Chapter 14
Introduction
• Compounds can have more than one double or
triple bond.
– Dienes are compounds with two double bonds
– If they are separated by only one single bond,
they are conjugated and their orbitals interact
There are three types of dienes:
• Conjugated: -C=C-C=C- alternating double and
single bonds
• Cumulated: -C=C=C- consecutive double bonds
(no intervening single bond)
• Isolated: -C=C-C-C=C- double bonds separated
by more than one single bond (more than one
intervening single bond)
• Conjugated systems have distinctive properties.
Example: The conjugated diene 1,3-butadiene has
properties that are very different from those of the
nonconjugated diene, 1,4-pentadiene
• The term "conjugated" or "conjugated system"
typically is applied to extended systems.
• Polyenes - are compounds with many alternating
single and double bonds
- are conjugated hydrocarbons with
many double bonds
• Examples:
a. beta-carotene/ vitamin A
b. lycopene: the red pigment in tomatoes
• Conjugated systems are common in nature
and in biologically important molecules.
enone (Alkene + Ketone)
• Ultraviolet Spectroscopy (UV) – determines if a conjugated
p electron system is present
• One of the characteristics of conjugated systems is the
absorbance of u.v. light by the p electron system.
Red pigment
1.
Preparation and Stability of
Conjugated Dienes
Conjugated dienes are generally prepared by:
i.
base-induced elimination of HX from an allylic
halide
ii. industrial catalytic dehydrogenation
iii. industrial scale dehydration
i.
Base-induced elimination of HX from an allylic halide
Example: Allylic bromination of Cyclohexene with
NBS followed by elimination (tBOC)
ii.
Industrial catalytic dehydrogenation
Example: 1,3- Butadiene, a substance used
industrially to make polymers, is prepared by
thermal cracking of butane in the presence of a
catalyst (Chromium oxide/ aluminum oxide)
iii. Industrial scale dehydration
Example: Preparation of Isoprene via acid-catalyzed
double dehydration of 3-Methyl-1,3-butanediol
Bond Length
•
The central single bond in a conjugated diene is
shorter than the single bond in a nonconjugated
diene or an alkane.
Example: The C2-C3 single bond in 1,3-butadiene is
shorter than the C2-C3 bond in butane
Stability
•
Conjugated dienes are more stable than nonconjugated
dienes as evidenced by their heats of hydrogenation.
•
More highly substituted alkenes are more stable (release
less heat of hydrogenation) than less substituted ones.
Stability: 1,3-butadiene vs 1,4-pentadiene
•
Hydrogenating 1,3-butadiene releases 16 kJ/mol less
heat than 1,4-pentadiene
•
Conjugation of the double bonds in 1,3-butadiene
gives the extra stability of approximately 16 kJ/mol
+
2
+
•
2 H2
2 H2
cataly st
2
DHo = 2(-126 kJ/mol)
= -252 kJ/mol)
cataly st
DHo = -236 kJ/mol)
The unusual stability of 1,3-butadiene (and also other
conjugated systems) is due to resonance energy (also
called resonance stabilization or delocalization energy).
Practice Problem: Allene, H2C=C=CH2, has a heat of
hydrogenation of -298 kJ/mol (-71.3 kcal/mol).
Rank a conjugated diene, a nonconjugated
diene, and an allene in order of stability
2.
Molecular Orbital Description of
1,3-Butadiene
The unusual stability of conjugated dienes
can be explained by:
•
Valence Bond Theory
•
Molecular Orbital Theory
Valence Bond Theory
•
According to the valence bond theory, the stability of
conjugated dienes is due to orbital hybridization:
25% s character
33% s character
Electrons in sp2 orbitals are closer to the nucleus.
Thus sp2-sp2 s bonds are shorter and stronger.
