom.
The Criteria for Aromaticity
56
4. A molecule must satisfy Hückel’s rule, and contain a particular number of electrons.
• Benzene is aromatic and especially stable because it contains 6 electrons.
• Cyclobutadiene is antiaromatic and especially unstable because it contains 4 electrons.
Hückel's rule:
The Criteria for Aromaticity
• Hückel’s rule refers to the number of electrons, not the number of atoms in a particular ring.
58
Why 4n +2? When electrons fill the various molecular orbitals, it takes
two electrons (one pair) to fill the lowest-lying orbital and four electrons (two pairs) to fill each of n succeeding energy levels
This is a total of 4n + 2
59
• The combination of two p orbitals can be constructive—that is, with like phases interacting—or destructive, that is, with opposite phases interacting.
• When two p orbitals of similar phase overlap side-by-side, a bonding molecular orbital results.
• When two p orbitals of opposite phase overlap side-by-side, a * antibonding orbital results.
Bonding and Antibonding Orbitals
60
• Two atomic p orbitals combine to form two molecular orbitals.• The bonding MO is lower in energy than the two p orbitals.• The * antibonding MO is higher in energy because a destabilizing node
results, which pushes nuclei apart when orbitals of opposite phase combine.
Figure 17.8
Formation of π and π* Molecular Orbitals
61
• Since each of the six carbon atoms in benzene has a p orbital, six atomic p orbitals combine to form six MOs.
Figure 17.9
Molecular Orbitals for Benzene
62
Inscribed Polygon Method of Predicting Aromaticity
63
• This method works for all monocyclic completely conjugated systems regardless of ring size.
• The total number of MOs always equals the number of vertices of the polygon.
• The inscribed polygon method is consistent with Hückel's 4n + 2 rule—there is always one lowest energy bonding MO that can hold two electrons and the other bonding MOs come in degenerate pairs that can hold a total of four electrons.
Inscribed Polygon Method of Predicting Aromaticity
64
Figure 17.10
Inscribed Polygon Method of Predicting Aromaticity
65
• Buckminsterfullerene (C60) is a third elemental form of carbon. • Buckminsterfullerene is completely conjugated, but it is not aromatic since it
is not planar (CAREFULL!!!)• It undergoes addition reactions with electrophiles in much the same way as
ordinary alkenes.
Buckminsterfullerene—Is it Aromatic?
66
Compounds With 4n Electrons Are Not Aromatic (May be Antiaromatic)
Planar, cyclic molecules with 4 n electrons are much less stable than expected (antiaromatic)
They will distort out of plane and behave like ordinary alkenes 4- and 8-electron compounds are not delocalized (single and double
bonds) Cyclobutadiene is so unstable that it dimerizes by a self-Diels-Alder
reaction at low temperature Cyclooctatetraene has four double bonds, reacting with Br2, KMnO4, and
HCl as if it were four alkenes
67
Aromatic Ions The 4n + 2 rule applies to ions as well as neutral species Both the cyclopentadienyl anion and the cycloheptatrienyl
cation are aromatic The key feature of both is that they contain 6 electrons in a
ring of continuous p orbitals
68
Aromaticity of the Cyclopentadienyl Anion
1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalization
Removal of H+ at the CH2 produces a cyclic 6-electron system, which is stable
Removal of H- or H• generates nonaromatic 4 and 5 electron systems
Relatively acidic (pKa = 16) because the anion is stable
69
Cycloheptatriene Cycloheptatriene has 3 conjugated double bonds joined by
a CH2 Removal of “H-” leaves the cation The cation has 6 electrons and is aromatic
70
• 1H NMR spectroscopy readily indicates whether a compound is aromatic. • The protons on sp2 hybridized carbons in aromatic hydrocarbons are
highly deshielded and absorb at 6.5–8 ppm, whereas hydrocarbons that are not aromatic absorb at 4.5–6 ppm.
NMR and Aromaticity
71
• Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4n + 2 electrons.
• Hydrocarbons containing a single ring with alternating double and single bonds are called annulenes.
• To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene.
Larger Aromatic Rings
72
• [10]-Annulene has 10 electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other.
• Thus, the ring puckers to relieve this strain.• Since [10]-annulene is not planar, the 10 electrons cannot delocalize
over the entire ring and it is not aromatic.
Hückel’s Rule and Number of Electrons
Biochemically Relevant Aromatics
NH2
O
OH
phenylalanine
NH2HO
O
OH
tyrosine
NH2
HN
OOH
tryptophan
HN
indole
HN
NN
N
purine
N
N
pyrimidine
Amino Acids
Biologically Relevant Aromatics
NN
O
NH2
R
H H
HR
NH2
O
Aromatic
NADH NAD+
Nicotinamide adeine dinucleotide, the biolgical hydrogenator