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Conjugated Systems

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Title: Conjugated Systems


1
Conjugated Systems
Chapter 20
2
20.1 Conjugated Dienes, Table 20.1
  • Comparison of heats of hydrogenation and relative
  • stabilities of conjugated and unconjugated dienes

3
Stability of Conjugated Dienes
  • conjugation of the double bonds in 1,3-butadiene
    gives an extra stability of approximately 17 kJ
    (4.1 kcal)/mol.

1-butene
butadiene
4
Conjugated Carbonyls ?,?-unsaturated
  • systems containing conjugated double bonds, not
    just those of dienes, are more stable than those
    containing unconjugated double bonds.

5
20.2 A. 1,2- and 1,4-Addition (HBr)
  • Addition of 1 mol of HBr to butadiene at -78C
    gives a mixture of two constitutional isomers.
  • we account for these products by the following
    two-step mechanism.

6
1,2- and 1,4-Addition (HBr)
  • the key intermediate is a resonance-stabilized
    allylic carbocation.

7
1,2- and 1,4-Addition (Br2)
  • Addition of 1 mole of Br2 to butadiene at -15 C
    also gives a mixture of two constitutional
    isomers.
  • we account for the formation of these 1,2- and
    1,4-addition products by a similar mechanism.
  • Note at 40C distribution favors the
    1,4-product.


1,3-Butadiene

8
Experimental Information
  • for addition of HBr at -78C and Br2 at -15C,
    the 1,2-addition products predominate at higher
    temperatures (40 to 60C), the 1,4-addition
    products predominate.
  • if the products of the low temperature addition
    are warmed to the higher temperature, the product
    composition becomes identical to the higher
    temperature distribution the same result can be
    accomplished using a Lewis acid catalyst, such as
    FeCl3 or ZnCl2.
  • if either pure 1,2- or pure 1,4- addition product
    is dissolved in an inert solvent at the higher
    temperature and a Lewis acid catalyst added, an
    equilibrium mixture of 1,2- and 1,4-product
    forms the same equilibrium mixture is obtained
    regardless of which isomer is used as the
    starting material.

9
B. 1,2- and 1,4-Addition
  • We interpret these results using the concepts of
    kinetic and thermodynamic control of reactions.
  • Kinetic control the distribution of products is
    determined by their relative rates of formation
  • in addition of HBr and Br2 to a conjugated diene,
    1,2-addition occurs faster than 1,4-addition.

10
1,2- and 1,4-Addition
  • Thermodynamic control the distribution of
    products is determined by their relative
    stabilities.
  • in addition of HBr and Br2 to a butadiene, the
    1,4-addition product is more stable than the
    1,2-addition product.

11
1,2- and 1,4-Addition
  • Figure 20.3 Kinetic vs thermodynamic control

12
1,2- and 1,4-Addition
  • Is it a general rule that where two or more
    products are formed from a common intermediate,
    that the thermodynamically less stable product is
    formed at a greater rate?
  • No.
  • whether the thermodynamically more or less stable
    product is formed at a greater rate from a common
    intermediate depends very much on the particular
    reaction and reaction conditions.

13
20.3 A. UV-Visible Spectroscopy
  • Absorption of radiation in these regions give us
    information about conjugation of carbon-carbon
    and carbon-oxygen double bonds and their
    substation.

14
UV-Visible Spectroscopy, Fig 20.4
  • typically, UV-visible spectra consist of one or a
    small number of broad absorptions

15
UV-Visible Spectroscopy
  • Beer-Lambert law the relationship between
    absorbance, concentration, and length of the
    sample cell (cuvette).
  • A absorbance (unitless) a measure of the
    extent to which a compound absorbs radiation of a
    particular wavelength.
  • e molar absorptivity (M-1cm-1) a
    characteristic property of a compound
  • values range from zero to 106 M-1cm-1
  • C concentration in Molarity (M)
  • l length of the sample tube (cm).

16
UV-Visible Spectroscopy
  • the visible spectrum of b-carotene (the orange
    pigment in carrots) dissolved in hexane shows
    intense absorption maxima at 463 nm and 494 nm,
    both in the blue-green region.

