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CHEM 2332H
Christopher J.Douglas

Chapter 15 Book Notes Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy 15.1: Introduction • Conjugated: double bonds that can interact w/ each other if separated by 1 single bond o ex: penta-1,3-diene o more stable • Isolated double bonds: 2+ single bonds separating them little interaction o ex: penta-1,4-diene o less stable 15.2: Stabilities of Dienes • Heats of hydrogenation used to compare relative stabilities of alkenes • Isolated double bonds: heat of hydrogenation =~ sum of heats of hydrogenation for individual double bonds o conjugated dienes: heat < sum of individual double bonds more stable • Cumulated double bonds: double bonds are right next to each other, no single bond in between o ex: penta-1,2-diene (also called allenes, after simplest member of the class H 2=C=CH ); 2as large heat of hydrogenation, larger than any other pentadiene o less stable than isolated double bonds • Overall stability: conjugated diene > isolated diene > internal alkyne > terminal alkyne > cumulated diene 15.3: Molecular Orbital Picture of a Conjugated System • Resonance energy (aka conjugation energy, delocalization energy, stabilization energy): energy amt of extra stability (ex: that given by conjugated vs isolated double bonds) o explained by molecular orbitals • 15.3A) Structure and Bonding of Buta-1,3-diene o heat of hydrogenation of buta-1,3-diene = about 17 kJ/mol less than 2*but-1-ene o buta-1,3-diene’s resonance energy = 17 kJ/mol o has planar conformation, p orbitals on 2 pi bonds = aligned o Single bond here = shorter than normal alkane single bond b/c of more s character b/c of sp orbitals and mostly b/c of pi bonding overlap and partial double- bonding character o double bonding overlap possible in planar conformation o overlapping delocalized electrons over whole molecule some pi overlap and some pi bonding btwn single and double bond length o Lewis structures can’t depict this need molecular orbitals that represent entire conjugated pi system, not just 1 bond at a time • 15.3B) Constructing t2e Molecular Orbitals of Buta-1,3-diene o All 4 C’s = sp hybridized, have overlapping p orbitals in planar conformation  each p orbital has 2 lobes w/ opposite phases o Bonding molecular orbital: overlapping lobes in phase (same “sign”)  aka constructive overlap o Antibonding molecular orbital: opposite phase lobes overlap in bonding region  destructive overlap wave functions cancel in bonding region  Node btwn nuclei: region of 0 electron density where both phases exactly cancel o Electrons have lower energy in bonding MO than original p orbitals and higher energy in antibonding MO  ground state of ethylene: 2 electrons in bonding MO, none in antibonding MO  stable molecules tend to have filled bonding MOs and empty antibonding MOs o Constructive overlap bonding interaction; destructive overlap antibonding interaction  # MOs = # atomic orbitals used to form MOs; energy = distributed above and below starting p orbital energy  Half = bonding MOs, half = antibonding o Molecular orbitals of buta-1,3-diene  C1-C4’s p orbitals overlap extended system of 4 p orbitals 4 pi molecular orbitals (2 bonding, 2 antibonding)  can draw like this even though not really linear o Lowest energy MO always consists entirely of bonding interactions  draw all + phases overlapping constructively on 1 face of molecule; - phases overlapping constructively on other face  In this case, MO places electron density on all 4 p orbitals, slightly more on C2 and C3 (shown by size) o Lowest energy MO, stable b/c  1) 3 bonding interactions, electrons delocalized over 4 nuclei  2) some pi bond character btwn C2 and C3 lowers energy of planar conformation short C2-C3 bond o 2 MO of butadiene: 1 vertical node in center of molecule  Bonding interactions btwn C1-C2 and C3-C4 bonds  weaker antibonding interaction btwn C2 and C3  2 bonding and 1 antibonding interaction  net result = bonding orbital (2 – 1 = 1…not exactly, but good enough, good to compare to others)  but less strong/more energy than all bonding previous one o 3) 2 nodes: bonding btwn C2 and C3; antibonding btwn C1-C2 and C3-C4; antibonding = vacant in ground state o 4) 3 nodes, all antibonding (antibonding btwn all pairs of adjacent atoms); highest energy; unoccupied in ground state o Butadiene has 4 pi electrons (stom double bonds), each MO can accommodate 2 electrons lowest energy (1 2) MO’s filled first  filled bonding orbitals and empty antibonding