Organic Reaction Mechanisms
Organic Reaction Mechanisms CHEM 542
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CHAPTER 5 STUDY AND DESCRIPTION OF ORGANIC REACTION MECHANISMS THERMODYNAMIC ASPECTS For reaction to proceed spontaneously the free energy of the products must be lower than free energy of the reactants AG must be negative Free energy is made up oftwo components enthalpy H and entropy S AGAHTAS AH difference in bond energies between the substrate and product including resonance increments strain differences in solvation AH is calculated as a sum of bond energies of all bonds broken subtracting from this the total of the bond energies of all bonds formed and adding increments corresponding to changes in resonance strain and solvation AS entropy differences are related to disorder and randomness of the system less ordered systems the greater entropy Usually less important compared to AH Preferred conditions low enthalpy amp high entropy Several aspects 1 Reaction in gasses entropy factor is more important in gaseous phase is higher degree of freedom and randomness ln liquids the entropy factor is less important and in solid state even less important 2 Stoichiometry ofthe reaction matters A B 9 C D reactions low entropic factors a Increasing number of molecules formed in a reaction A 9 B C corresponds to large gain in entropy Such reactions are therefore thermodynamically favored b Decreasing number of molecules formed during the reaction A B 9 C corresponds to loss in entropy which needs to be compensated by enthalpy factor 3 Despite the fact that cleavage reactions A 9 B C are entropically favored many possible cleavages are dif cult to perform because of the large increase in the enthalpy Remember the pyrolytic reactions I Example cleavage of ethane to two methyl radicals The 00 bond ca 80 kcalmol is broken and no new bond is formed to compensate for the enthalpy increase With increase in temperature the entropic factor T A8 takes over the enthalpy is independent on temperature 4 Acyclic molecules have more entropy than corresponding cyclic molecules because of the decreasing number of conformations Ring opening reactions proceed with gain in entropy ring closing reactions with loss of entropy CHAPTER 5 page 1 KINETIC ASPECTS The negative AG is a necessary but not suf cient condition for a reaction to proceed spontaneously Free energy of activation AG is another important factor transition state AG T G AG1 AGE Reversible reactions G1 G energy of TS1 is the same for fonNard and reverse rxns Transition state i possesses de nite geometry and charge distribution but no nite concentration accumulates since it is at the top ofan energy barrier system simply passes through the transition state The free energy is related to the equilibrium constants by the following equation AG R T InKi Therefore higher value of AG1 is associated with smaller rate constant The rates ofalmost all reactions increase with temperature Arrhenius equation Comes from i AnBF A 1 A B AB c A B The rate of decomposition of the transition state k T V AnBfF K transition coeficient k Boltzman s constant h Plank s constant The rate of reaction is de ned K k T t Rate of Reaction AB i i h and AB K A B CHAPTER 5 page 2 Then K e AG RT 1 Rate of Reaction e AG RT A B krA B KkT eAGiRT k r h This is why the higher value of AG1 is associated with smaller rate constant The temperature dependence ofthe reaction rates allows for determination of enthalpy and entropy components ofthe free energy of activation as follows kT 1 As R kr Kh eAHRTgtlte gtlgt Mi and A31 The magnitudes ofAHi and AS1 re ect the structure ofthe transition state Example dimerization of cyclopentadiene in the gas phase low AHi suggests that the reaction proceeds through concerted breakingformation of bonds Q i g Similarly intramolecular reactions would display negative entropy of activation as a result of formation of cyclic transition state which is characterized by a restricted number of conformations and limited degree of rotational freedom C 3 CD a CH The Cope Rearrangement is an example ofthis type ofreaction CHAPTER 5 page 3 KINETIC versus THERMODYNAMIC CONTROL Consider reaction in which one compound A may undergo two