Organic(Chemistry(1(–(Exam(#3((Chapter(6:(Addition(Reactions(of(Alkenes(6.1(–(Hydrogenation(of(Alkenes(• Hydrogenation:(the(addition(of(H2(• Hydrogenation(of(ethane(o (• ΔH°(is(the(heat(of(hydrogenation(and(for(all(alkenes,(hydrogenation(is(exothermic(or(−ΔH°(• MetalGcatalyzed(addition(of(hydrogen(is(normally(rapid(at(room(temperature,(and(the(alkane(is(produced(in(high(yield,(usually(as(the(only(product(6.2(–(Heats(of(Hydrogenation(• The(pattern(for(alkene(stability(determined(from(heats(of(hydrogenation(parallels(exactly(the(pattern(deduced(from(heats(of(combustion(• Hydrogenation(of(Alkenes((Step(Process)(o Step(1(–(Hydrogen(molecule(react(with(metal(atoms(at(the(catalyst(surface.(The(relatively(strong(hydrogenGhydrogen(σ(bond(is(broken(and(replaced(by(two(weak(metalGhydrogen(bonds(o Step(2(–(The(alkene(reacts(with(the(metal(catalyst.(The(π(component(of(the(double(bond(between(the(two(carbons(is(replaced(by(two(relatively(weak(carbonGmetal(σ(bonds(o Step(3(–(A(hydrogen(atom(is(transferred(from(the(catalyst(surface(to(one(of(the(carbons(of(the(double(bond(o Step(4(–(The(second(hydrogen(atom(is(transferred,(forming(an(alkane.(The(sites(on(the(catalyst(surface(at(which(the(reaction(occurred(are(free(to(accept(additional(hydrogen(and(alkene(molecules(6.3(–(Stereochemistry(of(Alkene(Hydrogenation(• Although(two(hydrogens(are(not(transferred(simultaneously,(they(both(add(to(the(same(face(of(the(double(bond(• Syn(addition:(when(two(atoms(or(groups(add(to(the(same%face(of(a(double(bond(• Anti(addition:(when(atoms(or(groups(add(to(opposite%faces(of(the(double(bond(o (• Stereoselectivity(o Alkene(hydrogenation(can(give(us(several(products(from(one(reactant(o Reactions(such(as(a(catalytic(hydrogenation(that(takes(pace(at(the(“less(hindered”(side(of(a(reactant(are(examples(of(steric(effects(on(reactivity(6.4(–(Electrophilic(Addition(of(Hydrogen(Halides(to(Alkenes(• (• Increasing(reactivity(of(hydrogen(halides(in(addition(to(alkenes(o HF(<<(HCl(<(HBr(<(HI(• Electrophilic(Addition(of(Hydrogen(Halide(to(an(Alkene((Step(Process)(o Step(1(–(Protonation(of(the(carbonGcarbon(double(bond(by(the(hydrogen(halide(§ (o Step(2(–(CarbocationGanion(combination(§ (• Electrophilic(addition:(triggered(by(the(acid(acting(as(an(electrophile(toward(the(π(electrons(of(the(double(bond(• The(characteristic(chemical(property(of(a(C=C(structural(unit(is(susceptibility(to(attack(by(electrophiles(6.5(–(Regioselectivity(of(Hydrogen(Halide(Addition:(Markovnikov’s(Rule(• Markovnikov’s(rule(o When(an(unsymmetrically(substituted(alkene(reacts(with(a(hydrogen(halide,(the(hydrogen(adds(to(the(carbon(that(has(the(greater(number(of(hydrogens,(and(the(halogen(adds(to(the(carbon(having(fewer(hydrogens(o Hydrogens(want(more(hydrogens(o