Delocalized electrons: Benzene
Resonance and Delocalized Electrons • Will the real benzene please stand up? • Drawing resonance structures • Relative stabilities of resonance structures • Resonance energy • The effect of resonance on stability of cations & radicals • Effects of electron delocalization on reactivity • Molecular orbitals: another way to look at resonance stabilization
Delocalized electrons: Benzene The puzzle…This much was known: Benzene is C6H6 All hydrogens are identical (one monosubstituted product) Three different disubstituted products are obtained Does not undergo “normal” addition reactions like alkenes OR
OR
OR...?
OR
One of these structures fits all the criteria, except… Benzene was also known to be unusually stable! OR
OR
OR
OR...?
Kekulé proposed a rapid equilibrium:
1
In the 30’s, x-ray diffraction showed: Benzene is a planar, 6-membered ring All C-C bonds are equal in length, in between the length of C-C and C=C
Benzene • Each π electron is shared by all six carbons • The π electrons are delocalized
Resonance Contributors and the Resonance Hybrid
Resonance contributors are imaginary, but the resonance hybrid is real
2
π electrons cannot delocalize in nonplanar molecules
Localized Versus Delocalized Electrons CH3
NH 2
CH3
CH
CH2
localized electrons
localized electrons
O δCH3C
delocalized electrons
O δ-
Drawing Resonance Contributors
3
Rules for Drawing Resonance Contributors 1. Only electrons move—never move the atoms 2. Only π electrons and lone - a pir electrons move 3. The total number of electrons in the molecule does not change 4. The numbers of paired and unpaired electrons do not change (do not separate paired electrons)
To obtain resonance structures, use curved arrows to move the electrons around the molecule. The first electrons can be moved in one of the following ways: 1. Move π electrons toward a positive charge or toward a π bond (an sp2 hybridized atom) 2. Move lone - p air electrons toward a π bond 3. Move a single nonbonding electron toward a π bond
Resonance contributors are obtained by moving π electrons toward a positive charge: CH3CH
CH
CHCH3
CH3CH
δ+ CH3CH CH CH2
CH2
CH
CHCH3
δ+ CHCH3 resonance hybrid
CH2
CH2
CH2
δ+ CH2 δ+
δ+
resonance hybrid δ+
4
Moving π electrons toward a π bond
Moving a nonbonding pair of electrons toward a π bond
Note • Electrons move toward an sp2 carbon but never toward an sp3 carbon • Electrons are neither added to nor removed from the molecule when resonance contributors are drawn • Radicals can also have delocalized electrons if the unpaired electron is on a carbon adjacent to an sp2 atom
5
The Difference Between Delocalized and Localized Electrons
delocalized electrons
CH2
CH
CHCH3
CH2
CH2 X CH
CH
CHCH3
an sp3 hybridized carbon cannot accept electrons
CH2CHCH3 localized electrons
Not all resonance structures have equal stability. Features that decrease the predicted stability of a contributing resonance structure … 1. An atom with an incomplete octet 2. A negative charge that is not on the most electronegative atom 3. A positive charge that is not on the most electropositive atom 4. Charge separation
6
Resonance contributors with separated charges are less stable O R
C
OOH
R
more stable
OH+
C
O R
C
OO-
R
C
O
equally stable
If possible, electrons always move toward the more electronegative atom
Most important contributor Very minor contributor
More important contributor
When there is only one way to move the electrons,
CH2 CH
OCH3
CH2 CH
OCH3
movement of the electrons away from the more electronegative atom is better than no movement at all because electron delocalization makes a molecule more stable
7
Resonance Energy • A measure of the extra stability a compound gains from having delocalized electrons
Benzene is stabilized by electron delocalization
Summary • The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid • The greater the number of relatively stable resonance contributors, the greater is the resonance energy • The more nearly equivalent the resonance contributors, the greater is the resonance energy
8
Resonance-Stabilized Cations
Relative Stabilities of Allylic and Benzylic Cations
Relative Stabilities of Carbocations
9
Relative Stabilities of Radicals
Some Chemical Consequences of Electron Delocalization
Before knowing about resonance stabilization, we wouldn’t have predicted a rearrangement from a 2˚ carbocation to another 2˚ carbocation
Delocalized electrons can affect the reactivity of a compound Relative reactivities toward HBr
CH3
CH3 >
CH2 C OCH3
A
>
CH2 C CH3
B
CH3 CH2 C CH2OCH3
C
10
Compound A is the most reactive … A.
CH3 CH2 C
CH3
CH3 HBr
OCH3
CH3 C
CH3 C
OCH3
OCH3 +
Br-
CH3 CH2 C OCH3
Why is B more reactive than C? B.
