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Organic reactions overview. Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections 6.1, 6.2, 6.4-6, 6.8-10, 7.10, 10.8. Organic Reactions. Types of Reactions: Addition Elimination Substitution Rearrangement Oxidation Reduction See handout. Reaction Mechanisms.

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organic reactions overview

Organic reactions overview

Dr. Clower

CHEM 2411

Spring 2014

McMurry (8th ed.) sections 6.1, 6.2, 6.4-6, 6.8-10, 7.10, 10.8

organic reactions
Organic Reactions
  • Types of Reactions:
    • Addition
    • Elimination
    • Substitution
    • Rearrangement
    • Oxidation
    • Reduction
  • See handout
reaction mechanisms
Reaction Mechanisms
  • The details of how reactions occur
    • Bonds broken
    • Bonds formed
    • Electron rearrangement
    • Order of steps
    • Kinetics (rate)
    • Thermodynamics (energy)
    • Role of solvent, catalysts, etc.
bond breaking
Bond Breaking
  • Symmetrical/radical/homolytic
    • One electron to each atom
    • Fishhook arrow
    • Result in formation of free radicals
  • Unsymmetrical/polar/heterolytic
    • Both electrons to one atom
    • Regular curved arrow
    • Electrons move to more electronegative atom
bond formation
Bond Formation
  • Symmetrical/radical/homogenic
    • One electron from each atom
  • Unsymmetrical/polar/heterogenic
    • Both electrons from one atom
    • What is the nucleophile? What is the electrophile?
nucleophiles and electrophiles
Nucleophiles and Electrophiles
  • Nucleophile
    • Electron pair donor
    • Contain lone pair or p e-
  • Electrophile
    • Electron pair acceptor
    • Positive or partial positive charge
  • Remember electrons always move from nucleophile to electrophile
drawing mechanisms
Drawing Mechanisms
  • Curved arrows
  • Some guidelines:
  • Electrons move from nucleophile to electrophile
  • Nucleophile is negative or neutral (Nu: or Nu:-)
  • Electrophile is positive or neutral (E or E+)
  • Obey the octet rule
  • See Mechanisms worksheet
energy diagrams
Energy Diagrams
  • Change in energy as reaction proceeds
  • A one-step reaction:
  • Label:
  • Axes
  • Starting material
  • Product
  • Transition state
  • DG/DH
  • DGǂ/Ea
  • Where does bond breaking occur?
  • Where does bond making occur?
  • How do you know if the reaction is endothermic or exothermic?
transition state
Transition State
  • One transition state per step
  • Highest energy species in the step
  • Unstable; cannot be isolated
  • Resembles species (starting material or product) that is closest in energy
    • Hammond’s postulate
    • In an endothermic step the TS resembles the product
    • In an exothermic step the TS resembles the reactant/starting material
activation energy
Activation Energy
  • DGǂ or Ea
  • Energy difference between starting material and transition state
  • Minimum energy needed for reation to occur
  • High activation energy = slow reaction
  • Rate-determining step (RDS)
    • The slowest step
    • The step with the largest activation energy
energy diagrams1
Energy Diagrams
  • A two-step reaction:
  • Label:
  • Axes
  • Starting material
  • Product
  • Transition states
  • DG/DH
  • DGǂ/Ea for each step
  • Intermediate
intermediate
Intermediate
  • Energy minimum between two transition states
  • Higher energy than starting material or product
  • Usually cannot isolate (unstable)
  • Types of intermediates:
    • Free radicals
    • Carbocations
i ntermediates
Intermediates
  • Which carbocation is most stable? Least stable?
  • Why?
    • Inductive effect
      • Donation of electrons through bonds (R groups)
    • Hyperconjugation
      • Donation of electrons through orbitals
  • Other stable carbocations are

resonance-stabilized

an example reaction
An Example Reaction
  • HBr+ ethylene → bromoethane
  • What type of reaction is this?
  • What is the nucleophile? Electrophile? Look at structure:
    • Ethylene C=C has high electron density (4 e-); relatively easy to break p bond (weaker than s bond)
    • HBr is a strong acid (H+ donor) with partial positive charge on H
    • Electrons are donated from p bond of ethylene to H of HBr
    • Sigma bond of ethylene is not broken
mechanism
Mechanism
  • Two steps
  • Step 1:
  • Step 2:
mechanism1
Mechanism
  • The mechanism can be written as one scheme:
energy diagram
Energy Diagram
  • Label:
  • Axes
  • Starting material
  • Intermediate
  • Product
  • DG/DH
  • DGǂ/Ea for each step
radical reactions
Radical Reactions
  • Homolytic reactions
  • Not as common as polar reactions (heterolytic)
  • Mechanisms involve three steps
    • Initiation: start of the reaction; usually catalyzed by something
    • Propagation: continuation of the reaction; there can be many of these steps
    • Termination: end of the reaction
  • An example reaction: chlorination of methane
  • What type of reaction is this?
chlorination of methane
Chlorination of Methane
  • Initiation
    • Caused by irradiation with UV light
    • Break s bond to create reactive radicals
chlorination of methane1
Chlorination of Methane
  • Propagation
    • Chlorine radical reacts with methane to create methyl radical
    • Methyl radical reacts with Cl2 to give product and more Cl radical
    • New Cl radical repeats this propagation process (a chain reaction)
chlorination of methane2
Chlorination of Methane
  • Termination
    • Two radicals collide to form stable product
    • Break the reaction cycle
radical halogenation
Radical Halogenation
  • Used to synthesize alkyl halides from alkanes
  • One of only two alkane/cycloalkane reactions
    • Radical halogenation
    • Combustion (alkanes as fuel)
  • Requires heat (Δ) or light (hn) to initiate radical formation
  • Chlorination (Cl2) or bromination (Br2)
    • Iodine is too endothermic; fluorine is too reactive
  • Typically results in mixtures of products
halogenation of alkanes
Halogenation of Alkanes
  • Ex: ethane
  • Ex: butane
  • Why is this? Consider the intermediate structure…
halogenation of alkanes1
Halogenation of Alkanes
  • Substitution is favored at more substituted carbons
    • Tertiary > secondary > primary
    • The tertiary radical is more stable than the secondary radical
    • Regiochemistry
stereochemistry of halogenation
Stereochemistry of Halogenation
  • If the product contains a stereocenter, what is the stereochemistry?
  • This reaction will produce a racemic mixture. Why?
    • Look at radical intermediate: CH3─CH─CH2─CH3