competing reactions depending on the reaction conditions BAB c c B The reaction coordinate shows that product B is thermodynamically more stable than productC If neither reaction A90 nor A98 is reversible product C is formed faster and will be the predominant product mixture We say the product is kinetically controlled In the case where the reactions are reversible the product mixture will have different composition Thus two scenarios exist a lfthe reaction is stopped before the equilibrium is reached C is predominant product This type of reaction is said to kinetically controlled b lfthe reaction is allowed to approach equilibrium the predominant product is the thermodynamically more stable B This type of reaction is said to be thermodynamically controlled HAMMOND POSTULATE HP Since transition states have short zero lifetimes they cannot be directly observed and their geometry is obtained from inference HP For any single reaction step the geometry of the transition state for that step resembles the side of the reaction coordinate to which it is closer in free energy This hypothesis leads to two possible scenarios 1 For exothermic reactions the transition state resembles the reactants more than products 2 For endothermic reactions the transition state resembles the products more than the reactants Energy Exmhmnic Rmnjnn Reaction Coordinate CHAPTER 5 page 4 Example in the following case the transition state 1 will resemble the intermediate B In addition the transition state for the second step transition state 2 also will resemble the intermediate B more closely than the product C CURTlNHAMMETT PRINCIPLE The reaction rate ofa molecule that can exist in several different conformations in its ground state is de ned as k 2 xn kn n k overall observed reaction rate constant Xn molar ratio ofthe nth conformer kn speci c reaction rate ofthe nth conformer 2 AG AG AGE Gbi G Energy Product from A Product from Equilibrium Coordinate The product ratio product ofAproduct of B e39AGa39AGb GmRT Thus the least favorable conformation can give rise to the major product ifthat conformation has a more favorable transition state to its product than does the more favorable conformation to its product The difference in energies between the two conformations their populations does not matter CHAPTER 5 page 5 Example basecatalyzed transelimination of 1 2 diphenyl1 ethyl 135triethylbenzoate This reaction is known to give exclusively transstilbene Et Et Et 0 0 024343 t Buoe A t Buoe B y t BuOK y H H Ph H H Ph HAPh OCOR PhH OCOR H H Ph H It may seem that the prevalence of transstilbene is due to the preference of the conformation B In fact the composition of the stilbene isomers in the reaction mixture depends solely on the free energy ofthe activation ofthe respective transition states A and B CURTINHAMMETT PRINCIPLE transition state ButOA 4 AAG ButOB 1 AG A AG conformation A B According to Hammond s postulate the difference in the energies of transitions states might be inferred from the different energies of the products 2337 kJmol but not the starting materials In the cisisomer there are two bulky phenyl isomers at the same side of the 00 Likewise in the transition state the phenyl substituents are already shifted toward the spatial positions they would eventually adopt This is a destabilizing factor that renders such a transition state of higher energy compared to the one of the transition state corresponding to the transproduct PRINCIPLE OF MICROSCOPIC REVERSIBILITY During the course of reaction nuclei and electrons assume certain con gurations that correspond to the lowest possible energy lfthe reaction is reversible these positions must be the same for the fonNard and the reverse process as well lfthe reaction A 9 C goes through intermediate B then the reverse reaction must have the same mechanism and go through the intermediate B too CHAPTER 5 page 6 SUBSTITUENT EFFECTS HAM METT EQUATION Substituents have an effect on the compound reactivity On the qualitative level we know how to implement the inductive eld and resonance effects When we need to quantify these effects we need to use quantitative methods equations Hammett equation log k l k0 o p or log k log k0 o p k0 the rate