Notes

Outline

Chapter 11
Arenes and Aromaticity

Examples of Aromatic Hydrocarbons

11.1
Benzene

Some history


	1825 Michael Faraday isolates a new hydrocarbon from illuminating gas.
	1834 Eilhardt Mitscherlich isolates same substance and determines its empirical formula to be C_nH_n. Compound comes to be called benzene.
	1845 August W. von Hofmann isolates benzene from coal tar.
	1866 August Kekulé proposes structure of benzene.

11.2
Kekulé and the
Structure of Benzene

Kekulé Formulation of Benzene


	Kekulé proposed a cyclic structure for C₆H₆ with alternating single and double bonds.

Kekulé Formulation of Benzene


	Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures.

Kekulé Formulation of Benzene


	However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found.

Structure of Benzene


	Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length.

All C—C bond distances = 140 pm

All C—C bond distances = 140 pm


	140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene.

11.3
A Resonance Picture of Bonding in Benzene

Kekulé Formulation of Benzene


	Instead of Kekulé's suggestion of a rapid equilibrium between two structures:

Resonance Formulation of Benzene


	express the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds, but are delocalized over all six ring carbons.

Resonance Formulation of Benzene


	Circle-in-a-ring notation stands for resonance description of benzene (hybrid of two Kekulé structures)

11.4
The Stability of Benzene


	benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity"
	aromaticity is a level of stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it

Thermochemical Measures of Stability


	heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene"

Figure 11.2 (p 404)

Figure 11.2 (p 404)


	"expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene

Figure 11.2 (p 404)


	observed heat of hydrogenation is 152 kJ/mol less than "expected"
	benzene is 152 kJ/mol more stable than expected
	152 kJ/mol is the resonance energy of benzene

Figure 11.2 (p 404)


	hydrogenation of 1,3-cyclohexadiene (2H₂) gives off more heat than hydrogenation of benzene (3H₂)!

Cyclic conjugation versus noncyclic conjugation

Resonance Energy of Benzene


	compared to localized 1,3,5-cyclohexatriene
		152 kJ/mol
	compared to 1,3,5-hexatriene
		129 kJ/mol
	exact value of resonance energy of benzene depends on what it is compared to, but regardless of model, benzene is more stable than expected by a substantial amount

11.5
An Orbital Hybridization View
of Bonding in Benzene

Orbital Hybridization Model of
Bonding in Benzene


	Planar ring of 6 sp² hybridized carbons

Orbital Hybridization Model of
Bonding in Benzene


	Each carbon contributes a p orbital
	Six p orbitals overlap to give cyclic p system; six p electrons delocalized throughout p system

Orbital Hybridization Model of
Bonding in Benzene


	High electron density above and below plane of ring

11.6
The p Molecular Orbitals
of Benzene

Benzene MOs


	6 p AOs combine to give 6 p MOs
	3 MOs are bonding; 3 are antibonding

Benzene MOs


	All bonding MOs are filled
	No electrons in antibonding orbitals

The Three Bonding p MOs of Benzene