Diels alder and stability of conjucated dienes

STABILITY OF CONJUGATED DIENES
M.Maruthamuthu, M.Pharm.,
Assistant professor , Department Of
Pharmaceutical Chemistry , svcp.

Conjugated dienes are characterized by alternating carbon-
carbon double bonds separated by carbon-carbon single
bonds. Cumulated dienes are characterized by adjacent
carbon-carbon double bonds. While conjugated dienes are
energetically more stable than isolated double bonds.
Cumulated double bonds are unstable
Introduction

Conjugated diene stability
1. Conjugated dienes are more stable than non
conjugated dienes (both isolated and cumulated) due to factors
such as delocalization of charge through resonance and
hybridization energy.
2. This stability can be seen in the differences in the energies of
hydrogenation between isolated and conjugated alkenes.
3. Since the higher the heat of hydrogenation the less stable the
compound, it is shown below that conjugated dienes (~54
kcal) have a lower heat of hydrogenation than their isolated
(~60 kcal) and cumulated diene (~70 kcal) counterparts.

Conjugated double bonds are separated by a single bond. 1,3-dienes
are an excellent example of a conjugated system. Each carbon in 1,3
dienes is sp2 hybridized and therefore has one p orbital. The
four p orbitals in 1,3-butadiene overlap to form a conjugated
system.

The resonance structure shown below gives a good understanding of
how the pi electrons are delocalized across the four carbons in this
conjugated diene. This delocalization of electrons stablizes the
conjugated diene

The concepts we have been discussing come into focus in an
important reaction in synthetic organic chemistry, the Diels–Alder
reaction. In this reaction, a diene with its 4 π electrons reacts with an
alkene with its two π electrons to give a cyclohexene ring. Since the
product is a ring, it is a cyclo addition reaction. Since the 4 π
electrons of the diene and the two π electrons of the alkene participate
in the reaction.The simplest example of a Diels-Alder reaction is the
cyclo addition reaction of 1,4-butadiene and ethene to give
cyclohexene. In every case, a cyclohexene ring is a product
of a Diels–Alder reaction.
The Diels–Alder reaction has three important characteristics.
1. It is concerted. Thus, there are no intermediates.
2. It is a thermal reaction; that is, it is initiated by heat.
3. The transition state for the reaction contains 6 π electrons.

we saw that butadiene can exist in either an s-cis or an s-trans
conformation. For open-chain, conjugated dienes, the diene must
be in an s-cis conformation for the reaction to occur.
Reactants that contain conjugated double bonds in a ring, such as
cyclopentadiene, react much faster than open-chain conjugated
dienes. The alkene that reacts with the diene is called the
dienophile.
Ethene is a poor dienophile, and the reaction is much faster when
an electron withdrawing is conjugated to the dienophile.

Since the Diels–Alder reaction is concerted, the stereochemistry of
the dienophile is retained in the product. The groups that are trans in
the dienophile are trans in the product, and groups that are cis in the
dienophile are cis in the product.

And, since the reaction is concerted, groups that are trans in the
diene are also trans in the product, and groups that are cis in the
diene are cis in the product.

Reactants that contain conjugated double bonds in a ring, such
as cyclopentadiene, react much faster than open-chain
conjugated dienes. Reactions of a dienophile with a cyclic diene
give bicyclic products. The products in bicyclo[2.2.1] ring
systems have substituents on the opposite side of one-carbon
bridge; that is, they are endo.

Cylopentadiene reacts with itself over the course of a few hours
in a Diels–Alder reaction. This reaction is reversible, and if the
dimer is heated, cyclopentadiene is regenerated.

Electrophilic Addition to Conjugated Dienes
1,2- and 1,4-Electrophilic Addition Reactions
The addition of HBr to a conjugated diene is strikingly different.
Adding 1M equivalent of HBr at a low temperature yields two
products.

The energy of the transition state for 1,2-addition of HBr to 1,3-
butadiene is lower than the energy of the transition state for 1,4-
addition.
Energy Profile for 1,2- and 1,4-Electrophilic Addition
Reactions

1,2-Addition predominates at low temperature because there is not
enough energy for the system to reach the transition state for 1,4-
addition.
This energy difference is responsible for kinetic control of the addition
reaction. The 1,4-addition product is more stable than the 1,2-
addition product, but it forms more slowly.
At higher temperatures, the transition state leading to the more stable
product can be attained and leads to 1,4 addition. Thus, at high
temperatures, the product composition reflects the relative stabilities
of the products, not the relative energies of the transition states leading
to them.

The ratio of products depends on the temperature at which the
reaction is carried out. In a separate experiment, when either
of the two compounds is heated at 45 °C, an equilibrium
mixture with a ratio of 85:15 of 1 -bromo-2-butene to 3 -
bromo-1-butene forms.

