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University of Minnesota Twin Cities
CHEM 2331H
T.Andrew Taton

Honors Organic Chemistry Chapter 4 Book Notes The Study of Chemical Reactions 4.1) Introduction • Halogenation: substituting halogen for H in alkane; occurs in gas phase (no solvent) o in reality, alkanes= unreactive o but their rxns = relatively uncomplicated, we know their properties and structure • Mechanism: step-by-step pathway from reactants to products o Thermodynamics: tells how well rxn goes to products; energetics of rxn @ equilibrium o amt reactant and products @ equilibrium depends on their relative stabilities • Kinetics: variation of rxn rates w/ different conditions/concentrations of reagents o  helps us propose rxn mechanisms that = consistent w/ behavior we observe 4.2) Chlorination of Methane • Chlorination of methane = important industrial rxn, relatively simple mechanism o products composition depends on amt Cl added & rxn conditions o o If continued: • Why and how this certain rxn’s possible? Need to understand: o 1) Mechanism: complete, step-by-step description of exactly which bonds break, which bonds form, and in what order to give observed products o 2) Thermodynamics: study of energy changes that accompany chem/physical transformations;  compare stability of reactants & products and predict which compounds favored by equilibrium o 3) Kinetics: study of rxn rates  which products = formed fastest; how rate changes if we change rxn conditions • Application to chlorine; need to know how rxn works/what affects it; what we know: o 1) chlorination doesn’t occur @ room temperature w/o light; rxn begins when light falls on mixture/when it’s heated  needs energy to initiate its o 2) most effective wavelength of light = blue color that’s strongly absorbed by C2 gas; light absorbed by chlorine, activates chlorine so that it initiates rxn w/ methane o 3) the light-initiated rxn has high quantum yield; many molecules of product formed per photon absorbed ( mechanism must explain how many individual rxns result from single molecule absorbing single photon 4.3) The Free-Radical Chain Reaction • Chain reaction: mechanism that’s proposed for chlorination of methane; 3 steps o 1) Initiation step: generates reactive intermediate o 2) Propagation steps: reactive intermediate + stable molecule → product + another reactive intermediate  chain continues until reactant supply exhausted/reactive intermediate destroyed o 3) Termination steps: side rxs that destroy reactive intermediates; tend to slow/stop rxn • Substitution: ex w/ chlorination of methane: chlorine doesn’t add to methane, replaces one of the H atoms, H goes to HCl byproduct • 4.3A) The Initiation Step: Generation of Radicals o Initiation of chlorination: chlorine absorbs blue light, which has energy to split it into 2 Cl atoms o o fishhook-shaped half arrows show movement of single electrons (just as we used full arrows to show movement/pushing of electron pairs) o reactive intermediate: short-lived species that’s never present in high concentration b/c reacts as quickly as it’s formed  Cl = highly reactive o radicals/free radicals: species w/ unpaired electrons  odd/radical electron: the unpaired electron  radicals = electron-deficient b/c no octet  odd electron combines w/ electron from other atom  complete octet + form bond  radicals represented by structure w/ single dot, which = unpaired electron (even if more valence electrons exist) • 4.3B) Propagation Steps o Propagation: Formation of products w/ regeneration of reactive intermediates o Cl radical collides w/ methane molecule  abstracts (removes) H  1 electron from broken C-H bond stays on C, other combines w/ odd electron on Cl  H-Cl bond (one of the products) o Propagation starts w/ 1 free radical, ends w/ another free radical  characteristic of propagation step in chain rxn, rxn must continue b/c another reactive intermediate produced o o Step 2: o Methyl radical reacts w/ chlorine molecule  chloromethane  odd electron from methyl radical combines w/ one of electrons from Cl-Cl bond  Cl-CH ,3Cl atom left w/ odd electron o Steps 1 and 2 cycle with each other until reactant supply exhausted or other rxn consumes radical intermediates  explains why many molecules of methyl chloride and HCl formed per photon of light absorbed • 4.3C) Termination Reactions o Termination reaction: step that produces fewer reactive intermediates (free radicals) than it consumes; side rxn that consumes free-radical intermediates w/o producing any o Possible termination rxns for chlorination of methane: o Termination steps include combination of any 2 free radicals, rxn of radicals w/ wall of vessel/other contaminants  breaks chain that’s needed for rxn to continue even if some still produce desired product  amt product obtained from termination step relatively < contribution of propagation steps