orbitals => most stable  conjugated butadiene system slightly more stable than 2 ethylene double bonds o There actually = 2 planar conformations that allow overlap btwn C2 and C3  b/c of rotation about C2-C3 bond  considered single-bonded analogues of trans/cis isomers  called s-trans/s-cis conformations (s for single) o S-trans more stable than s-cis  less energy needed to rotate single bond than double bond  conformations easily interconvert @ room T 15.4:Allylic Cations • Conjugated compounds react, some intermediates retain some resonance stabilization o Common intermediates: allylic systems, esp allylic cations/radicals o Allylic cations and allylic radicals stabilized by delocalization • Allyl group: -CH 2CH=CH gro2p o when allyl bromide heated w/ good ionizing solvent, ionizes to allyl cation (allyl group w/ +charge) o Allylic cations: more substituted analogue of allyl cation; stabilized by resonance w/ adjacent double bond delocalizes +charge over 2 C’s • Can show delocalization by either resonance structures or consensus structure (concise, but doesn’t always get all details in) • Resonance stabilization  (primary) allyl cation = about as stable as simple secondary carbocation o Most substituted allylic cations have at least 1 secondary C bearing partial +charge about as stable as simple tertiary carbocations 15.5: 1,2- and 1,4-Addition to Conjugated Dienes • Electrophillic additions to conjugated dienes usually involve allylic cations as intermediates o allylic cation can react w/ nucleophile @ either of its + centers unlike simple carbocations o ex: HBr addition to buta-1,3-diene: mixture of 2 constitutional isomers st • 1 product: 3-bromobut-1-ene results from Markovnikov addition across one of double bonds o called 1,2 addition: electrophilic addition of HBr across double bond o doesn’t matter if they are actually carbons 1 and 2 nd • 2 product: 1-bromobut-2-ene: double bond shifts to C2-C3 position o called 1,4-addition o proton and bromide ion add @ ends of conjugated system to C’s w/ 1,4 relationship • Similar mechanism to other electrophilic additions to alkenes o proton = electrophile, adds to alkene most stable carbocation (for buta-1,3-diene, gives allylic cation which = stabilized by resonance delocalization of +charge over 2 C’s) o Bromide can attack intermediate @ either of the 2 C’s sharing +charge o attacking secondary C 1,2-addition o attacking primary C 1,4-addition • Mechanism: 1,2- and 1,4-Addition to a Conjugated Diene o Step 1: Protonation of one of the double bonds resonance-stabilized allylic cation o Step 2: nucleophile attacks @ either electrophilic C • Main point: presence of double bond in position to form stabilized allylic cation b/c they = likely to react via resonance-stabilized intermediates 15.6: Kinetic vs Thermodynamic Control inAddition of HBr to Buta-1,3-diene • The rxn = T dependent o @ very low T, 1,2-addition favored o @ high T (warm it up or do it again), 1,4-addition favored • most stable product isn’t always the major product o we’d expect 1-bromobut-2-ene (1,4-product) to be more stable b/c most substituted double bond o proven by the fact that if warm up mixture, and allow to equilibrate, this one’s favored • If look at energy diagram/graph, 1,4 has low energy product but high activation energy (that’s why works at higher T); 1,2 has higher product energy but lower activation energy (works @ low T) o • Kinetic Control @ -80 C o 1,2 has lower activation/lower energy transition state b/c Br attacks more substituted secondary C, which has more +charge b/c better stabilized than primary C o takes place faster (at all T) o Br attacking allylic cation = strongly exothermic, so reverse rxn has high activation energy; at low T, not as many collisions have this much energy, so rate of reverse rxn = almost 0 o product that’s formed faster = favored here o this situation called kinetic control, product favored here = kinetic product • Thermodynamic control at 40 C o o here, lot of collisions have enough energy for reverse rxn to happen o 1,2 has less energy in its reverse than 1,4 reverse so even if it’s formed faster, reverts back faster too than 1,4 product o eqbm set up, relative energy of each species determines its concentration o 1,4 = most stable so it predominates o called thermodynamic control or eqbm control and creates thermodynamic product • Rxns that don’t reverse easily kinetically controlled b/c no eqbm established o product w/ lowest-energy transition state predominates • Rxns that = easily