© Prentice Hall 2001Chapter 61 The Structure of Benzene Molecular Formula:C 6 H 6 Sum of Rings and  Bonds = 4 Acyclic compound with 4  Bonds Cyclic compound with 3  Bonds Bicyclic compound with 2  Bonds Tricyclic compound with a single  Bond Tetracyclic compound

© Prentice Hall 2001Chapter 62 The Structure of Benzene Two structures consistent with observation of a single monosubstitution product are:

© Prentice Hall 2001Chapter 63 The Structure of Benzene Only two disubstituted products are possible

© Prentice Hall 2001Chapter 64 The Structure of Benzene Four disubstituted products are possible

© Prentice Hall 2001Chapter 65 The Structure of Benzene In 1865 Kekulé suggested an answer He viewed benzene as a rapid equilibrium between two structures The Kekulé structure explained the formula for benzene and the number of substitution products, BUT it failed to explain benzene's unusual stability

© Prentice Hall 2001Chapter 66 The Structure of Benzene Confirmation that benzene is a six- membered ring came in 1901 when Sabatier hydrogenated it to form cyclohexane Later X-ray diffraction experiments showed that all carbon–carbon bonds in benzene are equivalent

© Prentice Hall 2001Chapter 67 Bonding in Benzene (a)Carbon–carbon and carbon–hydrogen  bonds (b)Carbon p-orbitals overlap with neighbors (c)Cloud of  electrons above and below ring (d)Electrostatic potential map for benzene

© Prentice Hall 2001Chapter 68 Bonding in Benzene Representations of Bonding in Benzene These representations do not specify the number of  electrons (which is six)

© Prentice Hall 2001Chapter 69 Delocalized Electrons and Resonance Typically there are two or more resonance structures, with localized electrons that together represent a delocalized structure The delocalized structure is called a resonance hybrid Resonance contributors are drawn with a double headed arrow between them

© Prentice Hall 2001Chapter 610 Rules for Drawing Resonance Structures Move  electrons toward a positive charge

© Prentice Hall 2001Chapter 611 Rules for Drawing Resonance Structures Move  electrons toward a (forming)  bond

© Prentice Hall 2001Chapter 612 Rules for Drawing Resonance Structures Move a nonbonding pair toward a (forming)  bond

© Prentice Hall 2001Chapter 613 Rules for Drawing Resonance Structures Move a single nonbonding electron toward a (forming)  bond

© Prentice Hall 2001Chapter 614 Rules for Drawing Resonance Structures As electrons move, they must move to an sp 2 carbon Electrons cannot move through or over an sp 3 carbon

© Prentice Hall 2001Chapter 615 The Resonance Hybrid When there is a choice, electrons tend to move toward the more electronegative atom

© Prentice Hall 2001Chapter 616 Resonance Energy

© Prentice Hall 2001Chapter 617 Resonance Energy Benzene is calculated to have 36 kcal/mol of resonance energy

© Prentice Hall 2001Chapter 618 Resonance Energy The resonance energy tells us how much more stable the compound with delocalized electrons is due to that delocalization The greater the number of relatively stable resonance contributors, the greater the resonance energy

© Prentice Hall 2001Chapter 619 Resonance Energy It is the number of relatively stable resonance contributors, not the total number of contributors, that determines the resonance energy

© Prentice Hall 2001Chapter 620 Resonance Energy The more nearly equivalent the resonance structures, the greater the resonance energy