Molecular Orbital Theory
•
According to the molecular orbital theory, the stability
of conjugated dienes is due to interaction between
the p orbitals of the two double bonds:
–
The bonding p-orbitals are made from 4 p orbitals that
provide greater delocalization and lower energy than in
isolated C=C
–
The single bond between the two double bonds is
strengthened by overlap of p orbitals
•
Two p orbitals combine to form two p molecular orbitals:
Bonding and Antibonding
•
Both electrons occupy the low energy, bonding orbital,
forming a stable bond.
higher in energy
lower in energy
•
In a conjugated diene, four adjacent p orbitals combine to
form four p molecular orbitals: two bonding and two
antibonding
fully additive
•
The four p electrons occupy the two bonding orbitals
The number of nodes between nuclei increases as the energy level of the orbital increases
p Molecular Orbitals: 1,3-butadiene vs 1,4-pentadiene
•
In a conjugated diene (1,3-butadiene), the lowest-energy
p MO (y1) has a favorable bonding interaction between
C2 and C3 that is absent in a nonconjugated diene
–
C2-C3 bond has partial double-bond character
–
C2-C3 bond is stronger and shorter than a typical single bond
In a conjugated diene, the p electrons are delocalized or
spread out over the entire p framework rather than localized
between two specific nuclei.
•
•
Electron delocalization always leads to greater stability.
•
Systems containing conjugated double bonds, not
just those of dienes, are more stable than those
containing nonconjugated double bonds.
O
O
2-Cyclohexenone
(more stable)
3-Cyclohexenone
(less stable)
3.
Electrophilic Additions to Conjugated
Dienes: Allylic Carbocations
• Conjugated dienes undergo electrophilic addition
reactions via a different mechanism than that
observed in nonconjugated dienes: 1,4-addition
General Mechanism of electrophilic addition reaction
• Attack on electrophile
(such as HX) by a p bond
of alkene (nucleophile)
• Formation of carbocation
and halide ion
• Reaction of nucleophilic
halide ion with carbocation
(an electrophile)
Markovnikov’s Regiochemistry
In the addition of HX to alkene:
– The H attaches to the carbon with the most H’s and X
attaches to the carbon with the most alkyl
substituents
– The more highly substituted (more stable)
carbocation is formed as the intermediate rather than
the less highly substituted one
Electrophilic additions: Alkenes and Nonconjugated Dienes
•
Alkenes and nonconjugated dienes give Markovnikov’s products
Electrophilic additions: Conjugated Dienes
•
Conjugated dienes give mixtures of products: 1,2 adduct and
1,4 adduct
1,2 adduct
1,4 adduct
Constitutional isomers
•
Conjugated dienes give mixtures of products: 1,2 adduct and
1,4 adduct
1,4 adduct
1,2 adduct
Constitutional isomers
Carbocations from Conjugated Dienes
Two possible carbocations of electrophilic addition to
conjugated dienes:
–
secondary allylic carbocation
–
primary nonallylic carbocation (NOT formed)
•
The key intermediate formed is the delocalized secondary
allylic carbocation because:
– it is more stable, stabilized by resonance between two forms
– it forms faster than a nonallylic carbocation
Products of Addition to Delocalized Carbocation
• Nucleophile can add to either cationic site:
– 1,2 and 1,4 addition products result
– The transition states for the two possible products are not
equal in energy
Practice Problem: Give the structures of the likely products (both
1,2 adducts and 1,4 adducts) for this reaction
Practice Problem: Give the structures of both 1,2- adducts and
1,4 adducts resulting from reaction of 1
equivalent of HCl with 1,3-pentadiene.
Practice Problem: Look at the possible carbocation intermediates
produced during addition of HCl to 1,3pentadiene, and predict which 1,2 adduct
predominates. Which 1,4 adduct
predominates?
Practice Problem: Give the structures of both 1,2 and 1,4 adducts
resulting from reaction of 1 equivalent of HBr
with the following substance:
4.
