ORGO CHEM

Bromination Reaction Overview

  • Non Methane Bromination: Previous knowledge focused on free radical bromination of methane.

  • Overall Reaction: Ethane reacts with di-bromine (Br₂) to produce ethyl bromide and hydrogen bromide (HBr).

  • Mechanism Steps:

    • Initiation

    • Propagation

    • Termination

  • Reagents Change: The specific reagents change for each reaction step based on the overall process.

Initiation

  • Radical Formation: The process begins with the formation of bromine radicals.

  • Light Source Needed: A brief light exposure is necessary to initiate radical formation.

  • Bromine Bond Cleavage: The Br-Br bond breaks, generating two bromine radicals.

Propagation Step

  • Hydrogen Abstraction: Ethane undergoes hydrogen abstraction, yielding an ethyl radical:

    • Ethane molecule interacts with bromine radical.

    • Hydrogen atom from ethane is extracted, forming an ethyl radical and HBr.

  • Cycle Continuation: The ethyl radical will react with another molecule of Br₂, leading to the production of another bromine radical and perpetuating the cycle.

Termination Step

  • Radical Combination: The termination involves the combination of radicals.

  • Possible Reactions:

    • Two bromine radicals can recombine to reform Br₂.

    • An ethyl radical and a bromine radical may react, yielding ethyl bromide.

    • Two ethyl radicals can combine to give butane.

Minor Products

  • Dibrominated Compounds: While reactions can lead to dibrominated products, they are often considered minor products due to their lower stability and occurrence relative to the formation of monobrominated products.

Selectivity of Halogenation

  • Chlorination vs. Bromination: Different products from chlorine placement in propane (at the end vs. the middle of the chain).

  • Expected Product Distribution:

    • At primary carbon: 6 hydrogen atoms contribute to a product ratio estimated as 3:1 based on statistical expectation.

    • Actual experimentation shows 75% primary vs. 25% secondary halogenation product distribution inverted in the case of bromination—yielding approximately 60% secondary vs. 40% primary.

  • Stability of Radicals:

    • The stability of the radical intermediate affects product distribution.

    • The radical formed is crucial in determining the speed of hydrogen atom abstraction.

Energy Diagrams and Radical Stability

  • Energy Barrier for Radicals:

    • Primary radicals are less stable due to lack of stabilizing factors (induction or hyperconjugation).

    • Secondary radicals are more stable as they benefit from two stabilizing alkyl groups.

  • Hammond's Postulate:

    • The rate-determining step's transition state can be analyzed with respect to the energies of reactants and products.

Bond Energies and Thermodynamics

  • Bond Dissociation Energy (BDE):

    • The BDE for breaking C-H and forming HCl and HBr influences the exothermic or endothermic nature of the reactions.

    • Reaction calculations show:

    • Chlorination (BDE for HCl): 103 kcal/mol, relatively stable.

    • Bromination first step for HBr is endothermic (BDE of 99 kcal/mol).

  • Overall Enthalpy Changes:

    • For chlorination: approximately -32 kcal/mol (exothermic).

    • For bromination: Initially endothermic, total contributions yield a more stable reaction than starting materials with total energy changes leading to net release of energy (exothermic on completion).

Selectivity in Different Halogens

  • Bromine and Chlorine Behavior:

    • Bromine displays preference for secondary hydrogen abstraction, leading to selective reactions.

    • Chlorine exhibits less selectivity, affecting the overall product distribution rate.

Summary and Key Outcomes

  • Radical Stability: The mechanism and product distributions rely heavily on the stability of radical intermediates formed during the halogenation process.

  • Energy Considerations: The energy required for bond dissociation and the heat of reaction are critical influences on the pathway and efficiency of radical formation.

  • Practical Implications: Understanding these mechanisms helps in predicting chemical behavior during halogenation reactions, relevant in various synthetic applications.