Chapter 16 Aromatics 1

Chapter 16 Aromatic Compounds

Discovery of Benzene

• Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. • Synthesized in 1834 by Eilhard Mitscherlich who

determined molecular formula to be C6H6. • Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. =>

Chapter 16: Aromatics Slide 16-2

1 Kekulé Structure

• Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested. • Failed to explain existence of only one isomer of 1,2- dichlorobenzene.

H C H H C C C C H C H => H

Chapter 16: Aromatics Slide 16-3

Resonance Structure

Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring.

Chapter 16: Aromatics Slide 16-4

2 Unusual Reactions

• Alkene + KMnO4 → diol (addition) Benzene + KMnO4 → no reaction.

• Alkene + Br2/CCl4 → dibromide (addition) Benzene + Br2/CCl4 → no reaction.

• With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain. =>

Chapter 16: Aromatics Slide 16-5

Unusual Stability Hydrogenation of just one double bond in benzene is endothermic!

Chapter 16: Aromatics Slide 16-6

3 Annulenes

• All cyclic conjugated hydrocarbons were proposed to be aromatic. • However, cyclobutadiene is so reactive that it dimerizes before it can be isolated.

• And cyclooctatetraene adds Br2 readily. • Look at MO’s to explain aromaticity. =>

Chapter 16: Aromatics Slide 16-7

MO Rules for Benzene

• Six overlapping p orbitals must form six molecular orbitals. • Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no nodes. • As energy of MO increases, the number of nodes increases. =>

Chapter 16: Aromatics Slide 16-8

4 MO’s for Benzene

Chapter 16: Aromatics Slide 16-9

Energy Diagram for Benzene

• The six electrons fill three bonding pi orbitals. • All bonding orbitals are filled (“closed shell”), an extremely stable arrangement.

Chapter 16: Aromatics Slide 16-10

5 MO’s for Cyclobutadiene

Chapter 16: Aromatics Slide 16-11

Energy Diagram for Cyclobutadiene

• Following Hund’s rule, two electrons are in separate orbitals. • This diradical would be very reactive.

Chapter 16: Aromatics Slide 16-12

6 Polygon Rule

The energy diagram for an annulene has the same shape as the cyclic compound with one vertex at the bottom.

Chapter 16: Aromatics Slide 16-13

Aromatic Requirements

• Structure must be cyclic with conjugated pi bonds. • Each atom in the ring must have an unhybridized p orbital. • The p orbitals must overlap continuously around the ring. (Usually planar structure.) • Compound is more stable than its open-chain counterpart. =>

Chapter 16: Aromatics Slide 16-14

7 Anti- and Nonaromatic • Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals around the ring, but the energy of the compound is greater than its open-chain counterpart. • Nonaromatic compounds do not have a continuous ring of overlapping p orbitals and may be nonplanar. =>

Chapter 16: Aromatics Slide 16-15

Hückel’s Rule

• If the compound has a continuous ring of overlapping p orbitals and has 4N + 2 electrons, it is aromatic. • If the compound has a continuous ring of overlapping p orbitals and has 4N electrons, it is antiaromatic.

Chapter 16: Aromatics Slide 16-16

8 [N]Annulenes

• [4]Annulene is antiaromatic (4N e-’s) • [8]Annulene would be antiaromatic, but it’s not planar, so it’s nonaromatic. • [10]Annulene is aromatic except for the isomers that are not planar. • Larger 4N annulenes are not antiaromatic because they are flexible enough to become nonplanar. =>

Chapter 16: Aromatics Slide 16-17

MO Derivation of Hückel’s Rule

• Lowest energy MO has 2 electrons. • Each filled shell has 4 electrons.

Chapter 16: Aromatics Slide 16-18

9 Cyclopentadienyl Ions

• The cation has an empty p orbital, 4 electrons, so antiaromatic. • The anion has a nonbonding pair of electrons in a p orbital, 6 e-’s, aromatic.

Chapter 16: Aromatics Slide 16-19

Acidity of Cyclopentadiene pKa of cyclopentadiene is 16, much more acidic than other hydrocarbons.

Chapter 16: Aromatics Slide 16-20

10 Tropylium Ion

• The cycloheptatrienyl cation has 6 p electrons and an empty p orbital. • Aromatic: more stable than open chain ion.

H H OH + => H , H2O +

Chapter 16: Aromatics Slide 16-21

Dianion of [8]Annulene

• Cyclooctatetraene easily forms a -2 ion. • Ten electrons, continuous overlapping p orbitals, so it is aromatic.

