ORGANIC CHEMISTRY II
ORGANIC CHEMISTRY II CHEM 334
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This 11 page Class Notes was uploaded by Willow Hessel on Monday October 26, 2015. The Class Notes belongs to CHEM 334 at University of South Carolina - Columbia taught by G. Handy in Fall. Since its upload, it has received 59 views. For similar materials see /class/229605/chem-334-university-of-south-carolina-columbia in Chemistry and Biochemistry at University of South Carolina - Columbia.
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Date Created: 10/26/15
Chapter 22 Conjugated Dienes l Electrophilic Addition to Conjugated Dienes a Conjugated dienes undergo twostep electrophilic addition reactios just as do simple alkenes However certain features are unique to the reactions of conjugated dienes b Addition of one equivalent of HBr to 13butadiene at 78 C gives a mixture of two constitutional isomers 3bromolbutene and lbromo2butene 780 C llgr If llgr If CH2CH CHCH2 HBr gt CH2CH CH CH2 CHg CHCH CH2 13Butadiene 3Bromo lbutene l Bromo 2 butene 90 10 12 addition 1 4 addition The designations quot12quot and quot14quot used here to describe additions to conjugated dienes do not refer to IUPAC nomenclature Rather they refer to the fouratom system of two conjugated double bonds and indicate that addition takes place at either carbons l and 2 or 1 and 4 of the fouratom system The bromobutenes formed by addition of 1 mole of HBr to butadiene can in turn undergo addition of a second mole of HBr to give a mixture of dibromobutenes Our concern at this point is only with the products of a single addition of HBr Addition of one equivalent of Br at 15 C also gives a mixture of 12 addition and l4addition products 1500 llgr Br 13r llgr CH2CH CHCH2 t Brz gt CHg CH CHCH2 CHg CHCH CH2 13Butadiene 34 Dirbromo lbutene 14Djbr0m02butene 54 46 12addition 12addition We can account for the formation of isomeric products in the addition of HBr in the following way Electrophilic addition is initiated by reaction of a terminal carbon of one of the double bonds with HBr to form an allylic carbocation intermediate best represented as a hybrid of two contributing resonance structures The addition is completed by rapid reaction of the allylic cation with bromide ion Reaction at one carbon bearing partial positive charge gives the 12addition product reaction at the other gives the l4addition product i Mechanism A resonance stabilized allylic carbocation H H a m I e I CH2CH CHCH2 H Br gt CH2CH CH CH2lt gtCH2CH CHECH2 Br Br I I 1 Ir CH2CH CH CH2 CH2 CHCH CH2 1 2addition 1 4 addition c Kinetic versus Thermodynaic Control of Electrophilic Addition i Electrophilic Addition to conjugated dienes gives a mixture of 12addition and 14addition products 780 C 13f IIi 13f IIi CH2CH CHCH2 HBr gt CH2CH CH CH2 CH2CHCH CH2 1 3 Butadiene 3 Bromo 1butene 1 Bromo 2 butene 90 10 12 addition 1 4 addition 150 C Br Br Br Br CH2CH CHCH2 Br2 gt CHg CH CHCH2 CHg CHCH CH2 13Butadiene 34 DirbrOInO 1 blltene 14 Dibromo2butene 54 46 1 2 addition 1 2 addition Additional experimental observations about the products of electrophilic additions to 13butadiene 1 For addition of HBr at 78 C and addition of Brz at 15 C the 12 addition product predominates over the 14addition product Generally at lower temperatures the 12addition products predominate over 14 addition products 2 For addition of HBr and Br at higher temperatures generally 40 60 C the 14addition products predominate 3 If the products of low temperature addition are allowed to remain in solution and be warmed to a higher temperature the composition of the products changes over time and becomes identical to the products obtained when the reaction is carried out at higher temperatures The same result can be accomplished at the higher temperature in a far shorter time by adding a Lewis acid catalyst such as FeCl3 or ZnClz to the mixture of low temperature addition products Thus under these higher temperature conditions an equilibrium is established between the 12 and l4addition products in which the l4addition product predominates 4 If either the pure 12 or pure l4 addition product is dissolved in an inert solvent at higher temperature and a Lewis acid catalyst added an equilibrium mixture of 12 and l4addition product forms The same equilibrium is obtained regardless of which isomer is used as the starting material Chemists interpret these results using the twin concepts of kinetic control and equilibrium control of reactions For reactions under kinetic rate control the distribution of products is determined by the relative rates of formation of each We see the operation of kinetic control in the following way At lower temperatures the reaction is essentially irreversible and no equilibrium is established between 12 and 14 addition products The 12addition product predominates under these conditions because the rate of 12addition is greater than the rate of 14 addition For reactions under thermodynamic equilibrium control the distribution of products is determined by the relative stability of each At higher temperatures the reaction is reversible and an equilibrium is established between the 12 and l4addition products The percentage of each product present at equilibrium is in direct relation to the relative thermodynamic stability of that product The fact that the l4addition product predominates at equilibrium