CH3 CH2 C
CH3 HBr
CH3
C.
CH3
CH3 C
+
Br-
CH3 HBr
CH2 C CH2OCH3
CH3 CH3 C
+
Br-
CH2OCH3
Note: Oxygen can donate a lone pair to stabilize a carbocation. Oxygen can also withdraw electrons inductively to destabilize a carbocation.
11
Why is RCO2H more acidic than ROH?
1. Electron withdrawal by the double - bonded oxygen decreases the electron density of the negatively charged oxygen, thereby stabilizing the conjugated base (the carboxylate)
2. Resonance stabilization of the conjugate base is greater than resonance stabilization of the acid
Phenol is a stronger acid than ethanol
Protonated aniline is a stronger acid than protonated ethanamine
OH
OH
NH3+ NH3+
12
Account for the Acidity of Phenol by Resonance Stabilization
Account for the Acidity of Protonated Aniline by Resonance Stabilization
A Molecular Orbital Description of Stability
• Bonding MO: constructive (in-phase) overlap • Antibonding MO: destructive (out-of-phase) overlap
13
The Molecular Orbitals of 1,3-Butadiene
Symmetry in Molecular Orbitals ψ1 and ψ3 in 1,3- butadiene are symmetrical molecular orbitals
ψ2 and ψ4 in 1,3- butadiene are fully asymmetrical orbitals
• The highest- energy molecular orbital of 1,3 - butadiene that contains electrons is ψ2 (HOMO) • The lowest- energy molecular orbital of 1,3 - butadiene that does not contain electrons is ψ3 (LUMO) • HOMO = the highest occupied molecular orbital • LUMO = the lowest unoccupied orbital
14
The Molecular Orbitals of the Allyl System
ψ2 is the nonbonding MO
Resonance structures of the allyl cation, the allyl radical, and the allyl anion (How do these relate to the MO diagrams? CH2
δ+ δ+ CH2 CH CH2
CH2 CH
CH2
δ δ CH2 CH CH2
CH2 CH
CH2
δδCH2 CH CH2
CH2 CH
CH2
CH2 CH
CH2 CH
CH2
CH2 CH
CH2
.
.
The Molecular Orbitals of 1,3,5-Hexatriene
15
Benzene has six π molecular orbitals
Benzene is unusually stable because of large delocalization energies
16
Delocalized electrons: Benzene The puzzle…This much was known: Benzene is C6H6 All hydrogens are identical (one monosubstituted product) Three different disubstituted products are obtained Does not undergo “normal” addition reactions like alkenes OR
OR
OR...?
OR
One of these structures fits all the criteria, except… Benzene was also known to be unusually stable! OR
OR
OR
OR...?
Kekulé proposed a rapid equilibrium:
1
In the 30’s, x-ray diffraction showed: Benzene is a planar, 6-membered ring All C-C bonds are equal in length, in between the length of C-C and C=C
Benzene • Each π electron is shared by all six carbons • The π electrons are delocalized
Resonance Contributors and the Resonance Hybrid
Resonance contributors are imaginary, but the resonance hybrid is real
2
π electrons cannot delocalize in nonplanar molecules
Localized Versus Delocalized Electrons CH3
NH 2
CH3
CH
CH2
localized electrons
localized electrons
O δCH3C
delocalized electrons
O δ-
Drawing Resonance Contributors
3
Rules for Drawing Resonance Contributors 1. Only electrons move—never move the atoms 2. Only π electrons and lone - a pir electrons move 3. The total number of electrons in the molecule does not change 4. The numbers of paired and unpaired electrons do not change (do not separate paired electrons)
To obtain resonance structures, use curved arrows to move the electrons around the molecule. The first electrons can be moved in one of the following ways: 1. Move π electrons toward a positive charge or toward a π bond (an sp2 hybridized atom) 2. Move lone - p air electrons toward a π bond 3. Move a single nonbonding electron toward a π bond
Resonance contributors are obtained by moving π electrons toward a positive charge: CH3CH
CH
CHCH3
CH3CH
δ+ CH3CH CH CH2
CH2
CH
CHCH3
δ+ CHCH3 resonance hybrid
CH2
CH2
CH2
δ+ CH2 δ+
δ+
resonance hybrid δ+
4
Moving π electrons toward a π bond
Moving a nonbonding pair of electrons toward a π bond
Note • Electrons move toward an sp2 carbon but never toward an sp3 carbon • Electrons are neither added to nor removed from the molecule when resonance contributors are drawn • Radicals can also have delocalized electrons if the unpaired electron is on a carbon adjacent to an sp2 atom