The reactions of 1,3-butadiene are reasonably typical of
conjugated dienes. The compound undergoes the usual reactions
of alkenes, such as catalytic hydrogenation or radical and polar
additions, but it does so more readily than most alkenes or dienes
that have isolated double bonds. Furthermore, the products
frequently are those of 1,2 and 1,4 addition

Formation of both 1,2- and 1,4-addition products occurs not only with halogens,
but also with other electrophiles such as the hydrogen halides.. The first step, as
with alkenes, is formation of a carbocation. However, with 1,3-butadiene, if the
proton is added to C1 (but not C2), the resulting cation has a
substantial delocalization energy, with the charge distributed over two carbons.
Attack of Cl⊖ as a nucleophile at one or the other of the positive carbons yields the
1,2- or the 1,4- addition produc

An important feature of reactions in which 1,2 and 1,4 additions
occur in competition with one another is that the ratio of the
products can depend on the
1.Temperature
2. The solvent, and also on the
3.Total time of reaction.
The reason for the dependence on the reaction time is that the
formation of the carbocation is reversible, and the ratio of products
at equilibrium need not be the same as the ratio of the rates of attack
of Cl⊖ at C1 and C3 of the carbocation. This is another example of a
difference in product ratios resulting from kinetic
control versus equilibrium control.

The fact is that at low temperatures the 1,2 product
predominates because it is formed more rapidly, and the back
reactions, corresponding to k−1 or k−3, are slow . However,
at equilibrium the 1,4 product is favored because it is more
stable, not because it is formed more rapidly.

Conjugated dienes also undergo addition reactions by radical-chain
mechanisms. Here, the addition product almost always is the 1,4
adduct. Thus radical addition of hydrogen bromide to 1,3-butadiene
gives l-bromo-2-butene, presumably by the following mechanism

Markovnikov's rule allows us to predict the products of most
addition reactions, but constitutional isomers form in some reactions.
For example, the addition of HCl to 3-methyl-1-butene gives not
only the expected product , 2-chloro-3-methylbutane, but also 2-
chloro-2-methylbutane.
Allylic rearrangement

What is the origin of this second, apparently unexpected product?
The answer is:
the carbocation formed in the first step of the reaction
subsequently rearranges to a more stable species, which then
reacts with chloride.
The expected product forms from the reaction of the nucleophilic
chloride ion with the secondary cation that forms when a proton
adds to the double bond by Markovnikov addition.
1. The isomeric product forms when a nucleophilic chloride ion reacts with a
tertiary carbocation
2. This carbocation forms when the hydrogen atom at C-3 moves, with its
bonding pair of electrons, to the adjacent secondary carbocation center
3. This rearrangement is called a 1,2-hydride shift because a hydride ion
(H– ) moves between adjacent carbon atoms.

1. Some of the secondary carbocation also reacts with chloride ion
without rearranging to give the expected product.
2. The relative amounts of rearranged and un rearranged products
depend on how efficiently the carbocation is captured by the
nucleophile versus the rate of the rearrangement process.
The reaction of 3,3-dimethyl-1-butene with HCl gives us an
example of this type of rearrangement.

Adding a proton to the less substituted carbon atom of the double
bond (Markovnikov’s rule again) gives a secondary carbocation.
This secondary carbocation reacts with the nucleophilic chloride ion
to give the expected product. The rearranged product forms by
reaction of chloride ion with a tertiary carbocation that forms by a
shift of a methyl group, with its bonding pair of electrons, from the
quaternary center to the adjacent secondary carbocation center. This
rearrangement is called a 1,2-methide shift because a CH3 unit
moves between adjacent carbon atoms.

Sample Solution
Write the structures of all of the possible addition products of
3,3-dimethylcyclohexene with HBr.
Adding a proton at C-2 gives a secondary carbocation. Capture of
this carbocation by bromide ion yields 1-bromo-3,3-
dimethylcyclohexane.

Adding a proton at C-1 gives a secondary carbocation at the original
C-2 atom. Capture of the carbocation by bromide ion can give 1-
bromo-2,2-dimethylcyclohexane.
However, the secondary carbocation can rearrange to two possible
tertiary carbocations. Migration of methyl followed by capture of
the carbocation gives 1-bromo-1,2-dimethylcyclohexane.

A 1,2-shift of a methylene group of the ring can also occur to give a
tertiary carbocation. Capture of the carbocation by bromide gives a
product containing a cyclopentane ring.
We can say, however, that when we consider carbocation
chemistry we should more or less "expect the unexpected."

Diels alder and stability of conjucated dienes

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Diels alder and stability of conjucated dienes