The Allyl Group
Conjugation in Alkadienes and
Allylic Systems
“conjugare”
conjugare”: from the Latin meaning
"to link or yoke together"
H
H
H
C
C
H
C
H
Vinylic versus Allylic
Vinylic versus Allylic
H
C
C
H
C
allylic
carbon
vinylic carbons
Vinylic versus Allylic
H
C
C
C
H
Vinylic hydrogens or substituents
are attached to vinylic carbons.
Question
H
C
C
C
H
Allylic hydrogens or substituents
are attached to allylic carbons.
How many allylic and vinylic hydrogens
respectively are in cyclohexene?
cyclohexene?
A) 2 and 4
B) 4 and 2
C) 2 and 2
D) 6 and 2
Allylic Carbocations
A tertiary allylic halide undergoes
solvolysis (SN1) faster than a simple tertiary
alkyl halide
CH3
CH3
Allylic Carbocations
H 2C
C
C
CH
C
+
Cl
CH3
CH3
C
C
Cl
CH3
123
1
relative rates: (ethanolysis, 45°
45°C)
Allylic Carbocations
Resonance
Therefore:
allylic carbocations are more stable than
other comparable carbocations
CH3
CH3
H 2C
CH
C+
CH3
CH3
C+
CH3
H 2C
CH
C+
CH3
CH3
1<2<3
2<3<1
3<1<2
2<1<3
CH
C
CH3
CH
δ+
C
CH3
Question
A)
B)
C)
D)
CH3
CH3
δ+
H 2C
Resonance:
H2C=CH—
C=CH— stabilizes C+ better than does CH3—
Rank the following carbocations in order of
increasing stability (least → most)
+
H 2C
Question
Which of the following compounds will yield the
same carbocation on ionization?
A) 1 and 3
B) 1 and 4
C) 3 and 4
D) 2 and 4
Hydrolysis of an Allylic Halide
CH3
H 2C
CH
Cl
C
CH3
SN1 Reactions of Allylic Halides
H 2O
Na2CO3
CH3
CH3
H 2C
CH
(85%)
Corollary Experiment
CH
H 2C
C
CH
CH
(85%)
(85%)
H 2C
+
CH3
CH
CH
C
CH3
HOCH2
CH
CH3
C+
CH3
CH3
CH
C+
CH
C
CH3
Question
Which compound can produce a mixture of two
alcohols that are constitutional isomers, if it
reacts via an SN1 mechanism?
A)
B)
C)
D)
CH3
CH
CH3
C
CH3
+
H 2C
H 2C
+
H 2C
CH3
More positive charge on tertiary carbon;
therefore more tertiary alcohol in product.
CH
ClCH2
and ClCH
CH3
CH3
CH3
H 2C
CH3
Cl
C
C
(15%)
OH +
C
HOCH2
(15%)
CH3
CH
CH3
(15%)
CH3
OH
C
C
Give the same products because they
form the same carbocation.
Na2CO3
CH3
H 2C
CH3
CH
CH3
CH3
H 2O
HOCH2
CH3
CH3
ClCH
ClCH2
+
OH
C
C
CH3
Allylic SN2 Reactions
Allylic halides also undergo SN2 reactions
faster than simple primary alkyl halides.
SN2 Reactions of Allylic Halides
H 2C
CH2 Cl
CH
H 3C
CH2 CH2 Cl
1
80
relative rates: (I-, acetone)
Allylic SN2 Reactions
Allylic SN2 reactions
Two factors:
Two factors:
Steric
Electronic
Trigonal carbon smaller than tetrahedral carbon.
Electron delocalization lowers LUMO energy
which means lower activation energy.
H 2C
CH
H 3C
CH2 Cl
CH2 CH2 Cl
H 2C
1
80
CH2 Cl
CH
H 3C
CH2 CH2 Cl
1
80
relative rates: (I-, acetone)
relative rates: (I-, acetone)
Allylic Free Radicals are Stabilized by
Electron Delocalization
Allylic Free Radicals
C•
C
C•
C
C
C
•C
C
C
Free Radical Stabilities are Related to
Bond-dissociation Energies
CH3CH2CH2—H
H 2C
CHCH 2—H
410 kJ/mol
368 kJ/mol
•
CH3CH2CH2 + H•
H 2C
•
CHCH 2 + H•
C—H bond is weaker in propene because
resulting radical (allyl) is more stable than
radical (propyl) from propane.
Question
Which of the following C-H bonds would have
the smallest bond dissociation energy?
A)
B)
C)
D)
N-Bromosuccinimide
Reagent used (instead of Br 2 / light or heat) for allylic
bromination.
bromination. (Milder reaction conditions.)
Allylic Halogenation
Br
O
heat
NBr +
Allylic halogenation is only useful when:
all of the allylic hydrogens are equivalent
NH
O
(82-87%)
Example
H
H
Cyclohexene
satisfies both
requirements.
All allylic
hydrogens are
equivalent.
H
and
the resonance forms of allylic radical
are equivalent.
+
CCl4
O
Limited Scope
O
H
•
H
H
H
H
H
•
Both resonance forms are equivalent.
H
Example
2-Butene
Example
CH3CH
CHCH 3
All allylic
hydrogens are
equivalent.
2-Butene
CH3CH
But
CHCH 3
forms
Br
CH3CH
CH
•
CH2
•
CH3CH
CH
Two resonance forms are not equivalent;
gives mixture of isomeric allylic bromides.
Question
What is the other product formed in the
reaction shown here?
A)
B)
C)
D)
All allylic
hydrogens are
equivalent.
CH2
CH3CH
CH
CH2
Br
and
CH3CH
CH
Two resonance forms are not equivalent;
gives mixture of isomeric allylic bromides.
CH2