Lecture 2 Monday January 5 2015 12 57 PM Ultimately the circled structure allows us to use on structure and show that the charge in the allylic cation is on two terminal carbons What happens when the ends of the allylic fragment are not the same Given that the tertiary cations are more stable than the primary ones we can conclude that the resonance structure on the left is dominant and that there will be more charge on the more substituted end of the allylic fragment Similarly radical will be more on the more substituted end that is where its more stable while the anion will be more on the less susbtituted end its more stable there Presence of the double bond effects the structure of cations anions and radicals that are found on the neighboring carbon allylic position The presence of the double bond can also effect the reactivity of functional groups in allylic position The idea is the same the presence of 2 electrons in the pi system can interact with the rest of the molecule and change its properties SN1 reactions involve the formation of the carbocation and with allylic electrophiles the presence of the double bond dramatically effects the outcome of the reaction The presence of the double bond effects which products are formed Because charge is on two carbons we can get two products In the following reaction water can add to one end of the pi system arrow a and give tertiary alcohol or to the other carbon arrow b to give the tertiary cation Based on the relative stabilities of primary and tertiary cations we can conclude that there is going to be more charge on the tertiary cation than on the primary one and as a result tertiary alcohol will be the major product and primary will be a minor one If we take the other isomer of the allylic chloride used in the previous reaction what will happen Well we ll again form the same allylic cation and once formed that cation will react in the exact same way like in the previous example As a result will get a same mixture of the primary and tertiary alcohol with tertiary being the major product and primary being the minor product Allylic rearrangement The substitution product in which the double bond changes the position is said to be formed by allylic rearrangement For export Page 1 Lecture 2 Wednesday January 7 2015 2 16 PM The presence of a double bond also effects the rate of the reaction Allyic chloride shown reacts 100X faster than the tertbutyl chloride That s because the tertiary allylic cation is more stable than the simple tertiary cation Resonance structure indicates that we can use pi electrons to partially fill the empty p orbital of the cation SN2 reactions in allylic systems Presence of a double bond also effects the rate of Sn2 reactions when the leaving group is in allylic position SN2 reactions of allylic electrophiles are faster than reactions of non allylic ones You can see that in the foloowing figure that shows the relative rates of Sn2 reactions of alkyl chlorides So the following reactions is faster than it would be in the absence of the double bond For export Page 2 Lecture 2 Wednesday January 7 2015 2 22 PM Why are Sn2 reactions faster Looking at the starting material and the product it looks like the double bond does not participate in the reaction It turns out that the electrons from a double bond can stabilize the transition state for the SN2 reaction Another way to think about it is that the presence of the double bond changes the LUMO of the alkyl halide and that changes the orbital interaction necessary for the reaction and stabilizes the transition state The picture below shows the LUMO of the allylic chloride Notice that the LUMO stretches across not only the carbon with the leaving group but also the carbons with the double bond Unlike in Sn1 reactions in SN2 reactions allylic rearrangement does not happen So double bond does not change the position Allylic radicals as intermediates in substitution reactions Allylic radicals are more stable than alkyl radicals As a result they are easier to generate Radical allylic substitution can be useful transformations For export Page 3 Lecture 2 Wednesday January 7 2015 Mechanism Initiation 3 35 PM Propagation Analogous bromination is also possible using NBS N bromosuccinimide N bromosuccinimide Again the reaction proceeds through the allylic radical intermediate As you can see hydrogen atoms from both position 3 and position 6 can be abstracted and we get both allylic radicals In this case their reaction with the Br leads to the formation of the same product In cases where the structure of the starting material is less symmetrical we can get a complex mixture of products For example In the following reaction we can get two different allylic radical intermediates which are of roughly similar stabilities They are both secondary allylic radicals Furthermore both radicals have an additional resonance structure As a result of all of that we can get 4 different bromide products Write the structure of the 4 possible products In this case the for products are formed roughly in the same amounts For export Page 4 That s why allylic radical substitution is useful only for systems where two allylic positions are the same and both resonance structures of the allylic radical are the same In other words only with very simple and highly symmetrical systems Lecture 2 Wednesday January 7 2015 3 44 PM Allylic anions Allylic anions are more stable than the analogous alkyl anions As a result allylic positions are more acidic than simple alkanes It is easier to deprotonate the allylic position than it is to deprotonate the alkane The satability of the allylic anions comes from the fact that the two electrons of the anion can be delocalized across the 3 atom fragments by conjugation with the double bond That is reflected in the shown resonance structure An additional stabilization of the allylic anion relative to the simple alkyl anion comes from the fact that the anion is next to the carbon that is sp2 hybridized and not sp3 hybridized see in red Remember sp2 carbon atoms are more electronegative than the sp3 carbons Dienes Dienes are molecules that have two double bonds When two double bonds are found in the same molecule they can have a profound effect on each other s stability and activity What effect they will have on each other depends on their relative position in the molecule Based on the relative position double bonds have in the diene dienes can be divided into Isolated