Halogens(want(less(hydrogens(6.6(–(Mechanistic(Basis(for(Markovnikov’s(Rule(• According(to(Hammond’s(postulate,(the(transition(state(for(protonation(of(the(double(bond(has(much(of(the(character(of(a(carbocation,(and(the(activation(energy(for(formation(of(the(more(stable(carbocation((secondary)(is(less(than(that(for(formation(of(the(less(stable(one((primary)(6.7(–(Carbocation(Rearrangement(in(Hydrogen(Halide(Addition(to(Alkenes(6.8(–(Addition(of(Sulfuric(Acid(to(Alkenes(• Acids(other(than(hydrogen(halides(also(add(to(the(carbonGcarbon(bond(of(alkenes((in(the(same(way)(• Markovnikov’s(rule(is(obeyed(because(the(mechanism(of(sulfuric(acid(addition(to(alkenes(is(analogous(to(that(described(earlier(for(the(electrophilic(addition(of(hydrogen(halides(• Hydration(of(alkenes(by(this(method(is(limited(to(monosubstituted(alkenes(and(disubstituted(alkenes(of(the(type(RCH=CHR(6.9(–(AcidGCatalyzed(Hydration(of(Alkenes(• Acid(cataylist(o (• AcidGCatalyzed(Hydration((Step(Process)(o Step(1(–(Protonation(of(the(carbonGcarbon(double(bond(in(the(direction(that(leads(to(more(stable(carbocation(o Step(2(–(Water(acts(as(a(nucleophile(to(capture(tertGbutyl(cation(o Step(3(–(Deprotonation(of(tertGbutyloxonium(ion.(Water(acts(as(a(Brønsted(base(• Microscopic(reversibility:(in(any(equilibrium,(the(sequence(of(intermediates(and(transition(states(encountered(as(reactants(proceed(to(products(in(one(direction(must(also(be(encountered,(and(in(precisely(the(reverse(oder(,(in(the(opposite(direction(6.10(–(Thermodynamics(of(AdditionGElimination(Equilibria(• Le(Châtlier’s(principle:((o A(system(at(equilibrium(adjusts(so(as(to(minimize(any(stress(applied(to(it(o Helps(us(predict(qualitatively(how(an(equilibrium(will(respond(to(changes(in(experimental(conditions(• At(a(constant(temperature(and(pressure,(the(direction(in(which(a(reaction(proceeds—that(is,(the(direction(in(which(it(is(spontaneous—is(the(one(that(leads(to(a(decrease(in(free(energy((G)(o ΔG(=(Gproducts(–(Greactants;(spontaneous(when(ΔG(<(0(o G(is(always(positive;(ΔG(can(be(positive(or(negative(o ΔG(=(ΔG°(+(𝑅𝑇𝑙𝑛!!!!!!!!;(R(=(8.314(!!"#𝐾(o When(ΔG°(is(negative,(the(reaction(is(exergonic.(When(ΔG°(is(positive,(the(reaction(is(endergonic(o 𝛥𝐺° = 𝛥𝐻 − 𝑇𝛥𝑆(o 𝛥𝐺° = −𝑅𝑇𝑙𝑛𝐾(6.11(–(HydroborationGOxidation(of(Alkenes(• AcidGcatalyzed(hydration(converts(alkenes(to(alcohols(according(to(Markovnikov’s(rule(• HydroborationGoxidation:(the(synthetic(method(used(to(accomplish(conversions(from(alkenes(to(alcohols(• Hydrogenboration(o A(reaction(in(which(a(boron(hydride(adds(to(a(carbonGcarbon(π(bond(o (6.12(–(Stereochemistry(of(HydroborationGOxidation(6.13(–(Mechanism(of(HydroborationGOxidation(• Hydroboration((Step(Process)(o Step(1(–(A(molecule(of(borane((BH3)(attacks(the(alkene.(Electrons(flow(from(the(π(orbital(of(the(alkene(to(the(2p(orbital(of(boron.(A(π(complex(is(formed(o Step(2(–(The(π(complex(rearranges(the(organoborane.(Hydrogen(migrates(from(boron(to(carbon,(carrying(with(it(the(two(electrons(in(its(bond(to(boron(• Retention(of(configuration:(migration(of(the(alkyl(group(from(boron(to(oxygen(• Oxidation(of(an(Organoborane((Step(Process)(o Step(1(–(Hydrogen(peroxide(is(converted(to(its(anion(in(b asic(solution(o Step(2(–(Anion(of(hydrogen(peroxide(acts(as(a(nucleophile,(attacking(boron(and(forming(an(oxygenGboron(bond(o