constant or equilibrium constant recorded for the compound bearing a substituent H the standard rate constant k the constant for a compound bearing a substituent X p the reaction constant arbitrarily set as 1 for ionization of substituted benzoic acids in water at 25 C o the substituent constant describes the effect of the substituent on free energy of activation 0 sums up the total electronic effects inductive eld resonance Works ne for para and metasubstituents Often fails for on hosubstituents since they conformationally distorted due to crowding from the adjacent group The Hammett equation is a freeenergy relationship We will demonstrate it for equilibrium constants it is also valid for rate constants as well Standard reaction Studied reaction AGO RT an0 AG RT an Since the Hammett equation can be written as log K log K0 o p and AGO AG 039 p 23 RT 23RT For any given reaction p R T and AGO are constants That means that 0 depends linearly on AG CHAPTER 5 page 7 Both constants c p are tabulated in Tables 45 and 46 39lnhle 45 Subslilurnt Constants grule 1quot I7 1739 n all HJ UNII 0 llquot U LI 3 CHJCU o 39 01 n 114 HJCO 0 36 047 I Q 0 30 I U H 13 09 030 13 1112 en 40 ll quot37 quot16 015 044 O l6 111 4109 15 4121 nrhnmuliimy lJ 1733 44 OH 0 20 0 In 1 1 a l 1 035 I 44 Cnlur C 03 2 fl ll HAD 70 15 Cynmx 39N DJ 07quot 139 156 1 HS Ezlmw 21150 01 4111 4132 F 39l C1145 408 Ol3 010 nru F Lll HS 70 l 0 5t 70 31 lbdrugen 0 D U 1 0 1391 Hy lrnxy 013 71113 39 Mclhdncanll ouyl 06 L c 060 LII 1010 110 l39ll 127 4L4 Methyl 0 ob u1x 004 n 12 1lu 07l SI 0 65 139 1 rim1 0115 00 Irl uurumt hyl 46 I153 LI 4 fl l39l l39rumI lnrrmunio r a W I Hy Tnmrthylsrljl 111131151 4104 0117 1 Aluc 01311111 v39 mu fmnn r nwm dflw 1m 0 x B memmazd 1 511mm 1 1391 quot 1quot 1 I v 11 II R W Tuh I 1 4 1 a 1mm x 0 L z 1 1 911111911111131 rehablc 1 1m mum r Drum RTB my Mm mm mm yum 1 o and up shown 1n marshc mar m rtgnrrl Table 46 Reaction Constants Ramon 1 ArE39OZH Arfl ll 11le ArkquotyH ArlZ39O3 H 1310 ArCll 0H 11Cll3C03 H water RUCHICHECO ArCl13Cl3C03 4 H wa1cr AIOH A10 1139 water kNllf 113111 I H water ACllll39 ArCHNH1 ll watcr M 11E1 011 A103 121011 r39lg 0E1 1 TM a NCEICOI L1011 r39ll l a 111 NCon HC39i Ant39erl l 1111 39 Alk39lM JUH 1111 A1NH PhCOFl 7 Aerlt X l39 llCl wrung11 Aardcrm Pmr 1 lmm l V1 Mr nz39 War I rm 1 1 1 The constants in Table 45 are am and up or 0 and o 39 Y and denote para and meta positions and are constants modified to reflect the resonance participation in charged transition states Substituents may participate directly in resonance structures The a 39 quantifies how much the particular substituent takes part in the resonance structures that directly involve an electronrich center CHAPTER 5 page 8 Example Ionization of phenols involves resonancestabilized phenoxide anions formed in the transition states resonance stabilizes TS Here the use Ofc 39 is more appropriate 9 XQOH NaOH gtX4 70 H20 The o describes how much the particular substituent takes part in resonance with an electronde cient reaction center Example 8N1 reaction of cumyl halides involves cations and consequently a positively charged transition state Here the use of c is more appropriate 43quot Nu X CI gt XO CB gt X Nu Because ofthese resonance effects Hammett equation requires corrections YukawaTsuno equation log klk po pr c7 o The parameter r is adjusted from reaction to reaction describes the degree of resonance participation of the substituent Large r corresponds to a reaction with strong resonance effect In contrast when there is no resonance effect r goes to zero YTE equation becomes HE The YTE equation expanded for both the 7 and o 39 is called LArSR equation log klk po rAch r39AoR39 The GE and oR39 are the substituent constants corrected for the resonance interaction with a positively and negatively charged electrondemandingelectronreleasing site The variables r and r39 are the measures ofthe extra resonance contribution There are more linear free energy equations each describes a different aspect solvent eld effect etc The Taft equation is used for aliphatic systems and separates steric from electronic effects METHODS OF DETERMINING MECHANISMS IDENTIFICATION OF PRODUCTS and products of side reactions