Chapter_15

Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy

Conjugated Systems

• Conjugated double bonds are separated by one single bond.
• Isolated double bonds are separated by two or more single bonds.
• Conjugated double bonds are more stable than isolated ones.

Diene Stability

• Heats of hydrogenation are used to compare the relative stabilities of alkenes.
• For conjugated double bonds, the heat of hydrogenation is less than the sum for the individual double bonds.
• The more stable the compound, the less heat released during hydrogenation.
• Conjugated double bonds have extra stability.
• Relative stabilities (increasing energy):
  • Conjugated diene: trans-penta-1,3-diene (225 kJ or 53.7 kcal)
  • Isolated diene: penta-1,4-diene (252 kJ or 60.2 kcal), trans-hexa-1,4-diene
  • Terminal alkyne: pent-1-yne (291 kJ or 69.5 kcal)
  • Internal alkyne: pent-2-yne (240 kJ or 57.3 kcal)
  • Cumulated diene: penta-1,2-diene (292 kJ or 69.8 kcal)

Structure of Buta-1,3-diene

• The C2—C3 single bond is shorter than 1.54 Å.
• Electrons are delocalized over the molecule.
• There is a small amount of overlap across the central C2— C3 bond, giving it a partial double bond character.

Molecular Orbitals (MOs)

• All atoms of buta-1,3-diene are sp2 hybridized with overlapping p orbitals.
• Each p orbital has two lobes with the wave function indicated by plus (+) and minus (–) signs (not electrical charges).
• When lobes overlap constructively (+ and +, or – and –), a pi bonding (\pi) MO is formed.
• When lobes overlap destructively (+ and –), a pi antibonding (\pi*) MO is formed.

Ethylene \pi MOs

• The combination of two p orbitals must give two molecular orbitals.
• Constructive overlap is a pi bonding (\pi) MO.
• Destructive overlap is an antibonding (\pi*) MO.

\pi_1 MO for Buta-1,3-diene

• Lowest energy
• All bonding interactions
• Electrons are delocalized over four nuclei.

\pi_2 MO for Buta-1,3-diene

• Two bonding interactions
• One antibonding interaction
• This is a bonding MO.
• Higher energy than \pi_1 MO and not as strong

\pi_3* MO for Buta-1,3-diene

• Two antibonding and one bonding interaction
• Two nodes
• Vacant in the ground state

\pi_4* MO for Buta-1,3-diene

• Three nodes
• Strongly antibonding
• Highest energy MO
• Vacant at ground state

MO for Buta-1,3-diene and Ethylene

• The bonding MOs of both buta-1,3-diene and ethylene are filled, and the antibonding MOs are empty.
• Buta-1,3-diene has lower energy than ethylene.
• This lower energy is the resonance stabilization of the conjugated diene.

Conformations of Buta-1,3-diene

• The s-trans conformer is more stable than the s-cis by 12 kJ/mol (2.8 kcal/mol).
• Easily interconvert at room temperature.

The Allylic Position

• The allylic carbon is the one directly attached to an sp2 carbon.
• Allylic cations are stabilized by resonance.

Allylic Cations

• The positive charge is delocalized over two carbons by resonance, giving the allyl cation more stability than nonconjugated cations.

Stability of Carbocations

• Stability of 1° allylic » 2° carbocation.
• Stability of 2° allylic » 3° carbocation.

1,2- and 1,4-Addition to Conjugated Dienes

• Electrophilic addition to the double bond produces the most stable intermediate.
• For conjugated dienes, the intermediate is a resonance-stabilized allylic cation.
• Nucleophile adds to either carbon 2 or 4, both of which have the delocalized positive charge.
• Addition of HBr to buta-1,3-diene produces 3-bromobut-1-ene (1,2-addition) and 1-bromobut-2-ene (1,4-addition).

Mechanism of 1,2- and 1,4-Addition

• Step 1: Protonation of one of the double bonds forms a resonance-stabilized allylic cation.
• Step 2: A nucleophile attacks at either electrophilic carbon atom.

