Chapter 4 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7 th Edition L. G. Wade, Jr. The Study of Chemical Reactions

Chapter 42 Introduction Overall reaction: reactants  products Mechanism: Step-by-step pathway. To learn more about a reaction:  Thermodynamics  Kinetics

Chapter 43 Chlorination of Methane Requires heat or light for initiation. The most effective wavelength is blue, which is absorbed by chlorine gas. Many molecules of product are formed from absorption of only one photon of light (chain reaction).

Chapter 44 The Free-Radical Chain Reaction Initiation generates a radical intermediate. Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule). Termination: Side reactions that destroy the reactive intermediate.

Chapter 45 Initiation Step: Formation of Chlorine Atom A chlorine molecule splits homolytically into chlorine atoms (free radicals).

Chapter 46 Propagation Step: Carbon Radical The chlorine atom collides with a methane molecule and abstracts (removes) an H, forming another free radical and one of the products (HCl).

Chapter 47 Propagation Step: Product Formation The methyl free radical collides with another chlorine molecule, producing the organic product (methyl chloride) and regenerating the chlorine radical.

Chapter 48 Overall Reaction

Chapter 49 Termination Steps A reaction is classified as a termination step when any two free radicals join together producing a nonradical compound. Combination of free radical with contaminant or collision with wall are also termination steps.

Chapter 410 More Termination Steps

Chapter 411 Lewis Structures of Free Radicals Free radicals are unpaired electrons. Halogens have 7 valence electrons so one of them will be unpaired (radical). We refer to the halides as atoms not radicals.

Chapter 412 Equilibrium Constant K eq = [products] [reactants] For CH 4 + Cl 2  CH 3 Cl + HCl K eq = [CH 3 Cl][HCl] = 1.1 x [CH 4 ][Cl 2 ] Large value indicates reaction “goes to completion.”

Chapter 413 Free Energy Change   G = (energy of products) - (energy of reactants)   G is the amount of energy available to do work.  Negative values indicate spontaneity.  G o = -RT(lnK eq ) = RT(log 10 K eq ) where R = J/K-mol and T = temperature in kelvins.

Chapter 414 Factors Determining  G  Free energy change depends on:  Enthalpy   H  = (enthalpy of products) - (enthalpy of reactants)  Entropy   S  = (entropy of products) - (entropy of reactants)  G  =  H  - T  S 

Chapter 415 Enthalpy  H o = heat released or absorbed during a chemical reaction at standard conditions. Exothermic (-  H) heat is released. Endothermic (+  H) heat is absorbed. Reactions favor products with lowest enthalpy (strongest bonds).

Chapter 416 Entropy  S o = change in randomness, disorder, or freedom of movement. Increasing heat, volume, or number of particles increases entropy. Spontaneous reactions maximize disorder and minimize enthalpy. In the equation  G o =  H o - T  S o the entropy value is often small.

Chapter 417 Bond-Dissociation Enthalpies (BDE) Bond-dissociation requires energy (+BDE). Bond formation releases energy (-BDE). BDE can be used to estimate  H for a reaction. BDE for homolytic cleavage of bonds in a gaseous molecule.  Homolytic cleavage: When the bond breaks, each atom gets one electron.  Heterolytic cleavage: When the bond breaks, the most electronegative atom gets both electrons.

Chapter 418 Homolytic and Heterolytic Cleavages

Chapter 419 Enthalpy Changes in Chlorination CH 3 -H + Cl-Cl  CH 3 -Cl + H-Cl Bonds Broken  H° (per Mole) Bonds Formed  H° (per Mole) Cl-Cl+242 kJH-Cl-431 kJ CH 3 -H+435 kJCH 3 -Cl-351 kJ TOTALS+677 kJTOTAL-782 kJ  H° = +677 kJ + (-782 kJ) = -105 kJ/mol

Chapter 420 Kinetics Kinetics is the study of reaction rates. Rate of the reaction is a measure of how the concentration of the products increase while the concentration of the products decrease. A rate equation is also called the rate law and it gives the relationship between the concentration of the reactants and the reaction rate observed. Rate law is experimentally determined.

Chapter 421 Rate Law For the reaction A + B  C + D, rate = k r [A] a [B] b  a is the order with respect to A  b is the order with respect to B  a + b is the overall order Order is the number of molecules of that reactant which is present in the rate- determining step of the mechanism.

