Conjugated Systems

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Conjugated Systems
Chapter 20
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20.1 Conjugated Dienes, Table 20.1

• Comparison of heats of hydrogenation and relative
• stabilities of conjugated and unconjugated dienes

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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
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Conjugated Carbonyls ?,?-unsaturated

• systems containing conjugated double bonds, not
  just those of dienes, are more stable than those
  containing unconjugated double bonds.

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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.

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1,2- and 1,4-Addition (HBr)

• the key intermediate is a resonance-stabilized
  allylic carbocation.

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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

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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.

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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.

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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.

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1,2- and 1,4-Addition

• Figure 20.3 Kinetic vs thermodynamic control

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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.

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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.

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UV-Visible Spectroscopy, Fig 20.4

• typically, UV-visible spectra consist of one or a
  small number of broad absorptions

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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).

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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.

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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.

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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.

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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)

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B. UV-Visible Electronic Transitions

• s -----------------------
• p -----------------------
• n -----------------------
• p -----------------------
• s -----------------------

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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.

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UV-Visible Spectroscopy

• Figure 20.5 A p to p transition in excitation
  of ethylene

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UV-Visible Spectroscopy

• Figure 20.6 A p to p transition in excitation
  of 1,3-butadiene

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UV-Visible Spectroscopy, Table 20.3

• Wavelengths and energies required for p to p
  transitions of ethylene and three conjugated
  polyenes.

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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

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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.

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Pericyclic reactions
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Pericyclic reactions
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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
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A. Diels-Alder Reaction

• the conformation of the diene must be s-cis

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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
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Diels-Alder Reaction
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C. Diels-Alder Reaction

• the Diels-Alder reaction can be used to form
  bicyclic systems

H

170C
H
Diene
Dienophile
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Diels-Alder Reaction

• exo and endo are relative to the double bond
  derived from the diene

exo (outside)
endo (inside)
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Diels-Alder Reaction

• for a Diels-Alder reaction under kinetic control,
  endo orientation of the dienophile is favored

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D. Diels-Alder Reaction

• the configuration of the dienophile is retained

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E. Diels-Alder Reaction

• the configuration of the diene is retained

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Diels-Alder Reaction

• Figure 24.1 Mechanism of the Diels-Alder reaction

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Diels-Alder Reaction
Orbital overlap must obey orbital symmetry rules.
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F. Diels-Alder Reaction

• The 1,3 substitution product is not observed.

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F. Diels-Alder Reaction

• A 1,3 substitution product is not observed.

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Hexatriene Molecular Orbitals

• 1,3,5-hexatriene

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