An Overview of Organic Reactions Reaction types: Classification by outcome Most reactions produce changes in the functional group of the reactants: 1. Addition (forward) Gain of atoms across a bond Example: C C C Cl Cl 3 C 3 3 C C C C 3 base, heat 2. Elimination (reverse) Loss of atoms across a bond Dehydration and dehydrohalogenation are eliminations Organic Oxidation & Reduction are variations of addition & elimination An organic oxidation O C 3 Na 2 Cr 2 O C 7 3 C C 3 O C C 3 An organic reduction 3 C C 3 C O NaB 4 3 C C O 3. Substitution a replacement reaction 3 C C Br - O 3 C C O 4. Rearrangement - change in the alkyl group structure; may occur during some substitution or elimination reactions C 3 C 3 3 C C C catalyst C 2 3 C C C C 3
Mechanisms Reactions can be classified according to the process by which bonds are made and broken, or organic reaction mechanism. Two major types: hemolytic (radical) and heterolytic (polar) 1. Radical reactions (Introduced in section 5.3 - more details in Chapter 10) In homolytic bond cleavage, each atom gets one of the bonding electrons Both are left with an odd electron: A : B A. + B. = free radicals "Fishhook" arrow denotes movement of one e- Since free radicals are unstable & reactive, further rxns occur (chain reaction) Example from nature: Steps in the mechanism of a radical substitution reaction: Initiation: Formation of the initial radical, usually by application of energy to a compound with a weak or unstable σ bond: Example: O O hν or 2. O Propagation: Reactions which produce more radicals, usually by substitution Occurs when a radical collides with a stable molecule Many different propagation steps are possible for each rxn.. C 3 Ex: Cl. + C 3 Cl +. C 3 + Cl Cl C 3 Cl + Cl. Termination: 2 radicals combine to form a stable bond Ex: Cl. +. C 3 C 3 Cl
2. Polar Reactions (sections 5.4 5.6) When bonds break unsymmetrically to leave both bonding electrons with one atom (heterolytic cleavage) the products are charged species: A : B A + + B - Polar reactions are much more common than radical reactions Occur due to attraction between + and charges of polar bonds in different functional groups Require the presence of a nucleophile (electron-rich species) and an electrophile (electron-poor species) Influences on bond polarity: Atomic electronegativity Solvents, acids or bases that can interact with bonded atoms Polarizability of atoms Flow of electrons is always from the nucleophile toward the electrophile Arrows show the path of e- A + + B: - A B Typical nucleophiles: Lewis bases Electron-rich atoms such as O, N Anions such as Cl-, Br- Typical electrophiles: Electropositive atoms Electron-poor atoms Lewis acids + and other cations
Example: Identify the electrophile and the nucleophile in this reaction. Use arrows to show the flow of electrons.
Classify each as one of the four reaction types Follow the flow of electrons and predict the products: Add curved arrows to indicate the flow of electrons
The Physical Chemistry of Reactions Reaction mechanisms show the steps taking place but usually do not show us the position of equilibrium, the rates of each step or energy changes that take place. Thermodynamics = Study of the properties of a system at equilibrium and its associated energy changes The position of equilibrium of any reaction can be described by its K eq : Reactants Products K eq = [products] / [reactants] The associated energy change = the Gibbs free energy change or G G = -RT ln K eq For a reaction to favor product formation, the energy of the products must be lower than the energy of the reactions (products more stable) Favorable: Unfavorable: G is negative, energy is released, exergonic reaction G is positive, energy absorbed, endergonic reaction The free-energy change depends on two other changes: G = T S A favorable reaction is one that yields products with strong bonds & less order 1. S = entropy change: degree of disorder + S = reaction which produces greater number of particles - S = reaction which produces less particles, more order 2. = enthalpy change heat of reaction can be obtained by using bond dissociation energies (D) Table 5.3 gives D values at standard temperature and pressure ( o ) = sum of D of bonds broken sum of D of bonds formed
Kinetics: ow fast does a reaction occur? Why is it fast or slow? Any reaction can be described by a rate expression: Ex: A B rate = k [A] Rate = Amount of product formed per unit time (measured experimentally) Rate constant (k) depends on how easily energy barrier to rxn can be overcome For any reaction to occur, there must be enough energy to attain the transition state between reactants and products Transition state = Bonds partly broken and partly formed Short-lived; not a stable species G = Energy of activation = energy difference between reactants and Transition state The smaller the G, the faster reaction occurs In a multi-step reaction, the slowest step (largest G) limits the entire process; often referred to as the rate-limiting step Factors affecting reaction rates: 1. Temperature 2. Concentration of reactants 3. Number of collisions between reactant particles 4. Fraction of collisions with the correct orientation 5. Fraction of those collisions with enough energy to overcome activation energy!
What do Reaction Energy Diagrams tell us about a reaction? Br Br 2 C C 2 3 C C 2 1. Relative stabilities of reactants vs. products 2. Whether reaction is exergonic or endergonic 3. Whether equilibrium favors reactants or products 4. ow much activation energy is required to get rxn going 5. Structure of the transition state 6. Whether any intermediate species form 7. ow many steps in the reaction 8. The relative rates of the steps in the reaction (which one is the rate-determining step?)
Problem 5.32 in McMurry This reaction is an isomerization that occurs when the reactant, an alkene, is treated with acid: At equilibrium, the reaction mixture contains about 30% reactant and 70% product. What is the approximate K eq for this reaction? The reaction occurs slowly at room temperature. What s the approximate G +? Draw an energy diagram for the reaction.