is because it is thermodynamically more stable than the 12 addition product Why is the 12addition product less stable formed more rapidly at lower temperatures First we need to look at the resonance stabilized allylic carbocation We must consider the degree of substitution of both the positive carbon and the carbon carbon double bond in each contributing structure 399 CH2CH CH CH3 lt gt CHZ CHCH CH3 Less Substituted Double Bond More Substituted Double Bond Secondary carbocation Primary carbocation A secondary carbocation is more stable than a primary carbocation and if the degree of substitution of the carbon bearing the positive charge were the more important factor then the Lewis structure on the left would make the greater contribution to the hybrid A more substituted double bond is more stable than a less substituted one and if the degree of substitution of the carboncarbon double bond were the more important factor then the Lewis structure on the right would make the greater contribution to the hybrid We know from other experimental evidence that the location of the positive charge in the allylic carbocation is more important than the location of the double bond Therefore in the hybrid the greater fraction of the positive charge is on the secondary carbon Reaction with bromide ion occurs more rapidly at this carbon giving 12addition simply because it has a greater density of positive charge iii Is the 12addition product also formed more rapidly at higher temperatures even though it is the l4addition product that preodominates under these conditions Yes The factors affecting the structure of a resonance stabilized allylic carbocation intermediate and the reaction of this intermediate with a nucleophile are not greatly affected by changes in temperature iv Why is the l4addition product the thermodynamically more stable product Generally the greater degree of substitution of a carboncarbon double bond the greater the stability of the compound or ion containing it Following are pairs of 12 and l4 addition products In each case the more stable alkene is the l4addition product 113r H3C IH CH3CHCH CH2 H CHZBr 3Br0m0 lbutene E lBromo 2butene less Stable alkene more stable alkene Br BI CH2 H CC BrCHZCHCHCH2 H CHZBr 34 Dibromo lbutene E l4 Dibromo2butene less stable alkene more stable alkene There are cases where the 12addition product is more stable and would be the product of thermodynamic control Addition of Bromine to l4dimethyl l3cyclohexadiene under conditions of thermodynamic control gives 34 dibromol4dimethylcyclohexene because its trisubstituted double bond is more stable than the disubstituted double bond of the l4addition product CH 3 Br CH3 Br Br CH3 BI Q high temperature CH3 Br CH3 CH3 l4 Addition Product 12 Addition Product less stable more stable V What is the mechanism by which the thermodynamically less stable product is converted to the thermodynamically more stable product To answer this we must look at the relationships between kinetic energy potential energy and activation energy On collision a part of the kinetic energy is transformed into potential energy and if the increase in potential energy is equal to or greater than the activation energy for reaction the reaction may occur At the higher temperatures for electrophilic addition of HBr and Br to conjugated dienes collisions are sufficiently energetic that ionization of the l2addition product occurs to reform the resonance stabilized allylic carbocation intermediate It can then react again with bromide ion to form the thermodynamically more stable l4addition product At lower temperatures however the increase in potential energy on collision is not suf cient to overcome the potential energy barrier to bring about this ionization vi Is it a general rule that where two or more products are formed from a common intermediate the thermodynamically less stable product is formed at a greater rate No Whether the thermodynamically more or less stable product is formed at a greater rate from a common intermediate depends on the particular reaction and the reaction conditions 2 The Diels Alder Reaction a In 1928 Otto Diels and Kurt Alder in Germany discovered another unique reaction of conjugated dienes they undergo cycloaddition reactions with certain types of carboncarbon double and triple bonds b The compound with the double or triple bond that reacts with the diene is called a dienophile and the product of a DielsAlder reaction is called the Diels Alder adduct The designation cycloaddition refers to the fact that two reactants add together to give a cyclic product c Examples 0 O oCHZ H C C gt C H l H CH3 CH3 CH2 CH2 12 Butadiene 3 Buten 2 one 4 Cyclohexenyl methyl ketone a diene a dienoph ile a Diels Alder adduct COZCHZCH3 4CH2 C02CH2CH3 H C C III gt HC T CO CH CH CH2 COZCH2CH3 2 2 3 12 Butadiene Diethyl acetylene dic arboxylate Dlethyll 4 Cycbhexadlene a diene a dienoph ile 1 2dlcarb0Xylate a D1elsAlder adduct Note that the four carbon atoms of the diene and two carbon atoms of the dienophile combine to form a siXmembered ring Note further that there are two more sigma bonds anf two fewer pi bonds in the product than in the reactants This exchange of two weaker pi bonds for two stronger sigma bonds is a major driVing force of the reaction d Mechanism It is one of the few reactions that can be used to form a siXmembered ring It is one