The product composition is often the most important key to the reaction mechanism It is very important that the suggested mechanism also explains the formation of side products HW read the history of the determination of the mechanism of the von Richter rearrangement CHAPTER 5 page 9 KINETIC STUDIES The study of the reaction order The reaction rate does not always correspond to the concentration of all reaction components To predict any viable mechanism we must know the rateconcentration dependence for all potential reagents even those that do not appear in the stoichiometric equation Example 1 In a kinetic study carried out at wide range of concentrations it was determined that the transmethylation of compound A is a second order reaction to the reactant A vk2A2 This means that the reaction proceeds via intermolecular as opposed to intramolecular mechanism 0 0 SOe O o S0 s SO O Bimolecular I 06 A I O T S0 S O 2methylsulf0nyl1methyl E sulfomumsulfonute thiobenzene reactant g roduct O U 39 S0 nlmo ECU ar e O 9 Example 2 Nitration of benzene does not depend on the benzene concentration That means that the attack ofthe nitration agent is not the ratelimiting step In fact the rate limiting step is the formation of the nitronium ion fast 2HNo3 H2NO3 NO339 slow H2NO3 Noz H20 faSt NO Ar Ar 2 NO2 fast ArNO NO339 Ar No2 HNO3 2 CHAPTER 5 page 10 Determination of the presence of an intermediate Intermediates are postulated in most mechanisms and we must often try to determine their presence and structure How can that be done 1 Isolation of an intermediate can sometimes be realized when the intermediate is especially stable The isolated intermediate must give the required product when subjected to the appropriate reaction conditions 2 Independent synthesis of the intermediate can sometimes be realized when the intermediate is especially stable It is necessary that this synthesis be through an alternative route to that of the reaction under examination otherwise the strategy is the same as 1 above Again the synthesized intermediate must yield the product of the reaction under examination 3 Detection of an intermediate spectroscopically shortlived intermediates by UVvisible IR and Raman longlived intermediates by NMR MS ESR CIDNP For example when we propose a radical mechanism we might be able to see radicals in the EPR or CIDNP spectra 4 Trapping of an intermediate is often more feasible when dealing with unstable intermediates If we propose the presence ofan intermediate that cannot be isolated 1 above but we know that the proposed intermediate if present would react with be trapped by certain other reagents to give very speci c products trapped intermediate we can employ this strategy ed regular regwar presum product product A gt gt C A D gt C E trap trapped B E intermediate trap trapped intermediate We run the reaction in the presence ofthe trapping agent and see if we nd the trapped intermediate in the product mixture Well known example proof of the benzyne intermediate by cyclopentadiene trapping However benzyne is a relative longlived intermediate and lives long enough to trap itself and form biphenylene Biphenylene I 00 Qle ob trapping agent CHAPTER 5 page 11 Another example of selftrapping is that of the xylylene intermediate during the photodecomposition of 1 4 dihydrophthalazine hv N E o 176 00 2 l 2 gt 2 N 39N2 If the intermediate is very shortlived or not intrinsically reactive with itself selftrapping will not be possible Under these circumstances high concentrations of reactive trapping agent must be employed Nquot Lb a zno atm j 7 o 0 Exchange experiments might be regarded as a special case oftrapping that is usually done with isotopic labeling The similar substrates substrates with similar reactivity or labeled and unlabeled substrates are mixed in the same reaction vessel and the product distribution is determined usually either by NMR or MS Example the Claisen rearrangement twice 13Clabeled starting compound yielded twice labeled product while a labelfree starting compound yielded a label free product Products with only a single label were not observed Therefore the Claisen rearrangement proceeds via intramolecular mechanism ll O O OH OH OH OH gt but not 