5
The Difference Between Delocalized and Localized Electrons
delocalized electrons
CH2
CH
CHCH3
CH2
CH2 X CH
CH
CHCH3
an sp3 hybridized carbon cannot accept electrons
CH2CHCH3 localized electrons
Not all resonance structures have equal stability. Features that decrease the predicted stability of a contributing resonance structure … 1. An atom with an incomplete octet 2. A negative charge that is not on the most electronegative atom 3. A positive charge that is not on the most electropositive atom 4. Charge separation
6
Resonance contributors with separated charges are less stable O R
C
OOH
R
more stable
OH+
C
O R
C
OO-
R
C
O
equally stable
If possible, electrons always move toward the more electronegative atom
Most important contributor Very minor contributor
More important contributor
When there is only one way to move the electrons,
CH2 CH
OCH3
CH2 CH
OCH3
movement of the electrons away from the more electronegative atom is better than no movement at all because electron delocalization makes a molecule more stable
7
Resonance Energy • A measure of the extra stability a compound gains from having delocalized electrons
Benzene is stabilized by electron delocalization
Summary • The greater the predicted stability of a resonance contributor, the more it contributes to the resonance hybrid • The greater the number of relatively stable resonance contributors, the greater is the resonance energy • The more nearly equivalent the resonance contributors, the greater is the resonance energy
8
Resonance-Stabilized Cations
Relative Stabilities of Allylic and Benzylic Cations
Relative Stabilities of Carbocations
9
Relative Stabilities of Radicals
Some Chemical Consequences of Electron Delocalization
Before knowing about resonance stabilization, we wouldn’t have predicted a rearrangement from a 2˚ carbocation to another 2˚ carbocation
Delocalized electrons can affect the reactivity of a compound Relative reactivities toward HBr
CH3
CH3 >
CH2 C OCH3
A
>
CH2 C CH3
B
CH3 CH2 C CH2OCH3
C
10
Compound A is the most reactive … A.
CH3 CH2 C
CH3
CH3 HBr
OCH3
CH3 C
CH3 C
OCH3
OCH3 +
Br-
CH3 CH2 C OCH3
Why is B more reactive than C? B.
CH3 CH2 C
CH3 HBr
CH3
C.
CH3
CH3 C
+
Br-
CH3 HBr
CH2 C CH2OCH3
CH3 CH3 C
+
Br-
CH2OCH3
Note: Oxygen can donate a lone pair to stabilize a carbocation. Oxygen can also withdraw electrons inductively to destabilize a carbocation.
11
Why is RCO2H more acidic than ROH?
1. Electron withdrawal by the double - bonded oxygen decreases the electron density of the negatively charged oxygen, thereby stabilizing the conjugated base (the carboxylate)
2. Resonance stabilization of the conjugate base is greater than resonance stabilization of the acid
Phenol is a stronger acid than ethanol
Protonated aniline is a stronger acid than protonated ethanamine
OH
OH
NH3+ NH3+
12
Account for the Acidity of Phenol by Resonance Stabilization
Account for the Acidity of Protonated Aniline by Resonance Stabilization
A Molecular Orbital Description of Stability
• Bonding MO: constructive (in-phase) overlap • Antibonding MO: destructive (out-of-phase) overlap
13
The Molecular Orbitals of 1,3-Butadiene
Symmetry in Molecular Orbitals ψ1 and ψ3 in 1,3- butadiene are symmetrical molecular orbitals
ψ2 and ψ4 in 1,3- butadiene are fully asymmetrical orbitals
• The highest- energy molecular orbital of 1,3 - butadiene that contains electrons is ψ2 (HOMO) • The lowest- energy molecular orbital of 1,3 - butadiene that does not contain electrons is ψ3 (LUMO) • HOMO = the highest occupied molecular orbital • LUMO = the lowest unoccupied orbital
14
The Molecular Orbitals of the Allyl System
ψ2 is the nonbonding MO
Resonance structures of the allyl cation, the allyl radical, and the allyl anion (How do these relate to the MO diagrams? CH2
δ+ δ+ CH2 CH CH2
CH2 CH
CH2
δ δ CH2 CH CH2
CH2 CH
CH2
δδCH2 CH CH2
CH2 CH
CH2
CH2 CH
CH2 CH
CH2
CH2 CH
CH2
.
.
The Molecular Orbitals of 1,3,5-Hexatriene
15
Benzene has six π molecular orbitals
Benzene is unusually stable because of large delocalization energies
16