17
B. UV-Visible Spectroscopy
  • Absorption of UV-Vis energy results in move-ment
    of an electron from a lower-energy occupied MO to
    a higher-energy unoccupied MO.
  • the energy of the uv-vis is sufficient to promote
    electrons from a pi (p) bonding MO to a pi
    antibonding (p) MO.
  • but is generally not sufficient to affect
    electrons in the much lower-energy sigma bonding
    (s) MOs
  • following are three examples of conjugated
    systems.

18
Orbitals of Conjugated Dienes
  • the pi system of butadiene is derived from the
    combination of four 2p atomic orbitals there are
    two bonding MOs and two antibonding MOs.

19
Orbitals of Conjugated Dienes
  • bond order
  • three nodes -3
  • (3 antibonding)
  • two nodes -1
  • (2 antibonding
  • and 1 bonding)
  • one node 1
  • (1 antibonding
  • and 2 bonding)
  • no nodes 3
  • (3 bonding)

20
B. UV-Visible Electronic Transitions
  • s -----------------------
  • p -----------------------
  • n -----------------------
  • p -----------------------
  • s -----------------------

21
UV-Visible Spectroscopy
  • UV-Visible spectroscopy of carbonyls.
  • simple aldehydes and ketones show only weak
    absorption in the UV due to an n to p electronic
    transition of the carbonyl group.
  • if the carbonyl group is conjugated with one or
    more carbon-carbon double bonds, intense
    absorption occurs due to a p to p transition.

22
UV-Visible Spectroscopy
  • Figure 20.5 A p to p transition in excitation
    of ethylene

23
UV-Visible Spectroscopy
  • Figure 20.6 A p to p transition in excitation
    of 1,3-butadiene

24
UV-Visible Spectroscopy, Table 20.3
  • Wavelengths and energies required for p to p
    transitions of ethylene and three conjugated
    polyenes.

25
24.6 Diels-Alder Reaction
  • Diels-Alder reaction a cycloaddition reaction of
    a conjugated diene and certain types of double
    and triple bonds
  • dienophile diene-loving
  • Diels-Alder adduct the product of a Diels-Alder
    reaction

26
Pericyclic reactions
  • Pericyclic reaction a reaction that takes place
    in a single step, without intermediates, and
    involves a cyclic redistribution of bonding
    electrons. There three classes of pericyclic
    reactions
  • 1. Cycloaddition two molecules combine to form
    a ring.
  • 2. Electrocyclic one molecule, an
    intramolecular cyclization.
  • 3. Sigmatropic an intramolecular rearrangement.

27
Pericyclic reactions
28
Pericyclic reactions
29
Diels-Alder Reaction
  • alkynes also function as dienophiles
  • cycloaddition reaction a reaction in which two
    reactants add together in a single step to form a
    cyclic product


Diels-Alder adduct
30
A. Diels-Alder Reaction
  • the conformation of the diene must be s-cis

31
B. Diels-Alder Reaction
  • reaction is facilitated by a combination of
    electron-withdrawing substituents on one reactant
    and electron-releasing substituents on the other


pressure
Cyclohexene
Ethylene
1,3-Butadiene
O
O

3-Buten-2-one
1,3-Butadiene
O
O

3-Buten-2-one
32
Diels-Alder Reaction
33
C. Diels-Alder Reaction
  • the Diels-Alder reaction can be used to form
    bicyclic systems

H

170C
H
Diene
Dienophile
34
Diels-Alder Reaction
  • exo and endo are relative to the double bond
    derived from the diene

exo (outside)
endo (inside)
35
Diels-Alder Reaction
  • for a Diels-Alder reaction under kinetic control,
    endo orientation of the dienophile is favored

36
D. Diels-Alder Reaction
  • the configuration of the dienophile is retained

37
E. Diels-Alder Reaction
  • the configuration of the diene is retained

38
Diels-Alder Reaction
  • Figure 24.1 Mechanism of the Diels-Alder reaction

39
Diels-Alder Reaction
Orbital overlap must obey orbital symmetry rules.
40
F. Diels-Alder Reaction
  • The 1,3 substitution product is not observed.

41
F. Diels-Alder Reaction
  • A 1,3 substitution product is not observed.

42
Hexatriene Molecular Orbitals
  • 1,3,5-hexatriene

two nodes
five nodes
one node
four nodes
no nodes
three nodes
43
Conjugated Systems
End Chapter 20
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