reversed = thermodynamically controlled unless something happens to prevent eqbm from being attained o lowest energy product predominates 15.7:Allylic Radicals • Allylic radicals also stabilized by resonance delocalization o substitution @ allylic position get resonance-stabilized allylic radical as intermediate • Mechanism: Free-RadicalAllylic Bromination o Initiation: Formation of radicals (Br 2plits to Br by light) o Propagation:  1) Br radical abstracts allylic H allylic radical  2) Allylic radical reacts w/ Br2molecule allyl bromide and new Br atom continues chain o Regeneration of Br : 2BS reacts w/ HBr Br used2in allylic bromination step • Stability ofAllylic Radicals o Abstraction of allylic H preferred b/c allylic free radical = resonance-stabilized o In fact, allyl radical is even more stable than tertiary butyl radical • Allylic cyclohex-2-enyl radical has unpaired electron delocalized over 2 secondary C’s even more stable than unsubstituted allyl radical nd o 2 propagation step can happen @ either radical C’s o If symmetrical, you get same product o If unsymmetrical, get product mixture b/c of allylic shift (double bond can appear in either position) (similar to 1,4-addition) • Bromination Using NBS o Higher concentrations: bromine adds to double bond (via bromonium ion) saturated dibromides o Allylic bromination: Br substitutes for H o If want substitution, need low bromine concentration and light/free radicals to initiate rxn o Free radicals = highly reactive, even small concentration can produce fast chain rxn o Might be convenient to use NBS or else if just use bromine, you risk having too high concentration  keeps low Br c2ncentration b/c reacts w/ HBr liberated in substitution Br 2and removes HBr byproduct, prevents from adding across double bond by its own free radical rxn) o Step 1: Free-radical allylic substitution (see mechanism) o Step 2: NBS converts HBr byproduct back into Br ; 2 o NBS rxn  allylic compound dissolved in CCl an4 1 equivalent NBS added  NBS denser than CCl and4not soluble so sinks to bottom  rxn initiated using sunlamp/radical initiator (peroxide)  NBS appears to rise to top of CCl l4yer (actually converted to succinimide, which = less dense than CCl ) 4  when all solid succinimide rises to top, turn off sunlamp, filter sol’n to remove succinimide  evaporate CCl t4 recover product 15.8: Molecular Orbitals of theAllylic System • 2 resonance forms of allylic radical suggest there’s half pi bonds btwn C1-C2 and C2-C3 • p orbitals of all 3 C’s must be parallel to have simultaneous (b/c a molecule = all it’s resonance forms at same time) pi bonding overlap btwn C1-C2 and C2-C3 o Allyl cation, allyl radical, and allyl anion have same geometric structure, only difference = # pi electrons • Just like w/ buta-1,3-diene forms 4 MOs, allyl system forms 3 MOs, each having different combo of bonding/antibonding MOs different energies • odd # MOs so can’t symmetrically divide into bonding/antibonding o nonbonding molecular orbital: neither bonding nor antibonding; electrons here have same energy as isolated p orbital • 0 electron density on center p orbital (C2) b/c 2 MO must have 1 node and that’s where it can be symmetrically suited o overall, it = nonbonding b/c everything cancels out 15.9: Electronic Configurations of theAllyl Radical, Cation, andAnion • Allyl radical has 3 pi electrons  2 electrons go in 1 (lowest energy) MO, and last one goes into 2nd o this is justified by resonance picture • Allyl cation doesn’t have unpaired electron like allyl radical does o we’ve removed half electron from C1 and C3 but not changed C2 nd • Allyl anion has additional electron in 2 lowest energy MO o nonbonding orbital’s electron density divided btwn C1 and C3 15.10: S N Displacement Reactions ofAllylic Halides and Tosylates • Allylic halides and tosylates = more reactivetoward S 2 Nxns; good for them • Transition state: trigonal C w/ p orbitals perpendicular o nucleophile attacks in one of the p lobe, and leaving group leaves from the opposite p lobe o allylic: resonance stabilization through p orbital (of pi bond) conjugation lower transition energy state lower activation energy faster • Allylic halides = so reactive that they couple w/ Grignard and organolithium reagents (doesn’t work well w/ unactivated halides) 15.11 The Diels-Alder Reaction • Diels-Alder reaction: alkenes and alkynes w/ electron-withdrawing groups add to conjugated dienes six-membered rings o o can control stereoch
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