Chapter 16: Aromatics Slide 16-22

11 Pyridine • Heterocyclic aromatic compound. • Nonbonding pair of electrons in sp2 orbital, so weak base,

pKb = 8.8.

Chapter 16: Aromatics Slide 16-23

Pyrrole Also aromatic, but lone pair of electrons is delocalized, so much weaker base.

Chapter 16: Aromatics Slide 16-24

12 Basic or Nonbasic?

Pyrimidine has two basic N N nitrogens.

N N H Imidazole has one basic nitrogen and one nonbasic.

N N Purine? N N => H Chapter 16: Aromatics Slide 16-25

Other Heterocyclics

=> Chapter 16: Aromatics Slide 16-26

13 Fused Ring Hydrocarbons

• Naphthalene

• Anthracene

• Phenanthrene

Chapter 16: Aromatics Slide 16-27

Reactivity of Polynuclear Hydrocarbons

As the number of aromatic rings increases, the resonance energy

per ring decreases, so larger PAH’s will add Br2.

H Br

Br H Br H H Br (mixture of cis and trans isomers) =>

Chapter 16: Aromatics Slide 16-28

14 Larger Polynuclear Aromatic Hydrocarbons • Formed in combustion (tobacco smoke). • Many are carcinogenic. • Epoxides form, combine with DNA base.

pyrene =>

Chapter 16: Aromatics Slide 16-29

Allotropes of Carbon • Amorphous: small particles of graphite; charcoal, soot, coal, carbon black. • Diamond: a lattice of tetrahedral C’s. • Graphite: layers of fused aromatic rings.

Chapter 16: Aromatics Slide 16-30

15 Diamond

• One giant molecule. • Tetrahedral carbons. • Sigma bonds, 1.54 Å. • Electrical insulator.

Chapter 16: Aromatics Slide 16-31

Graphite

• Planar layered structure. • Layer of fused benzene rings, bonds: 1.415 Å. • Only van der Waals forces between layers. • Conducts electrical current parallel to layers.

Chapter 16: Aromatics Slide 16-32

16 Some New Allotropes

• Fullerenes: 5- and 6-membered rings arranged to form a “soccer ball” structure.

• Nanotubes: half of a C60 sphere fused to a cylinder of fused aromatic rings.

Chapter 16: Aromatics Slide 16-33

Fused Heterocyclic Compounds

Common in nature, synthesized for drugs.

Chapter 16: Aromatics Slide 16-34

17 Common Names of Benzene Derivatives

OH CH OCH 3 NH2 3

phenol toluene aniline anisole

H O O O C CH2 C C C CH3 H OH styrene acetophenone benzaldehyde benzoic acid =>

Chapter 16: Aromatics Slide 16-35

Disubstituted Benzenes The prefixes ortho-, meta-, and para- are commonly used for the 1,2-, 1,3-, and 1,4- positions, respectively.

Chapter 16: Aromatics Slide 16-36

18 3 or More Substituents Use the smallest possible numbers, but the carbon with a functional group is #1.

OH O2N NO2 O2N NO2

NO2 NO2 1,3,5-trinitrobenzene 2,4,6-trinitrophenol =>

Chapter 16: Aromatics Slide 16-37

Common Names for Disubstituted Benzenes

O OH CH3 CH3 C OH CH3

CH3 H3C CH3 H C m-xylene mesitylene o-toluic acid 3 p-cresol

Chapter 16: Aromatics Slide 16-38

19 Phenyl and Benzyl

Phenyl indicates the benzene ring attachment. The benzyl group has an additional carbon.

Br CH2Br

phenyl bromide benzyl bromide

Chapter 16: Aromatics Slide 16-39

Physical Properties

• Melting points: More symmetrical than corresponding alkane, pack better into crystals, so higher melting points. • Boiling points: Dependent on dipole moment, so ortho > meta > para, for disubstituted benzenes. • Density: More dense than nonaromatics, less dense than water. • Solubility: Generally insoluble in water. =>

Chapter 16: Aromatics Slide 16-40

20 IR and NMR Spectroscopy

• C=C stretch absorption at 1600 cm-1. • sp2 C-H stretch just above 3000 cm-1. • 1H NMR at δ7-δ8 for H’s on aromatic ring. • 13C NMR at δ120-δ150, similar to alkene carbons.