Chapter 422 Activation Energy The value of k depends on temperature as given by Arrhenius: where A = constant (frequency factor) E a = activation energy R = gas constant, J/kelvin-mole T = absolute temperature E a is the minimum kinetic energy needed to react.

Chapter 423 Activation Energy (Continued) At higher temperatures, more molecules have the required energy to react.

Chapter 424 Energy Diagram of an Exothermic Reaction The vertical axis in this graph represents the potential energy. The transition state is the highest point on the graph, and the activation energy is the energy difference between the reactants and the transition state.

Chapter 425 Rate-Limiting Step Reaction intermediates are stable as long as they don’t collide with another molecule or atom, but they are very reactive. Transition states are at energy maximums. Intermediates are at energy minimums. The reaction step with highest E a will be the slowest, therefore rate-determining for the entire reaction.

Chapter 426 Energy Diagram for the Chlorination of Methane

Chapter 427 Rate, E a, and Temperature XE a (per Mole)Rate at 27 °CRate at 227 °C F5140,000300,000 Cl ,000 Br759 x I1402 x x 10 -9

Chapter 428 Conclusions With increasing E a, rate decreases. With increasing temperature, rate increases. Fluorine reacts explosively. Chlorine reacts at a moderate rate. Bromine must be heated to react. Iodine does not react (detectably).

Chapter 429 Primary, Secondary, and Tertiary Hydrogens

Chapter 430 Chlorination Mechanism

Chapter 431 Bond Dissociation Energies for the Formation of Free Radicals

Chapter 432 Stability of Free Radicals Free radicals are more stable if they are highly substituted.

Chapter 433 Chlorination Energy Diagram Lower E a, faster rate, so more stable intermediate is formed faster.

Chapter 434 Rate of Substitution in the Bromination of Propane

Chapter 435 Energy Diagram for the Bromination of Propane

Chapter 436 Hammond Postulate Related species that are similar in energy are also similar in structure. The structure of the transition state resembles the structure of the closest stable species. Endothermic reaction: Transition state is product-like. Exothermic reaction: Transition state is reactant-like.

Chapter 437 Energy Diagrams: Chlorination Versus Bromination

Chapter 438 Endothermic and Exothermic Diagrams

Chapter 439 Radical Inhibitors Often added to food to retard spoilage by radical chain reactions. Without an inhibitor, each initiation step will cause a chain reaction so that many molecules will react. An inhibitor combines with the free radical to form a stable molecule. Vitamin E and vitamin C are thought to protect living cells from free radicals.

Chapter 440 Radical Inhibitors (Continued) A radical chain reaction is fast and has many exothermic steps that create more reactive radicals. When an inhibitor reacts with the radical, it creates a stable intermediate, and any further reactions will be endothermic and slow.

Chapter 441 Carbon Reactive Intermediates

Chapter 442 Carbocation Structure Carbon has 6 electrons, positively charged. Carbon is sp 2 hybridized with vacant p orbital.

Chapter 443 Carbocation Stability

Chapter 444 Carbocation Stability (Continued) Stabilized by alkyl substituents in two ways: 1. Inductive effect: Donation of electron density along the sigma bonds. 2. Hyperconjugation: Overlap of sigma bonding orbitals with empty p orbital.

Chapter 445 Free Radicals Also electron-deficient. Stabilized by alkyl substituents. Order of stability: 3  > 2  > 1  > methyl

Chapter 446 Stability of Carbon Radicals

Chapter 447 Carbanions Eight electrons on carbon: 6 bonding plus one lone pair. Carbon has a negative charge. Destabilized by alkyl substituents. Methyl >1  > 2  > 3 

Chapter 448 Carbenes Carbon is neutral. Vacant p orbital, so can be electrophilic. Lone pair of electrons, so can be nucleophilic.

Chapter 449 Basicity of Carbanions A carbanion has a negative charge on its carbon atom, making it a more powerful base and a stronger nucleophile than an amine. A carbanion is sufficiently basic to remove a proton from ammonia.

Chapter 450 Carbenes as Reaction Intermediates A strong base can abstract a proton from tribromomethane (CHBr 3 ) to give an inductively stabilized carbanion. This carbanion expels bromide ion to give dibromocarbene. The carbon atom is sp 2 hybridized with trigonal geometry. A carbene has both a lone pair of electrons and an empty p orbital, so it can react as a nucleophile or as an electrophile.