LMZ/CHEM109A/ Winter 2010

Chapter 7: Electron Delocalization, Resonance structures and More Molecular Orbital Theory I. Benzene (sec. 7.1-7.2): not cyclohexatriene (literally) All six hydrogens are equivalent, and carbon-carbon bonds are of the same length, 1.39Å A. Benzene is more stable than a triene. Heat of hydrogenation

Hydrogenation

Hydrogenation

H = - 28.6 kcal/mol

H = - 49.8 kcal/mol (< 3x28.6)

" "

H2

3H2 B. Unlike alkenes, benzene does not undergo electrophilic addition reactions with bromine C. Bonding in Benzene: electron delocalization II. Resonance structures: electron delocalization (mainly π or n electrons) (sec. 7.4) a simple but powerful method to explain bonding and stability A. examples

B. Rules for drawing resonance structures (contributors)-hyperthetical structures 1. only electrons move, atoms never move; 2. electron numbers stay the same 3. the octet rule (nobel gas electron configuration) can not be exceeded (but could be

unfulfilled, e.g. carbocation) 4. only π or n electrons move (exceptions with hyperconjugation, where σ electrons

involved) e.g. 5. The weighted average of all the resonance structures (resonance hybrid) represents the real picture of the molecule. C. Predict relative stability of resonance structures factors make resonance structures with higher energy (with less stability) 1. an atom with an incomplete octet 2. a negative charge not on the most electronegative atom or a positive charge not on the least electronegative atom 3. charge separation D. Electron delocalization (resonance) offers stability (sec. 7.6) - resonance structure contribution to stability - the more stable a resonance structure is, the more contribution it has to the real molecule, the more the structure is stable, the more similar it is to the real molecule structurally/reactivity-wise. - the greater the number of relatively stable resonance structures, the more stable the molecule is (more resonance energy/delocalization energy) (not just the sheer number of resonance structures). E. Examples: -conjugated dienes vs nonconjugated dienes vs cumulative dienes (allene)

LMZ/CHEM109A/ Winter 2010

- allylic and benzylic cations III. Molecular Orbital Description of Stability - 1,3-butadiene and 1,4-pentadiene and 1,2-propadiene for delocalization, the p orbitals need to be parallel (double bonds coplanar) - HOMO (highest occupied molecular orbital) -- electrons in these orbitals are the most energetic and reactive except lone electron pairs (n electrons) - LUMO (lowest unoccupied molecular orbital) -- the most stable orbital for filling additional electrons. - 1,3,5-hexatrienes vs benzene all p orbitals are parallel in order to achieve side-by-side overlap, thus delocalization - benzene

aromatic unusually stable due to large delocalization energy IV. Delocalization on pKa - resonance structures and inductive effect V. Delocalization on reactivity: regioselectivity (sec. 7.10) - benzylic cation vs secondary carbocation - allyl cation vs secondary carbocation

LMZ/CHEM109A/ Winter 2010

VI. Thermodynamic control vs kinetic control thermodynamics: equilibrium; kinetics: reaction rate thermodynamic control: equilibrium control, reversible, stability control kinetic control: irreversible ; rate control - why 1,2-adduct less stable in the above case? why is it formed faster? kinetic difference is not due to: but, proximity effect

- In conjugated dienes, 1,2-adduct is always kinetic product, 1,4-adduct is not always the thermodynamic product -- It can be 1,2-adduct. VII. Diels-Alder Reactions: A type of cycloaddition reaction (Sec. 7.12) A. Introduction: Concerted Mechanism

H Z HZ

HZ+

Diene Dienophile Diels-Alder Adducts

Z = electron withdrawinggroup (e.g. -NO2, -CN, -CHO, -COOR)

1. Molecular orbital description 2. favored by electron withdrawing groups (e.g. –CHO) on dienophile and electron donating groups (e.g. -OMe) on dienes

H CN

+HCN

HCN+

e.g. 1

B. Stereochemistry: Cycloaddition reactions are stereospecific 1. Stereochemical relationships of substituents of dienophiles and dienes are retained.

cce

f

a

ba

b

d

b

b

a

a e

dfe

b

b

a

a c

fd

ec

d

a

ba

b

f

and

cedf

b

b

a

a

ecfd

b

b

a

a

LMZ/CHEM109A/ Winter 2010

H CN

+H

CN HCN

+C HN

CN HCNH

Me

Me

Me

Me

Me

Me

e.g. 2

2. Endo product is favored kinetically because of stabilization of transition state by secondary orbital interactions

H CN

+

HCN

HCN+

e.g. 3

H CHO+

e.g. 4major (endo)

H CHO

HCHO H

CHO

+O O

major (endo)

OH CHO

CHO H

+

e.g. 5

O

O

OO

HH O

O HH

OO

O+

major (endo)

O

O

O

O

O

O

e.g. 7 HCO2MeH

CHO

e.g. 6

+COOMe

COOMe

C. Kinetics: 1. Effect of substituents on dienophile (Reaction rate increases with the number of electron withdrawing groups on the dienophile.)

H Z

H H

H Z

H Z

Z Z

Z Z

Z Z

Z H

H Z

Z H

Z Z

H H

or or< < <

Z = electron withdrawing group (e.g. -NO2, -CN, -CHO, -COOR, -CF3)

Relative rate:

2. Effect of conformation of diene

1

4

1

4C1 and C4 are too far apart in the transoid form for reaction wtih dienophile

C1 and C4 are in approprate distance in the cissoid form for reaction with dienophile

1

4C1 and C4 are locked in approprate distance for reaction with dienophile

overall: butadiene reacts slower than cyclopentadiene in Diels-Alder reactions

OK Bad!

1,3-butadiene

2E,4E-hexadiene2Z,4Z-hexadiene