2. Dienes:-
In organic chemistry, dienes is a hydrocarbon that
contains two carbon pi bonds (double bonds).
Conjugated dienes are functional groups, with a
general formula of CnH2n-2. Dienes and alkynes are
functional isomers. Dienes occur occasionally in
nature but are widely used in the polymer industry.
3. Classification of dienes:-
Dienes can be divided into three classes, depending
on the relative location of the double bonds:
1. Cumulated dienes/Allenes or cumulenes:
have the double bonds sharing a common atom as in a
group of compounds called allenes.
Eg:
penta-1,2-diene
2. Conjugated dienes:
have conjugated double bonds separated by one single
bond.
Eg:
penta-1,3-diene
C
H2 C CH CH2 CH3
C
H2 CH CH CH CH3
4. continued….
3. Unconjugated/Non-conjugated/isolated dienes:
have the double bonds separated by two or more
than two single bonds. They are usually less stable
than isomeric conjugated dienes.
Eg:
penta-1,4-diene
- Compounds that contain more than two double
bonds are called polyenes. Polyenes and dienes,
share many of their properties.
C
H2 CH CH2 CH CH2
5. Stabilities of dienes: Delocalisation of
charge:
Conjugated dienes are more stable than non-
conjugated dienes (both isolated and cumulated)
due to factors like delocalization of charge through
resonance and hybridization energy. These stability
can be seen in the differences in the hydrogenation
between isolated and conjugated dienes of alkenes.
Since the higher the heat of hydrogenation the less
stable the compound it is shown below the
conjugated dienes (~54 kcal) lower heat of
hydrogenation than their isolated (~60 kcal) and
cumulated dienes (~ 70 kcal) counterparts.
6. Continued….
Here is an energy diagram comparing different types
of bonds with their heats of hydrogenation to show
relative stability of each molecule.
7. Stabilities of conjugated dienes:
The resonance explanation
Conjugated double bonds are separated by single bond 1,3-dienes
are an excellent example of conjugated system. Each carbon in 1,3-
dienes are sp2 hybridized and therefore has one p-orbital. The four p-
orbital in 1,3-dienes 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 carbon in this
conjugated diene. This delocalization of electrons stabilizes the
conjugated diene.
8. Stabilities of conjugated dienes:
The molecular orbital explanation:
The molecular orbital for 1,3-butadiene is shown below.
Note that the lobe of four p orbital component in each pi-
orbital are coloured and carry a plus and minus sign. This
distinction refers to different phases defined by a
mathematical wave equations for such orbital. Regions in
which adjacent orbital lobes undergoes a phase change
are called nodes. Orbital electron density is zero in such
region. Thus a single p-orbital has a node at the nucleus
and all pi orbital shown has a nodal plane that is defined
by the atoms of the dienes. These is the only nodal
surface in the lowest energy pi orbital, n1. Higher energy
pi orbital have an increasing number of nodes. Since 1,3-
butadiene has four pi electrons. The two bonding
molecular orbital are filled to explain the measurable
10. Preparation of dienes:-
1. Retro-diels Alder reaction (pyrolysis of cycloalkene)
: when cyclohexene vapours is passed over heated
nichrome alloy then 1,3-Butadiene is produced.
CH
CH
CH2
CH2
C
H2
C
H2
cyclohexene
ethene
C
H2 CH2
C
H2 CH
CH
C
H2
buta-1,3-diene
11. 2. Catalytic hydrogenation :
1,3-Butadiene is prepared by catalytic hydrogenation
of n-butane, 1-butene or 2-butene.
Preparation of dienes:
continued….
12. Preparation of dienes:
continued….
3. Dehydrogenation of butane-1,3-diol :
acid catalysed dehydration of butane-1,3-diol gives
1,3-butadiene.
1,3-butadiene
13. Preparation of dienes:
continued….
4. Dehydrohalogenation of 1,3-dichlorobutanes : when
1,3-dichloro-3-methylbutane is reacted with alcoholic
KOH then isoprene (2-methylbuta-1,3-diene) is
formed.
14. Chemical reactions of dienes:-
Some important chemical reactions of dienes are as
follows:
1. Electrophilic addition to dienes:
a) Addition of Halogens:
When 1,3-Butadiene is treated with 1 mole of
bromine it gives a mixture of 1,2 and 1,4- addition
products.
1,2-addition
product
1,4-addition
product
Br2
15. Mechanism :-
Step-I:
Br+ (electrophile) adds to the terminal carbon atom
and forms stable 2˚ carbocation. Addition to second
carbon does not occur because it gives less stable
1˚ carbocation.
I
II
16. Continued…..
Step-II :
In resonance hybrid positive charge is equally
distributed between carbon atoms(I and II), the
attack of bromine ion can occur on either of these.
17. Continued…..
b) Addition of Halogen acids: Halogen acids (HI, HCl,
HBr) react with 1,3-Butadiene gives a mixture of 1,2
and 1,4-addition products.
1,4-addition
product
1,2-addition
product
19. 2. Free radical addition
reaction:
Conjugated dienes undergo free radical addition
reactions to give a mixture of 1,2 and 1,4- addition
products.
Peroxide
20. Mechanism :
Step-I : Peroxide dissociates to give free radicals.
C6H5CO-O-O-COC6H5 C6H5CO-O
Benzoyl peroxide
C6H5· + CO2
Phenyl radical
C6H5· + Br-CCl3 C6H5-Br + CCl3 ·
Bromobenzene trichloromethyl
radical
21. Continued….
Step-II : The CCl3 radical add to one of the terminal
carbon atoms since it yields the resonance stabilized
allyl radical.
CH2=CH-CH-CH2 + CCl3 · CH2=CH- ·CH-
CH2-CCl3
·CH2-CH=CH-CH2-
CCl3
I
II
22. Continued….
Step-III : The allyl radical formed is step-II then
abstracts a bromine atom from BrCCl3 to complete
the addition and generation a new ·CCl3 radical
which propagates the chain. If addition occurs at C2
then 1,2-addition product and if addition occurs at C4,
then 1,4-addition product is formed.
Br
CH2=CH- ·CH-CH2-CCl3 + Br-CCl3 CH2=CH-CH-
CH2-CCl3
+ ·CCl3
CH2=CH- ·CH-CH2-CCl3 + Br-CCl3 BrCH2-CH-
CH=CH2-CCl3
1,2-addition
product
1,4-addition
product
24. 4. Diels alder reaction:
It is also known as [4+2] cycloaddition reaction. It involves
addition of a conjugated diene (4π electron system) to a
substituted alkene (2π electron system) also known as
dienophile to form a six memebered cyclic alkene. This
reaction is particularly useful in synthetic organic chemistry
as it is reliable method for preparation of six membered
system. Dienophiles may be any acetylenic compound
containing electron withdrawing groups or alpha, ß-
unsaturated, ester, anhydrides, aldehydes, ketones and nitro
compounds. Diels Alder reaction normally do not require any
catalyst and occurs on heating.
25. Mechanism :
Diels- Alder reaction is an example of concerted
pericyclic reaction as it proceeds via a single, cyclic
transition state with no intermediates generated
during reaction. The reaction proceeds through the
suprafacial interaction of a 4π electron system with a
2π electron system and this interaction is thermally
allowed.
Cyclic transition
state
26. Allylic rearrangement :
Free radical substitution in alkenes or at allylic
positions lead to allylic rearrangement. For example,
when 2-butene is reacted with NBS (n-bromo
succinimide) then, two products are expected, 3-
bromo-2-butene and 3-bromo-1-butene.
Major product, more stable with
substituted double bond.
But get rearranged product as well