o When chain rxn in progress, concentration of radicals ↓  radicals less likely to combine w/ other radicals than w/ reactants to give propagation step  termination steps more important @ end b/c fewer molecules of reactant  radicals more likely to combine w/ other radicals  chain rxn quickly stops 4.4) Equilibrium Constants and Free Energy • Energetics of individual steps of chlorination of methane • Thermodynamics: deals w/ energy changes accompanying chem/physical transformations o energy changes describe system properties @ equilibrium o energy/entropy variables can describe equilibrium • Equilibrium constant: for o K teeqs whether products/reactants more stable  energetically favored o K > eq rxn favored (as written left  right) o K < eq reverse rxn favored (written right  left) • Chlorination of methane has very high equilibrium constant  very close to 0 reactants @ equilibrium  “goes to completion” • Free energy (Gibbs Free energy): represented by G o ΔG = (free energy of products) – (free energy of reactants) o If product energy level < reactant energy level (downhill rxn), rxn’s energetically favored, negativeΔG value • Standard Gibbs free energy change (ΔG ): degree symbol means reactants and products = o pure substances-ΔGo/RTir most stable states @ 25 C and 1 atm o K = eq o  ΔG = -RTlnK = -2.eq3RTlog K 10 eq o R = 8.314 J/Kmol or 1.987 cal/Kmol o T = absolute temperature in K, 298 K o e = 2.718 o o RT @ 25 C = 2.48 kJ/mol or 0.592 kcal/mol • If - ΔG value (energy released), rxn favored (K > 1eq If +ΔG value (energy added), rxn = unfavorable (K 99%) 4.5) Enthalpy and Entropy o o o • ΔG =ΔH - TΔS o ΔG = (free energy of products) – (free energy of reactants) o o ΔH = (enthalpy of products) – (enthalpy of reactants) o ΔS = (entropy of products) – (entropy of reactants) • at low temperatures, enthalpy is larger than entropy (contributes more than entropy) • 4.5A) Enthalpy o Change in enthalpy (ΔH ) = heat of rxn: amt heat evolved/consumed in rxn; units = kJ/mol or kcal/mol  = measure of relative product & reactant bond strength  rxns favor low enthalpy (strong bonds) products o Exothermic: weaker bonds broken, stronger ones formed, heat = evolved  ΔH is negative o  enthalpy term makes favorable negative contribution toΔG  endothermic = opposite o chlorination of methane = highly exothermic, enthalpy decrease drives rxn • 4.5B) Entropy o Entropy: randomness, disorder, freedom of motion  rxns tend to favor products w/ greater entropy  positive entropy = more freedom of motion  favorable (negative) o contribution to ΔG ; o Many cases, enthalpy change > entropy change, so affects ΔG more, drives rxn  so even negative entropy change doesn’t make rxn unfavorable  sometimes entropy change = so small, almost 0 4.6) Bond-Dissociation Enthalpies • Measuring heat of rxn: put known amts of methane and chlorine into bomb calorimeter, use hot wire to start rxn  temperature rises (exothermic, negative heat change  calculate  105 kJ/mol or 25 kcal/mol • Can add/subtract energies in forming/breaking bonds to approximate heat of rxn w/o actually measuring • Bond-dissociation energy/enthalpy (BDE): amt enthalpy required to break particular bond homolytically (each bonded atom retains one of bond’s 2 electrons; heterolytically: one atom retains both electrons) • Homolytic cleavage (radical cleavage)  free radicals; heterolytic cleavage (ionic cleavage)  ions o heterolytic cleavage enthalpies depend strongly on solvent ability to solvate resulting ions o homolytic cleavate enthalpy doesn’t vary w/ solvent/lack thereof  used to define BDEs • Bonds formed  energy released, bonds broken  energy consumed  BDEs always = positive o o ΔH = ∑(BDE of bonds broken) - ∑(BDE of bonds formed) 4.7) Enthalpy Changes in Chlorination • Can use table to find enthalpy of chlorination of methane • Alternative mechanism: Cl radical can react w/ methane to chlorinate it and produce H radical (as opposed to forming HCl and methyl radical)…but that’s endothermic, so initial mechanism is the better, lower energy option 4.8) Kinetics and the Rate Equation • Kinetics: study of rxn rates (how fast rxn goes =ly important as position of its equilibrium, just b/c – value of ΔG , not necessary that rxn actually occurs) • Rate of reaction: measure of how fast products appear & reactants disappear; measured by increase of [product]/decrease of [reactant] over time • Rate equation (rate law): relationship btwn [reactants] and observed rxn rates o [reactants] ↑  more often reactants collide  greater chance of rxn o each rxn has own rate, determined experimentally o For rxnA + B → C + D, rate = k[A] [r] a b o k =rrate constant, a, and b determined experimentally o rate eqn depends on rxn mechanism and rates of individual steps • Powers a and b = order of rxn w/ respect to respective reactant (can be 0, 1, or
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