0quot k Isotopic labeling experiments Experiments used to determine the fate of speci c carbon or hydrogen atoms isotopes of any type of atom can be used in this type of experiment during the course of a reaction The starting compound has an isotopic label tracer in or near the reaction center and the locationdistribution of that label determined in the products CHAPTER 5 page 12 Example The Claisen rearrangement which we already know proceeds via an intramolecular mechanism might proceed via a tight ion pair or a concerted mechanism the expected outcome of which is noted below Possible Mechanism Concerted 0M om OH O OH Observed at 5 Tight ion Pair Do o 6 OH 5 500 50 50 o gt Isotopic effects Exchange of one stable isotope for another eg H D T does not usually change the chemical reactivity of a compound What it may change is the reaction rate This is mostly observed for the HDT where the quantitative effects are largest due to the isotopes having the largest relative mass difference Primary isotopic effects This is the effect observed when a bond being made or broken in the transition state involves the isotope in question In general heavier isotopes have lower zero point vibrational energies Energy ofthe oth vibratinal state is E0H1500 cm391 and E0D1100 cm391 Thus more energy is required to break the 0D bond than the 0H bond Consequently the activation energy for the heavyatom reaction will be slightly higher than that forthe lightatom reaction transition state Bond dissociation in transition state Energy Reac on Coordinate CHAPTER 5 page 13 Examples The oxidation of lsopropyl alcohol by aqueous chromic acid will display a significant primary isotope effect 0 o H c H c 3 e 9 H30 ClrlOH BASE H 3 lrl OH SLOW Horoga H OH HCrO4 H gt H o H Hzo gt BASE 0 39 O e o BASEH H30 H30 0 H30 The bromination of acetone by bromine does not depend upon the bromine concentration 80 the ratedetermining step must occur before the attachment of the bromine Replacement of acetonehe by acetoned5 causes a signi cant change in reaction rate KcHkcD 70 This clearly indicates that the breaking ofthe acetone CH bond is involved in the ratedetermining step H H H 0 ol Slow o fast gt H H k Br Br 630 o 7 Inverse isotope effect SIE Isotope effects in which KcHkcD lt 10 and that arise from the hydrogen becoming more tightly bound in the transition state than in the starting material H H H I e I39 X CA CB gt C CB gt CA CB Transition State CHAPTER 5 page 14 Secondary isotope effects Effects associated with isotopes that are in the vicinity of the reaction but not directly involved in the bond breaking or making steps These may be separated further into x and SlE s depending on whether the isotope one or two heavy atoms removed from the reaction center Examples The case below is a secondary isotope effect If the methinyl hydrogen had been replaced by deuterium the secondary isotope effect would have been or In the case shown below kcHkcD 134 and is most likely due to the different degrees of hyperconjugative stabilization of the transition states H20 Br K cH e H CH CH CH H30 CH3 H30 CH3 H30 CH3 H20 IBr Km 9 CIJH CH CH CH 030 003 D3C coa D3C coa Isotope effects can be observed with heavier atoms such as 13C but these are much smaller than hydrogen isotope effects because the 12C13C mass difference is relatively small compared to that of 1H2H Thus the reaction shown below has k12ck13c 1053 CHz Br CH2OCH3 G gt k12ck13c 1053 CHAPTER 5 page 15 The study of the product stereochemistry This approach quite often provides significant insights into reaction mechanisms Example reduction of acetylenes by alkaline metals in liquid ammonia might proceed via a radical anion mechanism singlet electron addition to alkyne or a dianion mechanism double electron addition to the alkyne Since this reaction provides only the transole n it must proceed through the dianion mechanism This observation is inconsistent with a radical anion mechanism which would afford both ole n isomers since these radicals easily isomerizes High energy barrier between isomers Radical A R R gt R39eR gt HR l 2 H 2 H J Low energy barrier between isomers would yield a mixture of ole n isomers Dianion The study of the relationship between