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 (. ID: 214085
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Chapter 17: Benzene and Aromaticity
Key points & objectives:
Aromatic molecules are cyclic, conjugated, flat, and unusually stable
4n + 2 electrons (
= 0, 1, 2, ….
Hydrocarbon aromatics: benzene, naphthalene,
using Frost diagrams (inscribed circles)
NMR signals – downfieldSlide3
st isolated by Michael Faraday in 1825
,” corrupt form of
the Arabic "
” for the “frankincense of Java”Slide4
Not “aromatic” in the technical senseSlide5
Antiseptic, embalming agent, incenseSlide6
relieve the pain of peripheral neuropathytrigger apoptosis in human colon and lung cancerSlide8
Tincture (ethanol extract) of vanilla
and a remedy for
(4n +2) pi electrons
Ring current (
Anesthetics & analgesics
Advil, and MotrinSlide11
Only complete UVA blockSlide12
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.
Sources and Names of Aromatic Hydrocarbons
From high temperature distillation of coal tarHeating petroleum at high temperature and pressure over a catalystSlide14
Aromatics are less reactive than AlkenesSlide15
Thermally stable, electrically insulating heat transfer liquid
Casting wax for lost wax process for making metal thingsSlide19
Mueller 1948 Nobel Prize in Medicine
Thermodynamic stability of benzene: Heats of HydrogenationSlide21
Most monosubstituted aromatics are named using -benzene as the parent name preceded by the substituent name (as a prefix; all one word):
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:Slide23
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 chaine.g., 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.
the benzyl group benzyl
bromide benzyl alcoholSlide25
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
These common names are used as base names when naming more their more highly substituted derivatives. Examples of these will be given later.Slide26
Mono-substituted Benzene Nomenclature: Common NamesSlide27
Disubstituted benzenes can be named in one of two ways. Each method describes the relative positions of the two groups on the benzene ring.
of the aromatic ring.
When numbering the ring carbons, carbon # 1 is always a substituted carbon.
The substituents are listed alphabetically.Slide28
ortho- (abbreviated o- ) = 1,2-disubstituted (two groups on adjacent carbons on the ring)Slide29
meta- (abbreviated m- ) = 1,3-disubstituted (two groups having one unsubstituted carbon between them)Slide30
para- (abbreviated p- ) = 1,4-disubstituted (two groups on opposite sides of the ring)Slide31
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.
Common Names of Disubs. Benzenes
There are a few nonsystematic (common) names for disubstituted benzenes that you should be familiar with:Slide33
Relative positions on a benzene ringortho- (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”)Slide34
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.
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.
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 SubstituentSlide39
Polycyclic Aromatic Hydrocarbons (PAH)
Metabolic byproducts of benzo [a] pyrene react with
DNA to form adducts, leading to carcinogenesis (cancer).Slide40
Three resonance forms and delocalized electronsSlide41
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.Slide42
Drugs that Contain a Benzene RingSlide43
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 aromaticitySlide46
Protonation of Pyrroles and PyridinesSlide47
A five-membered heterocycle with one nitrogen electron system similar to that of cyclopentadienyl anionFour 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 baseSlide48Slide49
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 barrierSlide50
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/molBenzene has 3 unsaturation sites but gives off only 206 kJ/mol on reacting with 3 H2 moleculesTherefore it has about 150 kJ more “stability” than an isolated set of three double bondsSlide51
Benzene’s Unusual Structure
All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bondsElectron density in all six C-C bonds is identicalStructure is planar, hexagonalC–C–C bond angles 120°Each C is sp2 and has a p orbital perpendicular to the plane of the six-membered ringSlide53
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 AromaticitySlide54
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 AromaticitySlide55
A molecule must be completely conjugated.Aromatic compounds must have a p orbital on every atom.
The Criteria for AromaticitySlide56
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.
The Criteria for AromaticitySlide57
rule refers to the number of
electrons, not the number of atoms in a particular ring.Slide58
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 levelsThis is a total of 4n + 2Slide59
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 OrbitalsSlide60
Two atomic p orbitals combine to form two molecular orbitals.The bonding p 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.
Formation of π and π* Molecular OrbitalsSlide61
Since each of the six carbon atoms in benzene has a p orbital, six atomic p orbitals combine to form six MOs.
Molecular Orbitals for BenzeneSlide62
Inscribed Polygon Method of Predicting AromaticitySlide63
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 AromaticitySlide64
Inscribed Polygon Method of Predicting AromaticitySlide65
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?Slide66
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 alkenes4- 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 temperatureCyclooctatetraene has four double bonds, reacting with Br2, KMnO4, and HCl as if it were four alkenesSlide67
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 orbitalsSlide68
Aromaticity of the Cyclopentadienyl Anion
1,3-Cyclopentadiene contains conjugated double bonds joined by a CH2 that blocks delocalizationRemoval of H+ at the CH2 produces a cyclic 6-electron system, which is stableRemoval of H- or H• generates nonaromatic 4 and 5 electron systemsRelatively acidic (pKa = 16) because the anion is stableSlide69
Cycloheptatriene has 3 conjugated double bonds joined by a CH2Removal of “H-” leaves the cationThe cation has 6 electrons and is aromaticSlide70
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 AromaticitySlide71
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 RingsSlide72
-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 -annulene is not planar, the 10 electrons cannot delocalize over the entire ring and it is not aromatic.
Hückel’s Rule and Number of ElectronsSlide73
Biochemically Relevant Aromatics
Biologically Relevant Aromatics
Nicotinamide adeine dinucleotide, the biolgical hydrogenator