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Reactions of Alkenes

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Electrophilic Addition To Diene

(Conjugated Dienes And Allene)�

DR. K. K. NANDI

DEPARTMENT OF CHEMISTRY

BKC COLLEGE

CHE2201, Chapter 14

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Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

  • The behaviour of conjugated dienes in electrophilic addition reactions is markedly different from that of typical alkenes

CHE2201, Chapter 14

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Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

  • Conjugated dienes undergo electrophilic reactions to produce a mixture of products
    • 1,4-addition products are formed due to the intermediate action of allylic carbocations

CHE2201, Chapter 14

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Products of Addition to Delocalized Carbocation

  • The reaction between Br– the allylic cation occurs either at C1 or C3 as both carbons share the positive charge

CHE2201, Chapter 14

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Worked Example

  • Give the structures of both 1,2 and 1,4 adducts resulting from reaction of 1 equivalent of HBr with the following compound

CHE2201, Chapter 14

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Worked Example

  • Solution:

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CHE2201, Chapter 14

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Kinetic versus Thermodynamic Control of Reactions

  • Addition to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct
  • At higher temperature, the product ratio changes and the 1,4 adduct predominates

CHE2201, Chapter 14

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Kinetic versus Thermodynamic Control of Reactions

  • Consider a reaction with B and C as products

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    • B forms faster than C
      • ΔGǂB < ΔGǂC
    • C is more stable than B
      • ΔG˚C > Δ G˚B

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CHE2201, Chapter 14

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Energy Diagram for Competing Reactions

CHE2201, Chapter 14

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Kinetic versus Thermodynamic Control of Reactions

  • Reactions under kinetic control
    • Product of an irreversible reaction depends only on relative rates and not on stability
      • B is the major product
  • Reactions under thermodynamic control
    • Product of a readily reversible reaction depends only on stability and not on relative rates
      • C is the major product

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CHE2201, Chapter 14

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Kinetic versus Thermodynamic Control of Reactions

  • Applying the principles of kinetic control and thermodynamic control to the electrophilic addition reactions of conjugated dienes

CHE2201, Chapter 14

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Worked Example

  • Determine why 1,4 adducts of 1,3-butadiene are generally more stable than 1,2 adducts
    • Solution:

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    • Disubstituted bonds are more stable than monosubstituted double bonds

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CHE2201, Chapter 14

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