Kinetic vs Thermodynamic
Control of Reactions
Electrophilic addition to a conjugated diene leads to
a mixture of 1,2 and 1,4 addition products in varying
amounts depending on the reaction conditions:
•
1,2 adduct is usually formed faster and is said to
be the product of kinetic control
•
1,4 adduct is usually more stable and is said to
be the product of thermodynamic control
Example: The addition reaction of HBr to 1,3-butadiene
has unusual temperature dependence
– At low temperatures (0° C), the 1,2-addition products
predominate
– At higher temperatures (40°C), the 1,4-addition
products predominate
A reaction energy diagram for two competing reactions
•
B forms faster than C: DG‡B < DG‡C
•
C is more stable than B: DGoC > DGoB
Kinetic Control
•
Under kinetic control, the product of an irreversible reaction
depends on relative rates of formation.
–
If a reaction is irreversible or far from equilibrium, then the
relative concentrations of products depend on how fast each
forms, which is controlled by the relative free energies of the
transition states leading to each (Kinetic Control)
Thermodynamic Control
•
Under thermodynamic control, the product of a readily
reversible reaction depends on thermodynamic stability
–
At completion, all reactions are at equilibrium and the relative
concentrations are controlled by the differences in free energies
of reactants and products (Thermodynamic Control)
Example: Electrophilic Addition of HBr to 1,3-butadiene
• Under kinetic control (at lower temperatures), there
is a limited amount of energy available, sufficient only
to overcome the lowest activation energy barrier,
which leads to the 1,2-addition product.
• Under thermodynamic control (higher temperatures),
there is enough energy to overcome the larger
activation energy barrier, which leads to the 1,4
adduct.
• If the temperature is high enough, both reaction can
reach equilibrium, in which case the more stable
product (1,4-addition according to Zaitsev's rule)
will predominate.
Practice Problem: The 1,2 adduct and the 1,4 adduct formed by
reaction of HBr with 1,3-butadiene are in
equilibrium at 40oC. Propose a mechanism by
which the interconversion of products takes
place.
Practice Problem: Why do you suppose 1,4 adducts of 1,3butadiene are generally more stable than 1,2
adducts?
5.
The Diels-Alder Cycloaddition
Reaction
The Diels-Alder cycloaddition reaction is unique
to conjugated dienes.
•
Conjugated dienes can combine with alkenes to
form six-membered cyclic compounds
•
Example:
The Diels-Alder reaction
– is a cycloaddition reaction, i.e one in which two
reactants add together in a single step to form a
cyclic product
– was discovered by Otto Paul Hermann Diels and
Kurt Alder in Germany in the 1930’s (awarded the
1950 Nobel Prize)
The Diels-Alder cycloaddition reaction
–
forms two C-C bonds in one step.
–
is one of only a few ring-forming reactions.
–
is said to be "pericyclic," not polar or free-radical
(Woodward and Hoffman in 1965)
–
is a single, one-step process with no intermediates
(concerted formation of two bonds)
The Diels-Alder cycloaddition
–
involves orbital overlap, change of hybridization
and electron delocalization in transition state
sp2
sp2
sp3
sp2
sp2
sp2
sp2
Head on (s) overlap of
two alkene p orbitals
with two p orbitals on
C1 and C4 of the diene
sp3
6.
Characteristics of the Diels-Alder
Reaction
The Diels-Alder cycloaddition
–
–
involves a dienophile and a diene
is stereospecific and regioselective
a diene
a dienophile
Diels-Alder adduct
The Dienophile
– is “diene-loving”
– has an alkene (C=C) or alkyne (CC) component
conjugated to an electron-withdrawing group, such
as C=O or CN
– has a double (C=C) or triple (CC) bond next to
the positively polarized carbon of an electronwithdrawing substituent
The Dienophile
– has an alkene (C=C) or alkyne (CC) component conjugated
to an electron-withdrawing group, such as C=O or CN
The Dienophile
– has a double (C=C) or triple (CC) bond next to the
positively polarized carbon of an electron-withdrawing
group
The electron-withdrawing group makes the double bond
carbons less negative
Stereospecificity of the Diels-Alder Reaction
•
The Diels-Alder reaction is stereospecific:
– It maintains relative relationships from reactant to
product
– There is a one-to-one relationship between
stereoisomeric reactants and products
• The Diels-Alder reaction is stereospecific:
– The two carbons of the dienophile add to the same
face of the diene.