of the few reactions that can be used to from two new carboncarbon bonds at the same time It is stereoselective C93 00 V h Limitations Stereochemistry i The diene must be able to assume an 2cz39s conformation ii We can illustrate the importance of conformation of the diene by reference to l3butadiene For maximum stability of a conjugated diene overlap of the four unhybridized 2p orbitals making up the pi system must be complete a condition that occurs only when all four carbon atoms of the diene lie in the same plane It follows then that if the carbon skeleton of l3butadiene is planar the siX atoms bonded to the skeleton of the diene are in the same plane There are two planar conformations of l3butadiene referred to as the strans and th scis conformations where the designation s refers to the carboncarbon single bond of the diene Of these the strans conformation is slightly lower in energy and therefore more stable Although stransl3butadiene is the more stable conformation scz39s 13 butadiene is the reactive conformation in DielsAlder reactions In the scis conformation carbon atoms 1 and 4 of the conjugated system are close enough to react with the carboncarbon double or triple bond of the dienophile and form a siXmembered ring 1 1 C CH HC CH H I I HCH HCH H strans conformation scis conformation slightly lower in energy slightly higher in energy i The Effect of Substituents on Rate i The simplest example of a DielsAlder reaction is that between l3butadiene and ethylene both gases at room temperature Although this reaction does occur it is very slow and takes place only if the reactants are heated at 200 C under pressure CH2 H C CH2 20000 H EH 2 pressure CH2 13 Butadiene Ethylene Cyclohexene ii DielsAlders are facilitated by a combination of electronwithdrawing substituents on one of the reactants and electronreleasing substituents on the other For example placing a carbonyl group electronwithdrawing because of the partial positive charge on its carbon on the dienophile facilitates the reaction CH2 CH O H C 14000 H I CCH3 gt CCH3 H C H CH2 CH2 13Butadiene 3Buten2one Electmn Releasing Electmn Withdrawing Gmups Gmups CH3 C CH2CH3 CH aldehyde 0 CHCH32 R ketone C CH 33 C R other alkyl groups COH carboxyl OR ether 0 C COR ester OCR ester NO2 InHO CEN cyano Placing electron releasing methyl groups on the diene further facilitates the reaction 0 0 H3O CH2 CCH3 30 C H3C CCH3 gt H HC CH 3 2 CH2 H3O 23Dimethyl 32Buten 2one 13butadiene j DielsAlder Reactions can be used to form Bicyclic Systems i Conjugated cyclic dienes in which the double bonds are of necessity held in an scis conformation are highly reactive in DielsAlder reactions Two particularly useful dienes for this purpose are cyclopentadiene and 13 cyclohexadiene In fact cyclopentadiene is reactive both as a diene and as a dienophile and on standing at room temperature it forms a DielsAlder adduct known by the common name dicyclopentadiene When dicyclopentadiene is distilled at its normal boiling point of 170 C a reverse DielsAlder reaction takes place and cyclopentadiene is reformed room temp gt 170 C The D1ene The D1enoph ile D1cyclopentad1ene endo isomer H The terms quotendoquot and quotexoquot are used for bicycli DielsAlder products to describe the orientation of substituents of the dienophile in relation to the two carbon dienederived bridge Exo substituents are on the opposite side from the dienederived bridge endo substituents are on the same side the double bond delived om the exo outside relative to the double bond diene V endo inside relative to the double bond For DielsAlder reactions under kinetic control the endo orientation of the dienophile is favored Treatment of cyclopentadiene with methyl propenoate methyl acrylate gives the endo adduct almost exclusively The exo adduct is not formed 0 H IIC02CH3 gt Cyclopentadiene Methyl propenoate Methyl bicyclo 22 lhept5 enendo2 carboxylate k The Conformation of the Dienophile is Retained i If the dienophile is a cis isomer then the substituents cis to each other in the dienophile are cis in the DielsAlder adduct Conversely if the dienophile is a trans isomer substituents that are trans in the dienophile are trans in the adduct II H COCH3 gt H OCH3 O Dimethyl cisZbutenedioate Dimethyl cis4cyclohexene12dicarboxy1ate Cu CH3OC H E COCH3 gt quotquotquotCOCH3 H OCH3 II 0 Dimethyl transZbutenedioate Dimethyl trans4cyclohexene12dicarboxy1atc l Mechanism The DielsAlder Reaction is a Pericyclic Reaction i As chemists probed for details of the DielsAlder reaction they discovered that there is no evidence for participation of either ionic or radical intermediates Thus the DielsAlder reaction is unlike any reaction studied thus far To account for the stereoselectivity of the DielsAlder reaction and the lack of evidence for any intermediates chemists have proposed that reaction takes place in a single step during which there is a cyclic redistribution of electrons During this cyclic redistribution bond forming and bond breaking are concerted simultaneous The reaction is pericyclic that is a reaction that takes place in a single step without intermediates and involves the cyclic redistribution of bonding electrons l gt 0 H exo C endo H CO J C 3 3 3 exo 0 OCH3 endo 1 New Bonds Form 2 Envelope Flap Moves Up 3 H moves to exo position C02CH3 moves to endo position
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