Benzene and the Concept of Aromaticity

other representations:

two equivalent resonance forms:

C

CC

C

CC

H

H

H

H

H

H

Bond Lengths in Benzene

C

CC

C

CC

H

H

H

H

H

H

• All carbon-carbon bond lengths in benzene are EQUAL (139 pm)

• This is intermediate between a typical C-C (single) bond (154 pm) and a typical C=C (double ) bond (134 pm)

• This is consistent with electron delocalization

(We usually draw benzene with alternating single and double bonds for ease of electron counting, but we must remember that the electrons are actually delocalized around the ring)

(Un)Reactivity (stability) of Benzene

• Highly unsaturated (r + db = 4), yet neither Br2 nor HBr adds across its multiple bonds as with alkenes.

• Reacts with Br2 in presence of FeBr3 catalyst by SUBSTITUTION rather than by addition (which is the way alkenes react with Br2).

H Br

HBr+

Br2

FeBr3

Br2

FeBr3

Br

+ HBr

Stability of Benzene

(same product)

Hypothetical ‘cyclohexatriene’

ActualBenzene

-231.8 kJ/mol

-119.7 kJ/mol

observe:-208.4 kJ/mol(150.7 kJ/molmore stable!)

expect: 3X-119.7= -359.1 kJ/mol

+ H2

+ 2H2

+ 3H2

+ 3H2

Pi Bonding in Benzene

antibonding

bonding1

2 3

54

6

**

*

(six 2p orbitals,each w/ 1 electron)

Delocalization in Benzene

Note complete delocalization of electrons!

Hückel Definition of Aromaticity

For a system to be aromatic, it must have:• 4n + 2 electrons (for n = any integer: 0, 1, 2, etc.)• in the periphery• of a monocyclic• planar• delocalized (conjugated) system

(Hückel’s # = (4n+2) = 2, 6, 10, 14, 18, etc. for various integral values of n)

Examples of Aromatic Systems

# e = 6 2 6 6

HH

H

HH

H

H H

H

HH

H

HH

HH

HH

H

H

HH

H

HH

H

HH

H

HH

HH

etc. etc. etc.

Examples of Aromatic Systems

1 more res. form

3 more res. forms

5 more res. forms

Some Non-Aromatic Systems

# e = 4 8 4 10 (not

planar!)

H

HH

Some Heteroaromatic Systems

pyridine pyrrole imidazole

(has a lp in sp2) (lp is in p orbital) (has a lp in sp2 hybrid orbital + a lp in

p orbital)

NN

H

N

N

H1

2

3

4

1

2

345

561

2

34

5

Bonding in Pyrrole & Imidazole

NH

lp in p orbital

N

H

4 electrons in bonds plus 2 lp electrons = 6 electrons (6 is a Hückel #)

Polycyclic Aromatic Compounds

10 e- 14 e- 14 e-

naphthalene anthracene phenanthrene

All three are found in coal. Note that in this Kekule

resonance form they obey Hückel’s rule; try others!

Spectra of Aromatic Compounds

IR: Ar C-H 3030 cm-1

Ar C=C 1600, 1500, (1450) cm-1 (2 or 3 sharp bands)

1H-NMR: Ar H 6.5-8.0 benzene = 7.27 )13C-NMR: Ar C 110-160 benzene = 128.5

UV: n - * 205 nm (strong) - * 255-275 nm (weak)

Summary: Attributes of Aromatics

• Exceptional Stability• Due to extended system of delocalized

electrons (conjugation)• Special (un)Reactivity

• Electrophilic substitution rather than addition• Bond Length Equalization

• All C-C bond lengths are equal in benzene; there are NOT single bonds and double bonds.

• Magnetic Properties• The circulating electrons establish a ‘ring

current’ which causes large downfield shifts of aryl protons.