– The stereochemistry of the dienophile is maintained,
and a single product stereoisomer results
cis dienophile reactant gives cis-substituted cyclohexene product
Cis dienophile reactant gives cis-substituted cyclohexene product
Trans dienophile reactant gives trans-substituted cyclohexene product
Regiospecificity of the Diels-Alder Reaction
• The Diels-Alder reaction is regiospecific:
– The diene and dienophile reactants align to produce
endo (rather than exo) product
Endo and exo are relative to the double bond derived from the diene
• Endo and exo indicate relative stereochemistry in
bicyclic structures
• Substituent on one bridge is:
– exo if it is anti (trans) to the larger of the other two bridges
– endo if it is syn (cis) to the larger of the other two bridges
• Endo products are formed because orbital overlap
increases when the diene and dienophile reactants
align so that the electron-withdrawing group of the
dienophile is underneath the diene.
Practice Problem: Predict the product of the following DielsAlder reaction:
Practice Problem: Predict the product of the following DielsAlder reaction:
The Diene
– must have the s-cis conformation (“cis-like” about
the single bond) to undergo the Diels-Alder reaction
(higher in energy)
(lower in energy)
The Diene
– must have the s-cis conformation because only in
the s-cis conformation are C1 and C4 of the diene
close enough to react through a cyclic transition state
(overlap with dienophile p orbitals)
– Dienes that cannot adopt the s-cis conformation are
unreactive in the Diels-Alder reaction
– Examples
– Other dienes that are fixed in the s-cis conformation
are highly reactive in the Diels-Alder reaction
– Example: Dimerization of 1,3-cyclopentadiene
The Diels-Alder cycloaddition
–
is facilitated by a combination of electron-withdrawing
substituents on one reactant and electron-releasing
substituents on the other
–
Example:
a diene
a dienophile
Diels-Alder adduct
The Diels-Alder cycloaddition
–
is facilitated by a combination of electron-withdrawing
substituents on one reactant and electron-releasing
substituents on the other
Electron-releasing
Group
-CH3, alkyl groups
-OR (ether)
-OOCR (ester)
Electron-withdrawing
Group
-CN (cyano)
-CHO (aldehyde, ketone)
-COOH (carboxyl)
-COOR (ester)
-NO2 (nitro)
Practice Problem: Which of the following alkenes would you
expect to be good Diels-Alder dienophiles?
Practice Problem: Which of the following dienes have an s-cis
conformation, and which have an s-trans
conformation? Of the s-trans dienes, which can
readily rotate to s-cis?
Practice Problem: Predict the product of the following DielsAlder reaction:
7.
Diene Polymers: Natural and
Synthetic Rubbers
• Conjugated dienes can be polymerized
cis
trans
• Polymerization: is 1,4 addition of growing chain
to conjugated diene monomer
• The initiator for the reaction can be:
– a radical or
– an acid
Natural Rubber
• Two naturally occurring polymers of isoprene are:
– natural rubber (Z isomer)
– gutta-percha (E isomer)
• The repeating unit has 5 carbons
head-to-tail polymer of isoprene
Synthetic Rubber
• Synthetic rubbers are produced commercially by
diene polymerization
• Example: Neoprene (a polymer of chloroprene)
Synthetic rubber
(weather resistant)
Vulcanization
• Natural and synthetic rubbers are too soft to be
used in products unless hardened by vulcanization
• Vulcanization
– was discovered by Charles Goodyear
– involves heating the crude polymer with small
amount of sulfur to produce strong material
• Sulfur forms bridges between hydrocarbon
chains (cross-links), locking the chains
Practice Problem: Draw a segment of the polymer that might be
prepared from 2-phenyl-1,3-butadiene.
Practice Problem: Show the mechanism of the acid-catalyzed
polymerization of 1,3-butadiene.
8.