stereochemistry of the starting material and product By comparing the stereochemistry of the SM vs products we can make judgments about the intermediatetransition state Example 1 The Meisenheimer s rearrangement ofthe optically active amineoxide a well known example of the synelimination Erythro 9 Z isomer Threo 9 E isomer H H C CH CH 3 3 Ph CH3 H CH3 Ph CH3 3 ph H gtA c CH PH NCHg 9 CH3 lt 3 CH H CH H e CH3 ch CH3 GO 3 Threostartin GO 3 peo material g EPMduCt CHAPTER 5 page 16 The study of catalysis Much information about the mechanism of the reaction can be obtained from the knowledge of which substances catalyze or inhibit a reaction or do neither Note catalysts do not change AG They lower AG1 by providing an alternative pathway for the reaction to proceed This pathway generally involves several small steps in place of a single large step Catalysis Catalyst is an agent able to create a new reaction pathway new reaction coordinate for a given reaction The catalyst is not present in stoichiometry amounts While it may be consumed in an early step in the reaction sequence it will be released in a later step of that same sequence Catalysts are not consumed in reactions Catalysts change the rates of reactions by lowering AGI it does not affect AGO and affects both the forward and reverse rates equally Specific acidobasic catalysis H or its solvated form eg H30 acts in the ratedetermining step as a catalyst In the speci c basic catalysis it is OH39 Nondissociate forms of acidsbased are catalytically inactive Proton transfer between electronegative atoms 0 N is fast Other than electrontransfer reactions protontransfer reactions are the fastest reactions This fast equilibrium between the substrate R and the conjugated acid solvated H step A precedes the ratelimiting step which is the reaction of the conjugated acid to products step B Step A fast k1 Step B slow k2 R H30 RH H20 RH p k 1 For the reaction rate we can write the following equation k1 k2 R H3O Reaction rate k k2 K R H30 1 K k1 k2 For a speci c basic catalyst the equation is very similar Step A fast RH OH39 k1 Step B slow k2 R39 H20 R39 p k 1 CHAPTER 5 page 17 Intramolecular catalysis A key feature a catalyst is a moiety group present in the molecule Intramolecular catalysis is entropically more favored compared to intermolecular catalysis A classical example hydrolysis of acetylsalicylic acid versus ethyl acetate pH03 acidcatalyzed hydrolysis of both substrates is quite similar pH35 A acetylsalicylic acid we observe a catalytic activity of the carboxylate anion the sigmoidal dependence inside the dashed box corresponds to the dissociation curve of the carboxylate B the rate of ethyl acetate hydrolysis decreases as the pH approaches mildly acidicneutral pH log k COOH pKa 34 CocoCH3 COOH COOH 2006V COOH r I oHeCE flt a qo Hgt O E i OCOCH3 O H OH 0 2 O o CH3COOH 2 4 68 pH pH58 A acetylsalicylic acid the carboxyl group is fully dissociated and its concentration remains constant so the rate of hydrolysis remains constant B the rate of ethyl acetate hydrolysis increases with increasing pH Br nsted Theory based on the notion of releasing accepting of a proton e e 9 AHAH gt LIHQI K Ka Ka H I AHAe a AH I39gt p 09 p og 1 Strength of an acid pKa value 9 to zero and negative values pKa s of HCO4 or H is 10 of H2804 or HBr only 9 while those of aliphatic carboxylic acids are ca 45 CHAPTER 5 page 18 FACTORS THAT AFFECT REACTION MECHANISMS Concentration Temperature Pressure The change in reaction pressure effects mainly the reaction rate and equilibrium but in some case also the regio and stereoselectivity ofthe reaction Pressures up to 01 GPa have only a small impact they have a practical application only with reactions of lowboiling components at a temperature exceeding their bp and in the liquidgas equilibria where the higher pressure helps to dissolve the gaseous component hydrogenations Pressure 110 GPa dependence of the reaction rate on pressure Van t Hoffs isotherm dan Avi dp T RT AVi activation volume describes the change ofvolume corresponding to the formation of the transition state Three situations to be considered AV1 gt O 9 increase in the reaction pressure slows down the reaction AVi lt O 9 increase in the reaction