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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5852_Библиотеки_им_академика_М_И_Перельмана.pdf
C H a P T e r 8 Dienes
(Two nodes)
(Three nodes)
Energy
H
H
H
CC
+
+
−
+
+
+
H
H
H
CC
−
−
H
H
H
H
H
CC
H
CC
+
−
+
−
+ −
+
−
Ψ
4
∗
H
H
H
H
H
CC
H
CC
+
−
+
−
+
−
+
−
H
H
H
CC
−
+
−
+
+
−
H
H
H
CC
+
+
H
H
H
CC
+
+
−
−
+
H
H
H
CC
−
−
+
(One node)
Ψ
2
H
H
H
CC
H
H
H
CC
Ψ
1
(No nodes)
H
H
H
CC
H
H
H
CC
H
H
Ψ
3
∗
H
H
H
CC
CC
C C
H
H
H
H
H
H
CC
The central carbons of 1,3-butadiene (Fig. 8.1) are close enough for overlap to occur between the
p orbitals of C
those of C
electrons to be delocalized over all four atoms (Y
and C3 atoms. This overlap is not as great as that between the orbitals of C1 and C2 or
2
and C4. The partial overlap of C2–C3 gives partial double bond character and allows the four p
3
bonding orbital). The molecular orbital shown in Fig. 8.2
1
is the lowest-energy molecular orbital of 1,3-butadiene.
311
Conformations of 1,3-butadiene
There are two possible planar conformations of 1,3-butadiene: the s-cis conformation and the s-trans
conformation.
Figure 8.2 Lowest-energy molecular orbital in 1,3-butadiene.
Figure 8.1 LCAO of four p orbitals of 1,3-butadiene.
312
or or
HH
HH
H
CC
C
H
C
HH
CC
H
C
HH
H
C
4,5-Dibromopentene
1,4-Pentadiene
Br
2
Br Br
CH
2
CH
2
CH
2
CH CHCH
2
CH
2
CH
2
CH CH
Excess Br
2
1,2,4,5 -Tetrabromopentane
Br Br Br Br
CH
2
CH
2
CH
2
CH CH
Br Br
CH
2
CH
2
CH
2
CH CH
PHarmaCeuTICal OrganIC CHemIsTry
Due to rotation about C2–C3 ‘single’ bond only two conformations, name s-cis (cisoid) and s-trans
(transoid) are possible. The symbol ‘s’ refers to the fact that the isomerism is with respect to the single
bond. Therefore, these are not true cis and trans forms of 1,3-butadiene. These can be interconverted
through rotation about the single bond. The s-trans conformation is the predominant one (95%) at
room temperature. This is probably because of greater steric repulsion between hydrogen atoms of
s-cis form. It may be noted, however, that the energy barrier for conversion of s-trans to s-cis form is
quite low (5 kcal/mole).
8.5 CHEMICAL PROPERTIES OF CONJUGATED DIENES
The dienes contain two double bonds in a molecule and are expected to show the characteristic electrophilic
addition reactions as studied under alkenes. The chemical properties of dienes depend upon the arrangement
of its double bonds. Isolated double bonds exert little effect on each other and add up two molecules of
attacking reagent in the same ways as alkenes and show a close resemblance to them. Conjugated dienes
on the other hand differ from simple alkenes because of their high reactivity and stability. They undergo
1,2 and 1,4 reactions.
8.5.1 Electrophilic Addition Reactions
as stated above, dienes undergo electrophilic addition reactions. When 1,4-pentadiene is allowed to react
with bromine, we get the expected products 4,5-dibromopentene-1 and 1,2,4,5-tetra bromopentane.
1. Isolated dienes:
C H a P T e r 8 Dienes
Allyl-type carbonium ion
(Secondary)
δ+ δ−
⊕
Θ
. .
Br
1
CH2 + Br
2
CH
3
CH
4
CH
2
CH
2
CH2 + BrCH CH Br
It is clear from the above reactions that in isolated dienes the double bonds react independently,
as if they were in two different molecules.
2. Conjugated dienes: Conjugated dienes such as 1,3-buta-diene when treated with 1 mole of bromine
give a mixture of two products, i.e. 3,4-dibromobutene-1 (called 1,2-addition product) and 1,4dibromo butene-2 (called 1,4-addition product). Very often, 1,4-addition product is the major
product.
