Evidence for resonance stabilization: enthalpies of hydrogenation
58
) ( )H) = 4 kcal / mol
54
E
Energy of delocalization
H2 Pt /C
kcal / mol
H2 Pt /C
Conjugation Energy
Resonance Energy
28
28
H2 Pt /C
H2 Pt /C
Evidence for resonance stabilization: bond lengths
Csp2 - Csp2 bond length: 1.34
H
H
C C
H
Csp2 - Csp2
bond length:
1.34
H
H
H
C C
H
H
C C
H
H
H
Csp2 - Csp2 bond length: 1.46
H
H
H C C
H
H
Csp3 - Csp3 bond length: 1.54
Consider (E)-1,3-pentadiene:
Evidence for resonance stabilization: conformational preferences
Just as there is a barrier to rotation about the C2=C3 of cis and trans-2-butene, there is a barrier to rotation about the
C2 - C3 in 1,3-butadiene:
H
H
H
H
H
H
H
s - trans
conformation
H
H
H
H
H
s - cis
conformation
E
3.9
kcal / mol
2.8
0
90
C1= C2--C3= C4 torsion, degrees
180
Synthesis of conjugated dienes:
• Just like alkene synthesis, only twice – conjugated dienes prefer to form as major products because they are
the most thermodynamically stable products
• Dehydrohalogenation of allylic halides
Br
CH3CH2O- Na+
CH3CH2OH, )
90 %
• Isolated dienes form only when it is impossible to form the conjugated isomer
What would the E1 outcome of dehydrobromination of 4-bromo-2,3,3-trimethylhexene be?
Br
?
CH3CH2OH, )
Reactions of conjugated dienes:
• Additions of HX
Br
HBr
+
Br
dark, -80 EC
81 %
19 %
Reactions of conjugated dienes:
• Additions of HX
Br
HBr
+
Br
dark, -80 EC
81 %
1,2-addition
Markovnikov addition product
19 %
1,4-addition
conjugate addition product
What’s the mechanism?
+ Br+
+
H Br
+ Br+
Br
Br
Looking at the carbocation intermediate more closely: what possible addition products can form?
H
4
C
3
C
H
+
H
H
+
C
4
2
CH3
3
C
H
H
4
C
H
C
2
1
+
.
+
Br-
Br
C
H
C
1
+
Br-
Br-
2
CH3
1
1,4-addition
.
.
Br
less stable isomer
3
C
+
1,3-addition
1,2-addition
H
+
.
+
CH3
+
never observed
Br
more stable isomer
Reactions of conjugated dienes:
• Additions of HX
Br
HBr
+
Br
dark, -80 EC
81 %
19 %
1,2-addition
major
1,4-addition
minor
Br
HBr
+
Br
dark, 45 EC
19 %
1,2-addition
minor
81 %
1,4-addition
major
Energy vs reaction coordinate diagram:
+
+ Br-
Br
+ HBr
Br
Energy vs reaction coordinate diagram:
+
+ Br-
Br
+ HBr
Br
Energy vs reaction coordinate diagram:
+
+ Br-
Br
+ HBr
Br
Energy vs reaction coordinate diagram:
For the second step, Eact < Eact so
the rate of 1,2-addtion > the rate of 1,4-addition
+
+ Br-
Br
1,2-addition
+ HBr
Br
1,4-addition
When the major product that forms is the fastest forming product, a reaction is said to be under
rate or kinetic control.
Energy vs reaction coordinate diagram:
For the second step, if the temperature of
the reaction is $Eact reverse then the 1,2addition product can equilibrate to the
the more stable 1,4-addition product
+
+ Br-
Br
1,2-addition
+ HBr
Br
1,4-addition
When the major product that forms is the most stable product, a reaction is said to be under
equilibrium, thermodynamic or stability control.
The Alder Rule
diastereoselection is such that endo products predominate:
O
O
OCH3
O
+
OCH3
exo
OCH3
or
O
OCH3
endo
O
OCH3
exo transition state: C=O carbon hoovers over CH2 of 1,3-cyclopentadiene
more stable endo transition state: C=O carbon hoovers
over forming C2–C3 C=C of the cyclopentadiene
endo adduct
Ultraviolet - Visible (UV-Vis) Spectroscopy
Consider the ground state electronic configuration of ethene:
C-C F*
C-H F*
E
C-C B*
(lumo)
C-C B
(homo)
C-H F
C-C F
H
H
C C
H
H
The smallest ∆E that will promote a bonding electron to an antibonding orbital is:
C-C F*
C-H F*
E
C-C B*
(lumo)
∆E = UV-Vis
C-C B
(homo)
C-H F
C-C F
H
H
C C
H
H
(with λ = 162 nm here)
λmax = 175
λmax = 245
λmax = 275
λmax = 215
Absorbance
wavelength, nm
O
= 280 nm
A: B 6 B*
O
= 204 nm
O
A: B 6 B*
B: n 6 B*
= 254 nm
C=O π electron transitions
LUMO
π*
254 nm
204 nm
HOMO
n
HOMO-1
π
Lawsone in pH 7.2 phosphate buffer
O
OH
O
λmax
215
265
290 (sh)
330
452
A
1.52
2.16
1.07
0.22
0.27