Chapter 15 Benzene

I. Benzene Structure and NomenclatureA. Structure of Benzene

1) Faraday in 1825 isolates a colorless liquid from whale oila) Empirical formula = CH (C need 4 bonds?)b) Very inert

2) Later, the molecular formula of C6H6 was determined and named benzenea) Degrees of unsaturation = 4b) 1,3,5-cyclohexatriene structure is proposedc) Not reactive as conjugated polyenes should be

3) Various possible structures are proposed:a) Dewar Benzeneb) Ladenburg Prismanec) Benzvalened) Claus Benzenee) All but (d) go to benzene

a b

c d

4) Reactivity of Benzene

a) Benzene is a relatively inert molecule: no reaction with Br2

b) Reaction in the presence of a catalyst with Br2 does give product

c) Problem: If the ring is really alternating double and single bonds, we should have gotten 1,2 addition (Br on C=C) and 1,6 addition (C—C)

H

H

H

H

H

HBr2

FeBr3

Br

H

H

H

H

H

FeBr3

Br2

Br

Br

H

H

H

H

Br

H

Br

H

H

H

Br

H

H

Br

H

H

Br

H

H

H

H

H

+ +

Symmetric productagrees with structure

Br

Br

H

H

H

H

Br

Br

H

H

H

H

1,2 product 1,6 product

a) The fact that we only have one 1,2-disubstituted product supports a resonance hybrid structure

B. Nomenclature

1) Aromatic Compounds = benzene and its substituted analogues

2) We draw them as a single resonance structure, but we always mean the resonance hybrid

3) Monosubstituted Benzenes are named with the substituent as prefix:

=

F NO2

fluorobenzene nitrobenzene t-butylbenzene

4) Disubstituted benzene have three possible arrangements

1) 1,2 is also known as ortho (o-)

2) 1,3 is also known as meta (m-)

3) 1,4 is also known as para (p-)

5) Polysubstituted: number with the lowest set possible, label substituents as in cyclohexane nomenclature

Cl

Cl

Br

NO2

1,2-dichlorobenzeneo-dichlorobenzene

1-bromo-3-nitrobenzenem-bromonitrobenzene

1-ethyl-4-(1-methylethyl)benzenep-ethylisopropylbenzene

NO2

NO2

NO2

Br

1-bromo-2,3-dimethyl benzene

1,2,4-trinitrobenzene

1-ethenyl-3-ethyl-5-ethynylbenzene

6) Special cases: common names

a) Name with substituents before the common name

b) The substituent giving the common name is #1

OH CHO COOH COCH3 CH3

NH2OCH3CH2CH3

CH3H3C

CH3

CH3

phenol benzaldehyde benzoic acid acetophenone toluene

o-xylene mesitylene styrene anisole aniline

o-iodotoluene

CH3

I

2,4,6-tribromophenol

OH

Br Br

Br

CH2

Cl

m-chlorostyrene

7) A substituted benzene is called an arene

8) An arene as a substituent is called an aryl group (benzene itself is phenyl)

9) A phenylmethyl group is called benzyl

II. AromaticityA. Structure of benzene revisited

1) Ring of six sp2 hybridized carbons

a) Six p-orbitals give six MO’s with six electrons

b) -cloud above and below the plane of the molecule

c) Completely symmetric

d) Not a cyclohexatriene structure

benzyl alcohol

CH2OH

1-phenyl-2-methylcyclohexane

CH3

e) Orbital picture of benzene

2) Heats of Hydrogenation

H2

Pt

Ho = -28.6 kcal/mol

Ho = -54.9 kcal/mol

Ho = -49.3 kcal/mol Hocalculated = -78.9 kcal/mol

a) Resonance Energy of Benzene = 30 kcal/mol

b) Also called: Delocalization Energy, Aromaticity

C. MO Description of Benzene

1) 1,3,5-hexatriene

a) Similar to 1,3-butadiene

MO picture

b) 3 bonding MO’s are filled, so

conjugation stabilizes the molecule

c) 6 -MO’s with 6 -electrons

1,3,5-hexatriene

2) Benzene is a cyclic system, which changes how the MO’s are arranged

a) 6 -MO’s with 6 -electrons

b) The nodes intersect

c) There are degenerate orbitals

3) Energy comparison shows that the benzene structure is more stable

a) Benzene: 2 MO’s lowered in energy and 1 MO raised

b) Hexatriene: 1 MO lowered in energy and 2 MO’s raised

c) Overlap of terminal p-orbitals in 1, 2, and 3 determine energies

d) Aromatic Transition states favor concerted reactions:

O

Os

O

O

O

III. Spectroscopy of Aromatic SystemsA. UV-Vis Spectroscopy

1) The energy gap between HOMO and LUMO is large for aromatics because of extra aromatic stabilization

2) More energetic absorption is needed for electronic transition

a) max will be smaller than for trienes

b) Sample spectra

3) Substitution alters the energy levels and thus the spectrum and the color

1) Many dyes are aromatic compounds

2) Many sun-tan lotions contain PABA = p-aminobenzoic acid to block UV rays (max = 289 nm, = 18,600)

B. IR Spectroscopy

1) Typical aromatic IR bands

a) Aromatic C—H stretch = 3030 cm-1

b) Aromatic C—C = 1500-2000 cm-1

c) Aromatic C—H bending = 650-1000 cm-1

i. Can be used to determine substitution pattern

ii. C—H bending for different substitutions:

COOH

NH2

R R

R

R

R

R

R

690-710730-770

735-770 690-710750-810

790-840

O-xylene

C. 1H NMR Spectroscopy

1) Aromatic protons are highly deshielded due to ring current

a) Benzenes have C—H protons from 6.5-8.5 ppm

b) Alkenes are 4.6-5.7 ppm

2) Benzylic groups are not as deshielded. Ring current fades quickly.

3) Substitution pattern dictates the spectrum pattern

a) Benzene itself has only one singlet at 7.27 ppm

b) Spectral Examples

H2C CH CH3 1.68 ppm

CH3

CH2CH3

2.35 ppm

1.10 ppm

5) Coupling Constants

1) Ortho H’s = 9 Hz

2) Meta H’s = 3 Hz

3) Para H’s < 1 Hz

D. 13C NMR Spectroscopy

1) Ring current does not have a large effect on the carbons

2) Carbon shifts are very similar to the alkenes: 120-135 ppm

IV. Polycyclic Aromatic Hydrocarbons (PAH’s)A. Structure and Nomenclature

1) Two or more benzene rings share 2 or more Carbons to be a PAH

2) The general name for the series is the acenes (pentacene = 5 rings)

3) Angular fusion gives different compounds

CH3 21.3

137.8

129.3

128.5125.6 134.7

129.5

123.4

148.3NO2

Naphthalene Anthracene Phenanthrene Tetracene

B. Is Napthalene Aromatic?

1) White solid, mp = 80 oC, used as mothbolls

2) UV-Vis spectrum looks like a conjugated -system, but with more delocalization than in benzene

3) Structure = Symmetric like benzene

4) 1H NMR confirms that naphthalene is aromatic

C. Large acenes are also aromatic. The more benzene rings the better.

1) Anthracene

2) Phenanthrene