Mechanism: The formation of 3,4-dibromobutene-1 (1,2-addition) is quite expected but the formation of the
main product 1,4-dibromobutane-2 (1,4-addition) is unexpected and can only be explained by considering
the reaction mechanism. likewise, the addition of bromine to 1,3-butadiene also takes place in two steps.
Step I: The polarization of bromine molecule followed by abstraction of the electrophile bromonium ion
by a doubly bonded carbon atom takes place resulting in the formation of an allyl type of carbonium ion
and a negative bromide ion:
313
a primary carbonium ion can also be formed by the attack of Br+ ion at
but the allylic or secondary carbonium ion is preferred
because of its greater stability. This carbonium ion is
further stabilized by resonance. It is a resonance hybrid
of two contributing structures as depicted below:
Delocalization of positive charge
The two contributing structures differ from one another in location of the double bond and the positive
charge. The positive charge is not localized either at C
or C4 but is spread over both.
2
Learning Plus
Lycopene, β-carotene and antho cyanins are found in
the leaves of trees, but their characteristic colours are
usually obscured by the green colour of chlorophyll. In
the fall when chlorophyll degrades, the colours become
apparent.
314
1,2-Dibromobut-1-ene
(1,2-addition)
1,4-Dibromobut-2-ene
(1,4-addition)
Br
−
δ
+
δ
+
.... ....
CH
2
CH2BrCH CH
CH
2
CH2BrCHBrCH
CH2Br CH CH2BrCH
25°C
⊕
⊕
H
CH2CH CH CH
2
+ δ
− δ
CH
2
CH CH CH2 + H Cl
H
CH CH2CH CH
2
3-Chloro-1-butene (1,2-addition)
C1
−
C
H
Cl
CH2CH CH CH
2
⊕
C
H
CH2CH CH CH
2
⊕
C
H
CH2CH CH CH
2
C
H
Cl
CH2CH CH CH
2
Methyl allyl chloride (78%)
Crotyl chloride (22%)
1-Chloro-2-butene (1,4-addition)
PHarmaCeuTICal OrganIC CHemIsTry
Step II: The negative bromide ion (Br–) can attack either of the two positive carbons. addition of the
bromide ion at C2 will result in 1,2-addition whereas addition at C4 will lead to 1,4-addition:
The above mechanism clearly explains 1,2 and 1,4 addition to conjugated dienes.
The electrophilic attack that takes place at C-1 resulting in the formation of a resonance stabilized allylic
carbonium ion also explains the enhanced reactivity of conjugated dienes over isolated dienes.
1.
Addition of halogen acids: similarly, addition of hydrogen halides to 1,3-butadiene gives a mixture
of 1,2-addition and 1,4-addition products. The mechanism of this reaction is similar to that of addition
of bromine as discussed above. let us study the addition of HCl to 1,3-butadiene.
Role of temperature in electrophilic addition and rate control versus equilibrium control of a
reaction: The addition of HBr to 1,3-butadiene is interesting in another respect. The relative amounts
of 1,2 and 1,4-addition product that we obtain are dependent on the temperature at which we carry out
the reaction.
C H a P T e r 8 Dienes
3-Bromo-1-butene 1-Bromo-2-butene
HBr, 80°C
1,2-Addition
product
1,4-Addition
product
CH
3
CHCH CH
2
Br
CH3CH CHCH
2
Br
H
+
Br
(1,2-product)
(1,4-product)
Br
CH2CH CH CH
2
+ δ + δ
CH
3
CH CH CH
2
CH3CH CHCH2Br
CH3CHCH CH2
–
Br
–
When 1,3-butadiene and hydrogen bromide react at a low temperature (–80oC), the major reaction is
1,2-addition; we obtain about 80% of the 1,2 product and only about 20% of the 1,4 product.
at a higher temperature (40oC), the result is reversed. The major reaction is 1,4-addition; we obtain about
80% of the 1,4 product and only about 20% of the 1,2 product.
When the mixture formed at lower temperature is brought to the higher temperature, the relative amounts
of the two products change. The new reaction mixture eventually contains the same proportion of the
products given by the reaction carried out at the higher temperature.
