Chapter 15 Benzene I.Benzene Structure and Nomenclature A.Structure of Benzene 1)Faraday in 1825 isolates a colorless liquid from whale oil a)Empirical formula = CH (C need 4 bonds?) b)Very inert 2)Later, the molecular formula of C 6 H 6 was determined and named benzene a)Degrees of unsaturation = 4 b)1,3,5-cyclohexatriene structure is proposed c)Not reactive as conjugated polyenes should be 3)Various possible structures are proposed: a)Dewar Benzene b)Ladenburg Prismane c)Benzvalene d)Claus Benzene e)All but (d) go to benzene

4)Reactivity of Benzene a)Benzene is a relatively inert molecule: no reaction with Br 2 b)Reaction in the presence of a catalyst with Br 2 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)

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:

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

6)Special cases: common names a)Name with substituents before the common name b)The substituent giving the common name is #1

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.Aromaticity A.Structure of benzene revisited 1)Ring of six sp 2 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

e)Orbital picture of benzene 2)Heats of Hydrogenation  H o = kcal/mol  H o = kcal/mol  H o = kcal/mol  H o calculated = 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:

III.Spectroscopy of Aromatic Systems A.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 = cm -1 c)Aromatic C—H bending = cm -1 i.Can be used to determine substitution pattern ii.C—H bending for different substitutions:

O-xylene

C. 1 H NMR Spectroscopy 1)Aromatic protons are highly deshielded due to ring current a)Benzenes have C—H protons from ppm b)Alkenes are 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

5)Coupling Constants 1)Ortho H’s = 9 Hz 2)Meta H’s = 3 Hz 3)Para H’s < 1 Hz

D. 13 C NMR Spectroscopy 1)Ring current does not have a large effect on the carbons 2)Carbon shifts are very similar to the alkenes: 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

B.Is Napthalene Aromatic? 1)White solid, mp = 80 o C, 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) 1 H NMR confirms that naphthalene is aromatic

C.Large acenes are also aromatic. The more benzene rings the better. 1)Anthracene 2)Phenanthrene