diene

DIENES
Dienes are alkenes with 2 double bonds.
IUPAC: Same as alkene, but change -ene to -adiene and
use two numbers to locate the two double bonds (number
from the end of the chain which makes the smaller of
these numbers smaller).
Double bonds separated by more than one single bond are
isolated. Compounds with isolated double bonds have the
same chemical properties as alkenes.
Double bonds that alternate with single bonds,
eg C=C-C=C, are conjugated.
Double bond arrangements like C=C=C are cumulated.
Compounds containing two carbon-carbon cumulated
double bonds are called allenes.
Conjugated dienes differ from simple alkenes in that they
are more stable,
undergo 1,4-addition, and
are more reactive.

Stability
Alkene Type
RCH=CH 2
∆H hydrogenation
30 kcal/mole,exotherm.
R 2 C=CH 2 , RCH=CHR 28
R 2 C=CHR 27
For dienes with isolated double bonds, the ∆H hydrogenation
calculated by adding the appropriate values from the table
above together is very close to the experimental value.
For conjugated dienes, the experimental ∆H hydrogenation is
smaller than the calculated one, which is based on isolated
double bonds.
Conjugated Diene
Calculated
∆H h
Experimental
∆H h
CH 2 =CH-CH=CH 2 60 kcal/mole 57
CH 2 =C(CH 3 )-C(CH 3 )=CH 2 56 54

Therefore, conjugated dienes are more stable than
isolated dienes.
This stability is reflected in the fact that, where possible,
conjugated dienes are the preferred diene products of
elimination reactions ---
H
H H
C
H
C
C C
C
Cl H
KOH, alcohol
heat
Formation of the conjugated
diene takes precedence over
the Saytzeff rule.
+
H
C
C C
C C
H
major product - conjugated
H H
C
C
C
C
C
+ H 2 O + KCl
H
minor product - isolated
(Saytzeff product)
This stability can be explained by electron delocalization in
the conjugated diene
using the resonance model ---
or the molecular orbital model ---

Energy (Huckel) of Two Isolated vs Conjugated Double Bonds
1.68
_
π4
1.0
_
0.68
_
Energy 0
-0.68
_
-1.0
_
π∗
π
π3
π2
-1.68
_
Two isolated π-bonds.
Conjugated π-system.
π1
Eπ = −4.0 Eπ = −4.72

Electrophilic Addition to Conjugated Dienes:
1,2- vs. 1,4- Addition ---
1
2
3
4
YZ
1
Y
2 3
Z
4
1
Y
2
3 4
Z
1,2-addition
1,4-addition
Note the "shift"
of the π-bond.
eg CH 3 CH=CH-CH=CHCH 3 + HCl >
CH 3 -CH 2 -CHCl-CH=CHCH 3 via 1,2-addition
+
CH 3 -CH 2 CH=CH-CHCl-CH 3 via 1,4-addition

Mechanism ---
H H
H H
4 CH
1
3 HCl
2 4 CH
CH
3
3
CH
1 3
3
2
3
H H
H H H
ordinary 2o carbocation -
VERY LITTLE FORMED
+
H H
H H
1
2 4 CH
1
3 3
2
CH 3 4
CH 3
3
CH 3
H H
H H
H H
2o allylic type of carbocation; ~ stability of 3o carbocation.
Since the positive charge is shared between the carbons labeled 2 & 4
Cl - can attack either.

1,2- vs. 1,4-Addition: Rate vs. Equilibrium Control
1
2
3
4
HBr
Low temp (-80o C)
High temp (40o C)
1
2 3
H Br
1
2
3 4
1
4
2
80%
40oC
3 4
1
2 4
3
80%
H 20% H 20%
Br
Br
H
Br
At -80o the 1,2-isomer
converts to 1,4-isomer
very slowly. Since 1,2-
predominates here,
it is formed faster.
Reaction rates determine
product here:
Kinetic Control
At 40o the 1,2- and 1,4-
isomers interconvert, giving
an equilibrium mixture.
The 1,4-isomer is more
stable and predominates.
Equilibrium constant
determines product here:
Thermodynamic Control
In this case, the less stable product is formed faster.
How does this happen

Diels-Alder Reaction
X
Y
X
Y
diene
dieneophile
(X, Y: typically
one or both are
electron
withdrawing)
a substituted cyclohexene
There are other variations on this theme, eg---
X
X
Y
Y

Stereochemistry ---
1) With respect to the dieneophile, the addition is
stereospecifically syn, eg
H
Cl
H
Cl
H
Cl
H
Cl
H
Cl
Cl
H
H
Cl
Cl
H
+
Cl
H
H
Cl
2) With respect to the diene, the addition is
stereospecific and syn, eg
C 6 H 5
C 6 H 5
Cl Cl
Cl Cl C 6 H 5
Cl Cl
Cl Cl
C 6 H 5

3) The diene must be in the s-cis conformation for
reaction to occur. [The mechanism is concerted --- no
intermediates.]
close
enough
s-cis
close
enough
too
far
s-trans
too
far
Dienes unable to attain the s-cis conformation do not react, eg

4) If the diene is cyclic and the dieneophile has an
unsaturated substituent, the endo product is usually
favored over the exo product [the reaction is
stereoselective; one diasteromer (endo) is favored
over another (exo)].
a
3
4
cyclopentadiene
3
4
b
e
2 2
5
2
5
d
c
c
d
1
1
b
cyclopentadiene
e
a
4
3
1
5
d
H
H
exo-dicyclopentadiene
4
3
1
5
2
H
d
c
H
e
endo-dicyclopentadiene
e
c
b
b
a
a