It can also be shown that at the higher temperature and in the presence of HBr, the 1,2-addition product
rearranges to the 1,4 product and that equilibrium exists between them:
315
since the equilibrium favours the 1,4-addition product, it must be more stable. It is actually more stable
as it has more substituted olenic system.
This behaviour of 1,3-butadiene and HBr can be more fully understood if we examine Fig. 8.3.
316
E
Act
E
Act
Potential energy
1,4 addition (More stable)
1,2 addition
(Less stable)
Br
Br
Progress of reaction
PHarmaCeuTICal OrganIC CHemIsTry
The step that determines the overall outcome of the reaction is the step in which the resonance hybrid
allylic cation combines with a bromide ion:
This step determines the orientation of the reaction.
Figure 8.3 Energy of activation of 1,4 and 1,2 addition products
From Fig. 8.3, it is clear that the energy of activation leading to the 1,2 addition product is less than the
energy of activation leading to the 1,4 addition product, even though 1,4 product is more stable.
at low temperature, a larger fraction of collisions between the intermediate ions will have enough energy
to cross the lower energy barrier (leading to the 1,2 product), and a very small fraction of collisions will
have enough energy to cross the higher energy barrier (leading to the 1,4 product).
The key point here is that whichever barrier is crossed, product formation is irreversible because there is not
enough energy available to lift either product out of its deep potential energy valley. since 1,2 addition occurs
faster, the 1,2 product predominates and the reaction is said to be under rate control or kinetic control.
At higher temperature, the intermediate ions have sufcient energy to cross both barriers with relative ease.
Sufcient energy is also available to take the products back over their energy barriers to the intermediate
level of allylic cations and bromide ions. Thus both reactions are reversible.
The 1,2 product is still formed faster, but being less stable than the 1,4 product, it also reverts to the allylic
cation faster. Thus under these higher temperature conditions, the relative proportions of the energy barriers
are responsible for converting allylic cation to products. They reect the relative stabilities of the products
themselves. since the 1,4 product is more stable, it is formed at the expense of the 1,2 product because the
overall change from 1,2 product to 1,4 product is energetically favoured. such a reaction is said to be under
equilibrium control or thermodynamic control.
8.5.2 Free Radical Addition to Conjugated Dienes
Conjugated dienes undergo free radical addition reactions. In these reactions, conjugated dienes undergo
1,4 as well as 1,2-addition. These reactions take place in the presence of peroxides. let us take the addition
of BrCCl3 to 1,3-butadiene in the presence of a peroxide.
C H a P T e r 8 Dienes
Benzoyl peroxide Benzoate free
radical
Phenyl free
radical
Bromo trichloro
methane
Trichloro methyl
radical
°
CCl
3
C6H
5
+ Br
Br + CCl
3
°
C
6H5
Product formation in resonance stabilized allyl free radical
Step I: The peroxide decomposes homolytically to produce a free radical.
Step II: Free radical produced above abstracts bromine from BrCCl3 yielding C°Cl3 free radical.
Step III: The CCl3 free radical gets attached to one of the terminal carbon atoms of the conjugated diene
(1,3-butadiene). This is the preferred reaction, as it forms the more stable (secondary) allyl free radical,
which is further stabilized by resonance. It can be represented by two resonating structures that differ only
in the arrangement of the electrons. The radical does not correspond to either of the structure but to an
intermediate structure in which the odd electron is distributed over two carbon atoms.
317
Step IV: In order to complete the reaction, the allyl free radical abstracts a bromine atom from BrCCl3
producing addition products. The bromine atom of BrCCl
1,2- or 1,4-addition product. The newly formed
Allylic free radical (resonance hybrid)
can be attached to either C2 or C4, thus producing
3
o
CCl3 free radical carries on the chain process.
PH armaC euTICal OrganIC CHemIs T ry
318
Maleic anhydride
(Dienophile)
1,2,3,6-Terrahydrophthalic
anhydride
8.5.3 Diels–Alder Reaction
This reaction involves 1,4-cycloaddition of the conjugated system of a diene to a carbon–carbon double
bond of a dienophile activated by an adjoining polar bond. For example, maleic anhydride in benzene
solvent reacts at room temperature with 1,3-butadiene to form a 1,4-addition product without any catalyst
or condensing agent. The reaction involves a shift of the double bond in the diene.
ethylene, which is not activated by a polar double bond, reacts only under drastic conditions to form
cyclohexene.
some of the important dienophiles are maleic
anhydride, acraldehyde, crotonaldehyde, acrylic esters,
acetylene dicarboxylic acid (HOOC–C
cinnamaldehyde (C
CHCHO) and benzoquinone.
