Unsaturated hydrocarbons containing:
two double bonds: diene
three double bonds: triene
four double bonds: tetraene
many double bonds: polyene
Dienes
Chapter 8
Adapted from Turro & Breslow, Columbia University and Irene Lee,
Case Western Reserve University
Classification of Dienes
C
Nomenclature
isolated diene
(2E
(2E,5E
,5E)-2,5-heptadiene
)-2,5-heptadiene
conjugated diene
(2E
(2E,4E
,4E)-2,4-heptadiene
)-2,4-heptadiene
cumulated diene
C
3,4-heptadiene
3,4-heptadiene
If there is a tie, the double bond gets the lowest number
Relative Stabilities
of Dienes
Heats of Hydrogenation
252 kJ/mol
1,3-pentadiene
1,3-pentadiene is
26 kJ/mol more
stable than
1,4-pentadiene
1,4-pentadiene,,
but some of this
stabilization is
because it also
contains a more
highly substituted
double bond
Heats of Hydrogenation
226 kJ/mol
126 kJ/mol
252 kJ/mol
115 kJ/mol
226 kJ/mol
Heats of Hydrogenation
126 kJ/mol
126 kJ/mol
Heats of Hydrogenation
111 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
126 kJ/mol
111 kJ/mol
when terminal double bond is conjugated with
other double bond, its heat of hydrogenation is
15 kJ/mol less than when isolated
Heats of Hydrogenation
Heats of Hydrogenation
Cumulated double bonds have relatively
high heats of hydrogenation
126 kJ/mol
111 kJ/mol
this extra 15 kJ/mol is known by several terms
stabilization energy
delocalization energy
resonance energy
CH3CH2CH3
CH2 + 2H2
ΔH° = -295 kJ
H 2C
C
H 2C
CH2CH3 + H2
CH3CH2CH3
ΔH° = -125 kJ
Isolated diene
1,4-pentadiene
1,4-pentadiene
Bonding
in Conjugated Dienes
1,3-pentadiene
1,3-pentadiene
Conjugated diene
Isolated diene
Isolated diene
π bonds are
independent of
each other
π bonds are
independent of
each other
p orbitals overlap
to give extended π
bond
encompassing
four carbons
1,3-pentadiene
1,3-pentadiene
Conjugated diene
Conjugated diene
Conformations of Dienes
Isolated diene
less electron
delocalization;
less stable
H
H
H
H
more electron
delocalization;
more stable
Conjugated diene
H
H
H
s-trans
s-trans
H
H
H
HH
s-cis
s-cis
s prefix designates conformation around single bond
s prefix is lower case (different from CahnCahn-IngoldIngoldPrelog S which designates configuration and is upper
case)
Conformations of Dienes
H
H
H
H
H
H
Conformations of Dienes
H
H
H
s-trans
s-trans
H
HH
s-cis
s-cis
s prefix designates conformation around single bond
s prefix is lower case (different from CahnCahn-IngoldIngoldPrelog S which designates configuration and is upper
case)
s-trans
s-trans
Both conformations allow electron delocalization via
overlap of p orbitals to give extended π system
s-trans
s-trans is more stable than s-cis
s-cis
Interconversion of conformations requires two
π bonds to be at right angles to each other
and prevents conjugation
12 kJ/mol
kJ/mol
16 kJ/mol
kJ/mol
12 kJ/mol
kJ/mol
s-cis
s-cis
Bonding in Allenes
A cumulated diene is less stable than an isolated diene
Cumulated Dienes
C
C
C
cumulated dienes are less stable than
isolated and conjugated dienes
Structure of Allene
Structure of Allene
118.4°
118.4°
118.4°
118.4°
131 pm
131 pm
linear arrangement of carbons
nonplanar geometry
linear arrangement of carbons
nonplanar geometry
Bonding in Allene
Bonding in Allene
sp 2
sp
sp 2
Bonding in Allene
Bonding in Allene
Stereogenic Axis
Chiral Allenes
Allenes of the type shown are chiral
A
X
C
C
C
Y
B
A ℵ B; X ℵ Y
Have a stereogenic axis
analogous to difference between:
a screw with a right-hand thread and one
with a left-hand thread
a right-handed helix and a left-handed helix