pressure speeds up the reaction AVi O 9 the reaction rate is not affected by the pressure change The AV1 values are experimentally accessible Practically more accessible are the values ofthe ASi from which the AV1 may be calculated Synthetically important reactions that are known to proceed faster at higher pressures a hydrogenations b condensations atmospheric pressure CN 3 CO2Et gto 4 Ollt 023 15GPA 100 N c cycloadditions g T OCH3 OCH3 d reactions with dipolar transition state eg quarternizations Et E CH2CI2 EtNEt gt ClAlllgEt Ole E The main advantage of the use of high pressure is that the reaction proceeds at lower temperature thus limiting the formation of sideproducts CHAPTER 5 page 19 Solvent effects Determining the effect of the solvent on the reaction ratecourse may offer insight into the reaction mechanism We will talk about this in qreater detail in L quot reactions The description of solvent polarity there is no single measure ofthe solvent polarity p molecular dipole moment property ofthe individual molecule It does not account for intermolecular interactions 5 dielectric constant effect ofthe substance on the electric eld between two conductor plates with opposite charges It describes the orientation of molecules between conductors does not account for orientation of solvent molecules around chargedpolar solutes Empirical equationsfactors have been developed Most ofthem are based on the observed solvatochromism of certain dyeschromophores Kosower Z scale based on the solvatochromism of A ET3O scale based on the solvatochromism of B A B Kamlet and Taft introduced general polaritypolarizability index 1T which describes solvent s ability to stabilize chargedpolar species by its dielectric effect The ability of solvents to act as hydrogen bond acceptorsdonors is described by coef cients aB introduced by Kamlet amp Taft CHAPTER 5 page 20 Table of solvent parameters i 7 TA quotl39lynunvlrg i l mum l39 q A theoretical calculation of the AG of solvation isicom ated Simple empirical rules based on solvation theory have been developed 1 Reactions during which the formation of the transition state results in the formation or high concentration localization of the charge in a small volume are accelerated by increase solvent polari 2 Reactions proceeding with recombination or delocalization of the charge are accelerated by decreasing solvent polarity Reactions most affected by solvent polarity Nucleophilic substitutions solvolyses polar additions isotopic exchange reactions of the mesomeric ions enolates etc Three interesting examples CHAPTER 5 page 21 Example 1 CH3 RODRO39 CH3 lt gt i H lt t a ROHRO39 t A In nonpolar solvents the tight ion pair is formed and the HD exchange proceeds via a cyclic mechanism A Such reaction proceeds with retention ofthe con guration A T B c HO e e quotOR DIE retention of lIR con guration racemization inversion of con guration B In dipolar aprotic solvents the anion either nonsolvated or symmetrically solvated is formed B The carbanion racemizes before recombination and a racemic mixture is formed C In protic solvents the formed anion is solvated from the side opposite to the leaving proton C Such reaction proceeds with inversion ofthe configuration Example 2 A solventinduced change in the mechanism ofthe arylazosul de decomposition Benzene EtOH e e NN SR lt NN SR gt QNN SR The change between radicalionic mechanisms is caused by the increase in the probability ofthe heterolytic cleavage as a result of the effective solvation ofthe ion pair formed while preventing their recombination Nonpolar solvent benzene does not solvate the ion pair and does not prevent their recombination Example 3 Recation of mesomeric ions and enolates Onlkylotion gt e O O amp k9 ltl Cnlkylntion A In dipolar aprotic solvents enolates are dissociated cations are well solvated anions not so well solvated In the enolates negative charge is localized on oxygen and O alkylation is favored B In nonpolar aprotic solvents cations form ion pairs with enolate anions The Oatom is masked by the close proximity of the cation Attacking electrophiles attack the 00 bond and Calkylation is favored CHAPTER 5 page 22
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