Unsaturated hydrocarbons containing:
two double bonds: diene
three double bonds: triene
four double bonds: tetraene
many double bonds: polyene
Dienes
Question
2,3-Pentadiene, CH3CH=C=CHCH 3 is
A) a planar substance.
B) an allene.
allene.
C) a conjugated diene.
diene.
D) a substance capable of cis-trans
isomerism.
Nomenclature
(2E
(2E,5E
,5E)-2,5-heptadiene
)-2,5-heptadiene
(2E
(2E,4E
,4E)-2,4-heptadiene
)-2,4-heptadiene
C
3,4-heptadiene
3,4-heptadiene
Relative Stabilities
of Dienes
Conformations of Dienes
H
H
H
H
H
Conformations
of Conjugated Dienes
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
s-trans is more stable than s-cis
H
H
H
Interconversion of conformations requires two
π bonds to be at right angles to each other
and prevents conjugation
HH
H
s-trans
s-trans
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)
12 kJ/mol
Question
Which diene is shown in its s-cis conformation?
A)
B)
C)
Bonding in Allenes
D)
Bonding in Allene
sp 2
sp
sp 2
A cumulated diene is less stable than an isolated diene
Structure of Allene
Chiral Allenes
Allenes of the type shown are chiral
A
X
C
118.4°
118.4°
131 pm
C
C
Y
B
A ℵ B; X ℵ Y
linear arrangement of carbons
nonplanar geometry
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
Have a stereogenic axis