H C H C C H H C C C H

Benzene and

H

the Concept of Aromaticity

two equivalent resonance forms: other representations:

Bond Lengths in Benzene

H C H C H C C C C H H H (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) •

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

p

electron delocalization

(Un)Reactivity (stability) of Benzene

•

Highly unsaturated (r + db = 4), yet neither Br 2 nor HBr adds across its multiple bonds as with alkenes.

•

Reacts with Br 2 in presence of FeBr 3 catalyst by SUBSTITUTION rather than by addition (which is the way alkenes react with Br 2 ).

H Br 2 Br 2 Br + Br HBr + HBr FeBr 3 FeBr 3

+ H2 -119.7 kJ/mol

Stability of Benzene

Hypothetical ‘cyclohexatriene’ Actual Benzene + 3H2 expect: 3X-119.7= -359.1 kJ/mol + 2H2 -231.8 kJ/mol + 3H2 observe: -208.4 kJ/mol (150.7 kJ/mol more stable!) (same product)

(six 2p orbitals, each w/ 1 electron) Pi Bonding in Benzene

 * 6  * 4  * 5  2  3  1

antibonding bonding

Delocalization in Benzene

Note complete delocalization of p electrons!

Hückel Definition of Aromaticity For a system to be aromatic, it must have:

•

4n + 2

p

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

H H H

#

p

e = 6 2 6 6

Examples of Aromatic Systems

H H H H H H H H H H H H H H H H H H etc.

H H H H etc.

H H H H H etc.

H H H 1 more res. form 3 more res. forms 5 more res. forms

Some Non-Aromatic Systems

H HH

#

p

e = 4 8 4 10 (not planar!)

Some Heteroaromatic Systems

4 5 6 N 1 2 3 5 4 3 N H 1 2 5 4 N 3 N H 1 2

pyridine pyrrole imidazole (has a lp in sp 2 ) (lp is in p orbital) (has a lp in sp 2 hybrid orbital + a lp in p orbital)

Bonding in Pyrrole & Imidazole

H N 4 electrons in p bonds plus 2 lp electrons = 6 p electrons (6 is a Hückel #) lp in p orbital N H

Polycyclic Aromatic Compounds 10

p

e naphthalene 14

p

e anthracene 14

p

e 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 Ar C=C 3030 cm -1 1600, 1500, (1450) cm -1 (2 or 3 sharp bands) 1 H-NMR: 13 C-NMR: UV: Ar H Ar C n -

p

-

p

*

p

* 6.5-8.0

d (

benzene = 7.27

d

) 110-160

d (

benzene = 128.5

d)

205 nm (strong) 255-275 nm (weak)

Summary: Attributes of Aromatics

• • • •

Exceptional Stability

•

Due to extended system of delocalized

p

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

p

electrons establish a ‘ring current’ which causes large downfield shifts of aryl protons.