6H5
C COOH),
Besides 1,3-butadiene, the other dienes that may be used
in this reaction are cyclopentadiene, cyclohexadiene, furan
and anthracene.
These reactions are also commonly referred to as [4 + 2]
Learning Plus
Kurt Alder (1902–1958)
Alder worked with Diels and in 1928 they discovered the
Diels–Alder reaction. Alder was a professor of chemistry at
the University of Kiel and at the University of Cologne. He
received the 1950 Nobel Prize in chemistry, sharing it with
his mentor, Otto Diels.
cycloaddition reactions since they involve the addition of a
4p- electron system to 2p-electron system. The 4p-electron
system is a conjugated diene and the two-electron system is a dienophile and the product formed is a
six-membered cyclic alkene.
The Diels–alder reaction does not involve ionic or free radical intermediates and is remarkably insensitive
to the presence of solvents and catalysts. In fact, this reaction proceeds in a concerted manner involving a
cyclic transition state with simultaneous making of new bonds and breaking of old bonds.
electron-releasing groups in the diene are very helpful in these 1,4-cycloaddition as is clear in the
following reactions:
C H a P T e r 8 Dienes
O O
CCO
(Not formed)
CH
2
CH
2
CH CH CH3CH2CH2CH
3
CH
3
CH
3
CH CH
Na + C2H5OH Na + C2H5OH
Ni
1-Butene
(Minor)
2-Butene
(Major)
recent research has shown that the locations of electron-withdrawing and electron-releasing groups in
the dienophile and diene can be reserved without reducing the yields of the adducts. Dienes with electron
withdrawing groups have been found to react readily with dienophiles containing electron-releasing
groups.
additional facts about the reaction are as follows:
1. The reaction is a syn addition and the conguration of the dienophile is retained in the products.
2. The diene, of necessity, must react in the s-cis conformation rather than the s-trans.
3. The Diels–alder reaction occurs primarily and is an endo rather than exo fashion when the reactions
are kinetically controlled. Endo and exo are terms used to designate the stereochemistry of bridged
bicyclic rings like norbornane. The point of reference is the shortest
bridge. a group that is on the
same side of the six-membered ring as the carbon bridge is said to be exo; if it is in on the opposite
side, it is endo.
In the Diels–alder reaction of cyclopentadiene with maleic anhydride, the major product is the one in
319
which the anhydride linkage
has assumed the endo conguration. The favoured
chemistry seems to arise from favourable interaction between p electrons of the developing double bond in
diene and the electrons of unsaturated groups of the dienophile.
In the example above, the p electrons of the linkage of the anhydride interact with p electrons of the
developing double bond in cyclopentadiene.
8.5.4 Reduction
Conjugated dienes are more reactive than isolated dienes and this enhanced chemical reactivity of conjugated
diene over isolated diene is born out by the fact that conjugated diene is easily reduced by sodium and
alcohol giving 1,4-addition product whereas isolated dienes are not reduced.
Catalytic hydrogenation of conjugated dienes gives a mixture 1,2 and 1,4-addition products with the 1,4-
addition product predominating.
PH armaC euTICal OrganIC CHemIs T ry
320
1,2-addition
cis 1,4-addition
trans 1,4-addition
n
n
n
n
n
2-Methyl-1,3-butadiene
(Isoprene)
8.5.5 Polymerization
Conjugated dienes also undergo free radical polymerization
like alkene but the interesting feature is that their
polymerization may involve 1,2- or 1,4-addition to the
monomer. In the case of polymerization through 1,4addition of one monomer molecule to another, there may be
cis and trans addition. let us study the polymerization of
butadiene in the presence of peroxides:
Learning Plus
Polybutadiene itself is a very soft, almost liquid material
but the copolymers prepared from mixtures of butadiene
with styrene and/or acrylonitrile such as acrylonitrile
butadiene styrene (ABS), acrylonitrile butadiene (NBR)
and styrene-butadiene (SBR) are tough and elastic.
similarly substituted dienes also undergo free radical polymerization mostly by 1,4-addition, giving
mainly the all trans-polymer. For example,