Aromatic compounds
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Aromatic compounds

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Aromatic compounds




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Presentation on theme: "Aromatic compounds"— Presentation transcript:

Slide1

Aromatic compounds

Dr.

Sheppard

CHEM

2412

Summer 2015

Klein (2

nd

ed

.)

sections

: 18.1, 18.2, 18.8, 18.3, 18.4, 18.5

Slide2

Aromatic Compounds

Originally distinguished because of smellThen noticed trends in reactivityNow, highly unsaturated, stable compoundsUnreactive to many reagents that react with alkenesAromatic hydrocarbons = arenes (Ar-)Most famous is benzene

Slide3

Aromatic Compounds

Nomenclature Review

Physical Properties

Spectroscopy

Benzene Structure

Aromaticity

Slide4

I. Nomenclature (Review)

Monosubstituted benzenesSubstituent name + “benzene”

Slide5

Common Benzene Compounds

Slide6

Benzene Nomenclature

If substituent has greater than 6 carbons, it becomes the parent, and benzene is called a phenyl groupBenzene substituents:

(

Ph

‒ or

F

‒)

Slide7

Disubstituted Benzenes

ortho (1,2)meta (1,3)para (1,4)

Slide8

Naming Disubstituted Benzenes

If one substituent is part of a common name, that name is the parent and that substituent is at carbon 1If neither substituent is part of a common name, list the substituents in alphabetical order (first alphabetically is at carbon 1)If both substituents are part of common name, use this order of priority to determine the parent name:-CO2H > -CHO > -OH > -NH2 > -CH3

Slide9

Naming Polysubstituted Benzenes

With 3 or more substituents do not use ortho, meta, paraNumber ring to give smallest set of numbersIf a common name, use as parent (substituent at carbon 1)List substituents in alphabetical order

Slide10

II. Physical Properties

Melting pointBased on “packing”Benzene packs easily, so has a higher mp than other hydrocarbonsSubstituted benzenes: para > ortho and meta due to packingBoiling pointPolarity depends on substituentsHigher polarity = higher boiling point

mp (°C)-17-2554bp (°C)181173170

Slide11

III. Spectroscopy of Aromatics: IR

sp2 C-H absorption at 3030 cm-1 Ring absorptions at 1450-1600 and 1660-2000 cm-1Also peaks in fingerprint region can differentiate substitution pattern

Slide12

13C-NMR: 1H-NMR:

III. Spectroscopy of Aromatics: NMR

Slide13

IV. Benzene Structure and Stability

Cyclic, planar, hexagonal shapeConjugatedHybridization of carbons?Bond angles?All H’s are identicalAll C-C bonds are equivalentBond order = 1.5

Slide14

Benzene Reactivity

Unsaturated, but doesn’t behave like alkeneAlkenes:Benzene:Benzene will reduce at high pressure and temperature or with special catalyst:

Slide15

Explanation for Benzene Stability

Molecular orbital modelBonding and antibonding molecular orbitalsSkip thisResonance modelLots of orbital overlap and conjugation = very stable

Kekulé

structures

Hybrid structure

Slide16

V. Aromaticity

All aromatic structures are similar in stability and reactivityAll have structural similaritiesExample: Benzene is aromaticBut, 1,3-cyclobutadiene is not aromaticEven though it resembles benzene and is resonance-stabilizedReacts like an alkene (addition) not benzene (substitution)

Slide17

Criteria for Aromaticity

HückelBased on molecular orbital calculationsCyclicPlanarUnhybridized p orbital on each atom of the ring(4n + 2) electrons in the p orbitals

n

4n + 2

0

2

1

6

2

10

3

14

4

18

Slide18

Examples

BenzeneIs it cyclic?Is it planar?Does it have an unhybridized p orbital on each atom of the ring?How many electrons does it have in the p orbitals? Is that equal to (4n + 2)?

Slide19

Examples

[14] annulene[#] is number of atoms in ringAnnulene = cyclic, conjugated hydrocarbonIs it cyclic?Is it planar?Does it have an unhybridized p orbital on each atom of the ring?How many electrons does it have in the p orbitals? Is that equal to (4n + 2)?

Slide20

Aromaticity

Molecules that are not aromatic (do not satisfy the 4 criteria listed above) are either:AntiaromaticNonaromaticThe reactivity of both antiaromatic and nonaromatic molecules will be like alkenes (addition reactions)

Slide21

Antiaromatic

CyclicPlanarUnhybridized p orbital on each atom of the ring(4n) electrons in the p orbitalsExamples:

Slide22

Nonaromatic

Either not cyclic, or not conjugated, or not planarExamples:Not cyclic:Not conjugated:Not planar:

Slide23

Aromaticity of Ions

Cyclopentadienyl cationCyclicPlanarUnhybridized p orbitals4 electrons = 4nAntiaromaticCyclopentadienyl anionCyclicPlanarAtom with : can become sp2 hybridizedLone pair in p orbitalUnhybridized p orbitals6 electrons = 4n+2AromaticVery stable, likely to form (H lost is more acidic than other hydrocarbon H’s)Cyclopentadiene pKa = 16; cyclopentane pKa > 50

Slide24

Aromaticity of Heterocycles

Heteroatoms with lone pairsDetermine which electrons are part of aromatic systemExamples:PyridineFuranRemember aromatic amines are weaker bases than aliphatic aminesPartially due to resonance delocalizationAlso due to lone pair electrons counting toward aromaticityPyrrole loses aromaticity when protonatedWhat about pyridine?

Slide25

Polycyclic aromatic compounds

Fused ring systemsNaphthaleneIndole

Fused benzene rings

Slide26

Aromatic, antiaromatic, or nonaromatic?