Lecture 6 Friday January 15 2016 4 54 PM This is just about how we call things Banzylic position is special Similar to the allylic position the reactive intermediates cations radicals and anions in benzylic position are conjugated connected to the pi system of the benzen ring The conjugation allows resonance delocalization of the cation radical or anion through the ring which in all cases leads to stabilization of the intermediate So benzylic intermediates are more stable than simple alkyl intermediates and are bi more stable than the allylic intermediates It is worth noticing that the cation radical or anion is delocalized just in the ortho and para position and not into meta positions That is because the corresponding MO has a node in meta positions similar to the node on the middle carbon in allylic intermediates The stability of benzylic intermediates suggest their higher reactivity in a number of organic reactions Sn1 with benzylic electrophiles is faster than with simple alkyl electrophiles because rate of the cation formation is limiting and the stability of the benzylic cation makes allows faster formation For this think about Hammond postulate formation of the cation is ednothermic so TS is similar to the product Unlike SN1 with allylic substrates we do not have to worry about multiple product formation The only products formed are the ones that have the aromatic ring intact Remember aromatic ring is worth 40 kcal mol in energy So substitution always in benzylic position For export Page 1 Lecture 6 Friday January 15 2016 4 54 PM Sn2 is also much faster and the explanation is the same as with allylic systems LUMO is delocalized and that lowers TS energy Benzylic radicals much more stable than simple alkyl radicals that s way in a molecule that has a benzylic C H bonds it is possible to selectively substitue that bond over other types of C H bonds with high selectivity That s why radical substitution on a benzylic substrates is a very useful reaction Here is an example For export Page 2 Lecture 6 Friday January 15 2016 4 54 PM Industrially important Used a lot As long as the alkyl group off of benzene ring has a benzylic C H bond it will be oxidized to the acid The week C H bond serves as a weak link that starts the process For export Page 3 Lecture 6 Friday January 15 2016 4 54 PM Same as with conjugated diens the conjugation works for us and we can make conjugated alkenyl benzenes by either E1 or E2 process and in all cases if it is possible conjugated alkene will be formed Because of the aromaticity double bonds in benzene are much more stable than other double bonds so we can selectively do chemistry on the alkene without touching the ring Here are some examples of such reactions The presence of the benzene ring will effect regioselectivity In all cases where there is a chance for that the reactive intermediate cation or radical will be formed in the benzylic position with high selectivity see last 2 reactions For export Page 4