Hóa học - Chapter 4: The study of chemical reactions

Chapter 4©2010, Prentice HallOrganic Chemistry, 7th EditionL. G. Wade, Jr.The Study of Chemical ReactionsChapter 4*IntroductionOverall reaction: reactants  productsMechanism: Step-by-step pathway.To learn more about a reaction: Thermodynamics Kinetics Chapter 4*Chlorination of MethaneRequires 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 4*The Free-Radical Chain ReactionInitiation 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 4*Initiation Step: Formation of Chlorine Atom A chlorine molecule splits homolytically into chlorine atoms (free radicals).Chapter 4*Propagation Step: Carbon RadicalThe chlorine atom collides with a methane molecule and abstracts (removes) an H, forming another free radical and one of the products (HCl).Chapter 4*Propagation Step: Product FormationThe methyl free radical collides with another chlorine molecule, producing the organic product (methyl chloride) and regenerating the chlorine radical.Chapter 4*Overall ReactionChapter 4*Termination StepsA 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 4*More Termination StepsChapter 4*Lewis Structures of Free RadicalsFree 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 4*Equilibrium ConstantKeq = [products] [reactants]For CH4 + Cl2  CH3Cl + HCl Keq = [CH3Cl][HCl] = 1.1 x 1019 [CH4][Cl2]Large value indicates reaction “goes to completion.”Chapter 4*Free Energy ChangeDG = (energy of products) - (energy of reactants)DG is the amount of energy available to do work.Negative values indicate spontaneity. DGo = -RT(lnKeq) = -2.303 RT(log10Keq) where R = 8.314 J/K-mol and T = temperature in kelvins. Chapter 4*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 4*Enthalpy DHo = heat released or absorbed during a chemical reaction at standard conditions.Exothermic (-DH) heat is released.Endothermic (+DH) heat is absorbed.Reactions favor products with lowest enthalpy (strongest bonds).Chapter 4*Entropy DSo = 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 DGo = DHo - TDSo the entropy value is often small. Chapter 4*Calculate the value of DG° for the chlorination of methane.DG° = –2.303RT(log Keq)Keq for the chlorination is 1.1 x 1019, and log Keq = 19.04At 25 °C (about 298 ° Kelvin), the value of RT is RT = (8.314 J/kelvin-mol)(298 kelvins) = 2478 J/mol, or 2.48 kJ/molSubstituting, we have DG° = (–2.303)(2.478 kJ/mol)(19.04) = –108.7 kJ/mol (–25.9 kcal>mol)This is a large negative value for DG°, showing that this chlorination has a large driving force that pushes it toward completion.Solved Problem 1SolutionChapter 4*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 4*Homolytic and Heterolytic CleavagesChapter 4*Enthalpy Changes in ChlorinationCH3-H + Cl-Cl  CH3-Cl + H-Cl Bonds BrokenDH° (per Mole)Bonds FormedDH° (per Mole)Cl-Cl+242 kJH-Cl-431 kJCH3-H+435 kJCH3-Cl-351 kJTOTALS+677 kJTOTAL-782 kJDH° = +677 kJ + (-782 kJ) = -105 kJ/molChapter 4*KineticsKinetics 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 4*Rate LawFor the reaction A + B  C + D,  rate = kr[A]a[B]ba is the order with respect to Ab is the order with respect to Ba + b is the overall orderOrder is the number of molecules of that reactant which is present in the rate-determining step of the mechanism. Chapter 4*Activation EnergyThe value of k depends on temperature as given by Arrhenius: where A = constant (frequency factor) Ea = activation energy R = gas constant, 8.314 J/kelvin-mole T = absolute temperatureEa is the minimum kinetic energy needed to react.Chapter 4*Activation Energy (Continued)At higher temperatures, more molecules have the required energy to react.Chapter 4*Energy Diagram of an Exothermic ReactionThe 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 4*Rate-Limiting StepReaction 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 Ea will be the slowest, therefore rate-determining for the entire reaction. Chapter 4*Energy Diagram for the Chlorination of MethaneChapter 4*Rate, Ea, and TemperatureXEa(per Mole)Rate at 27 °CRate at 227 °CF5140,000300,000Cl17130018,000Br759 x 10-80.015I1402 x 10-192 x 10-9Chapter 4*Consider the following reaction:This reaction has an activation energy (Ea) of +17 kJ/mol (+4 kcal/mol) and a DH° of +4 kJ/mol (+1 kcal/mol). Draw a reaction-energy diagram for this reaction.We draw a diagram that shows the products to be 4 kJ higher in energy than the reactants. The barrier is made to be 17 kJ higher in energy than the reactants.Solved Problem 2SolutionChapter 4*ConclusionsWith increasing Ea, 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 4*Primary, Secondary, and Tertiary HydrogensChapter 4*Chlorination MechanismChapter 4*Bond Dissociation Energies for the Formation of Free RadicalsChapter 4*Tertiary hydrogen atoms react with Cl• about 5.5 times as fast as primary ones. Predict the product ratios for chlorination of isobutane.There are nine primary hydrogens and one tertiary hydrogen in isobutane.(9 primary hydrogens) x (reactivity 1.0) = 9.0 relative amount of reaction(1 tertiary hydrogen) x (reactivity 5.5) = 5.5 relative amount of reactionSolved Problem 3SolutionChapter 4* Even though the primary hydrogens are less reactive, there are so many of them that the primary product is the major product. The product ratio will be 9.0:5.5, or about 1.6:1.Solved Problem 3 (Continued)SolutionChapter 4*Stability of Free RadicalsFree radicals are more stable if they are highly substituted.Chapter 4*Chlorination Energy DiagramLower Ea, faster rate, so more stable intermediate is formed faster.Chapter 4*Rate of Substitution in the Bromination of PropaneChapter 4*Energy Diagram for the Bromination of PropaneChapter 4*Hammond PostulateRelated 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 4*Energy Diagrams: Chlorination Versus BrominationChapter 4*Endothermic and Exothermic DiagramsChapter 4*Radical InhibitorsOften 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 4*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 4*Carbon Reactive IntermediatesChapter 4*Carbocation StructureCarbon has 6 electrons, positively charged.Carbon is sp2 hybridized with vacant p orbital.Chapter 4*Carbocation StabilityChapter 4*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 4*Free RadicalsAlso electron-deficient.Stabilized by alkyl substituents.Order of stability:3 > 2 > 1 > methylChapter 4*Stability of Carbon RadicalsChapter 4*CarbanionsEight electrons on carbon: 6 bonding plus one lone pair.Carbon has a negative charge.Destabilized by alkyl substituents.Methyl >1 > 2  > 3 Chapter 4*CarbenesCarbon is neutral.Vacant p orbital, so can be electrophilic.Lone pair of electrons, so can be nucleophilic. Chapter 4*Basicity of CarbanionsA 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 4*Carbenes as Reaction IntermediatesA strong base can abstract a proton from tribromomethane (CHBr3) to give an inductively stabilized carbanion. This carbanion expels bromide ion to give dibromocarbene. The carbon atom is sp2 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.