Structure Determination in Conjugated
Systems: Ultraviolet Spectroscopy
• Mass Spectrometry (MS) – determines the size and formula
• Infrared (IR) Spectroscopy – determines the kinds of
functional groups present
• Nuclear Magnetic Resonance Spectroscopy (NMR) –
– determines the carbonhydrogen framework
• Ultraviolet Spectroscopy (UV) – determines if a conjugated
p electron system is present
• The ultraviolet (UV) region is higher in photon
energy than visible light
– The region from 200 to 400 nm (2 x 10-7 m to 4 x 10-7 m)
is most useful in organic chemistry
• Conjugated compounds can absorb light in the
ultraviolet region of the spectrum
– The energy absorbed corresponds to the amount
necessary to promote an electron from one orbital to
another
– The electrons in the highest occupied molecular orbital
(HOMO) undergo a transition to the lowest unoccupied
molecular orbital (LUMO)
Practice Problem: Calculate the energy range of electromagnetic
radiation in the UV region of the spectrum from
200 to 400 nm. Recall the equation
E = NA e =
NA hc
l
=
1.20 x 10-4 kJ/mol
l
Practice Problem: How does the energy you calculated in the
previous problem for UV radiation compare
with the values calculated previously for IR and
NMR spectroscopy?
9.
Ultraviolet Spectrum of 1,3Butadiene
• 1,3-butadiene has four p molecular orbitals with
the two lower-energy MOs occupied (y1 and y2)
• When 1,3-butadiene absorbs UV light, a p electron in the
highest occupied molecular orbital (HOMO) is “promoted”
to the lowest unoccupied molecular orbital (LUMO).
– This corresponds to a p  p* excitation
– This transition requires 217 nm UV light (lmax)
• A UV spectrum is a plot of absorbance versus
wavelength.
– A UV spectrum of purified molecule is obtained by irradiating
a sample with a linearly changing wavelength of UV light and
measuring the amount of light absorbed at each wavelength.
A = log
Io
I
• The Beer-Lambert Law gives:
A=exCxl
where A = Absorbance
= log(% of light transmitted through the sample)
e = molar absorptivity (extinction coefficient) in M-1cm-1
C = concentration in mol/L
l = pathlength in cm
• Absorbance for a particular compound in a specific solvent
at a specified wavelength is directly proportional to its
concentration
Practice Problem: If a pure vitamin A sample has an absorbance
at 325 nm of 0.735 in a 1.00 cm cell and e is
known to be 50,100 M-1 cm-1. What is its
concentration?
A=exCxl
Rearranging the equation,
C=
A
exl
0.735
C=
50,100 M-1 cm-1 x 1.00 cm
C = 1.47 x 10-5 M
10. Interpreting Ultraviolet Spectra:
The Effect of Conjugation
• tlmax is the wavelength where UV absorbance
for a compound is greatest
• It depends on:
– the energy difference between HOMO and LUMO
– the extent of conjugation
• llmax increases as conjugation increases (lower
energy)
– Energy difference between HOMO and LUMO
decreases as the extent of conjugation increases
Molecule
1,3-butadiene
1,3,5-hexatriene
1,3,5,7-octatetraene
lmax (nm)
217 nm
258 nm
290 nm
• Substituents on p system increase lmax
Practice Problem: Which of the following compounds would you
expect to show ultraviolet absorptions in the
200 to 400 nm range?
11. Conjugation, Color, and the
Chemistry of Vision
•
The visible region is about 400 to 800 nm, adjacent
to the UV region.
•
Extended systems of conjugation absorb in visible
region; they are colored
• Example: b-Carotene, 11 double bonds in conjugation,
lmax = 455 nm
• Example: b-Carotene is yellow-orange; it absorbs the
blue wavelength and transmits the rest
380
450
500
550
600
650
700
750
• Light-sensitive molecules responsible for vision are
conjugated systems.
(dietary)
(in liver)
• Visual pigments are responsible for absorbing light in
eye and triggering nerves to send signal to brain
trans-rhodopsin
found in rod cells (lightsensitive receptor cells
responsible for dim light
vision)
sends signal to brain
Chapter 14