Kinetic Versus Thermodynamic Control

• Kinetic Control at –80 °C
  • Transition state for the 1,2-addition has a lower Ea because it is a more stable secondary carbocation.
  • The 1,2-addition will be the faster addition at any temperature.
  • The nucleophilic attack of the bromide on the C2 allylic carbocation is irreversible at this low temperature.
  • The product that forms faster predominates (kinetic product).
  • Because the kinetics of the reaction determines the product, the reaction is said to be under kinetic control.
• Thermodynamic Control at 40 °C
  • The 1,2-addition is still the faster addition, but at 40 °C, the bromide attack is reversible.
  • The 1,2-product ionizes back to the allylic cation.
  • At 40 °C an equilibrium is established, which favors the most stable product.
  • The 1,4-addition is the most stable product (thermodynamic product) because it has a more substituted double bond.
  • Because the thermodynamics of the reaction determines the product, the reaction is said to be under thermodynamic control.

Allylic Radicals

• Stabilized by resonance.
• Radical stabilities:
• Substitution at the allylic position competes with addition to double bond.
• To encourage substitution, use a low concentration of reagent with light, heat, or peroxides to initiate free radical formation.

Mechanism of Allylic Bromination

• Initiation step:
  Br-Br \xrightarrow{hv} 2 Br•
• Propagation steps:
  H \xrightarrow{} allylic hydrogrens

Stability of Allylic Radicals

• Primary:
  CH3CH2-H \rightarrow CH3CH2• + H•, \Delta H = +423 \, kJ \, (+101 \, kcal)
• Secondary:
  (CH3)2CH-H \rightarrow (CH3)2CH• + H•, \Delta H = +413 \, kJ \, (+99 \, kcal)
• Tertiary:
  (CH3)3C-H \rightarrow (CH3)3C• + H•, \Delta H = +403 \, kJ \, (+96 \, kcal)
• Allyl:
  H2C=CH-CH2-H \rightarrow H2C=CH-CH2• + H•, \Delta H = +372 \, kJ \, (+89 \, kcal)

Resonance Stabilization

• During propagation, an allylic radical is formed that is stabilized by resonance.
• Either radical can form the final product.

Bromination Using NBS

• A convenient bromine source for allylic bromination (substitution) is N-bromosuccinimide (NBS), a brominated derivative of succinimide.
• NBS provides a low, constant concentration of Br_2.
• NBS reacts with the HBr by-product to produce Br_2 and to prevent HBr addition across the double bond.

Allyl System

• Geometric structure of the allyl cation, allyl radical, and allyl anion
• The three p orbitals of the allyl system are parallel to each other, allowing for the extended overlap between C1–C2 and C2–C3.

Molecular Orbitals of the Allylic System

• One resonance form shows the radical electron on C1, with a pi bond between C2 and C3.
• The other resonance form shows the radical electron on C3 and a pi bond between C1 and C2.
• No resonance form has an independent existence.

Molecular Orbitals of the Allyl Radical

• The radical electron occupies the nonbonding molecular orbital.

MOs for the Allylic Species

• Allyl cation (2\pi electrons)
• Allyl radical (3\pi electrons)
• Allyl anion (4\pi electrons)

SN2 Reactions of Allylic Halides and Tosylates

• Allylic halides and tosylates react quickly by the SN2 mechanism.
• The transition state of the SN2 mechanism is stabilized through conjugation with the p orbitals of the pi bond.
• This transition state has a lower activation energy and explains the enhanced reactivity of these allylic species.

SN2 Reactions with Organometallics

• Allylic halides and tosylates react with Grignards and organolithiums:

Diels-Alder Reaction

• Named after Otto Diels and Kurt Alder. They received the Nobel prize in 1950.
• The reaction is between a diene and an electron-deficient alkene (dienophile).
• Produces a cyclohexene ring.
• The Diels–Alder is also called a [4 + 2] cycloaddition because a ring is formed by the interaction of four pi electrons of the alkene with two pi electrons of the alkene or alkyne.

Mechanism of the Diels–Alder Reaction

• One-step, concerted mechanism
• A diene reacts with an electron-poor alkene (dienophile) to give cyclohexene or cyclohexadiene rings.

Stereochemical Requirements

• The diene must be in s-cis conformation.
• Diene’s C1 and C4 p orbitals must overlap with dienophile’s