Electrophilic Aromatic Substitution 04 09 2015 Ring attacks electrophilic carbon When you have an aromatic ring like benzene in the presence of an electrophile first step is exactly the same as an addition reaction we ve seen this before You get a carbocation intermediate Now if this were an addition reaction next the nucleophile would come along and attack the carbocation and give the end product However we do not see this in aromatic systems Why Because it loses its aromaticity So when we attack the electrophile we lose our aromaticity If the nucleophile comes in and attacks the carbocation that lost of aromaticity is present Instead the nucleophile acts as a base and plucks off the proton and returns the ring back to its aromaticity No longer an aromatic compound You loss aromaticity permanently So Nu comes in and plucks off the proton o Acts as a base and returns the ring back to its aromaticity Aromaticity is very stable and nature loves that On benzene Straight forward Doesn t matter H you pick You get a substituted benzene ring Things after benzene gets complicated How do functional groups effect substitution o 1 Rate how fast subs occurs o 2 Orientation o o Where does o So let s talk about rate first The first thing is we have to have a standard We set our standard as benzene When we start putting fx groups on the aromatic ring and we take that subs benzene ring and take another We start with benzene as the standard o Then we take that subs benzene ring with another subs does it happen faster or slower than benzene o We have classified all functional groups into two basic classes 1 Electron withdrawing Decreases the rate Why because you withdraw electron density away from the aromatic ring unto itself So it is a weaker nucleophile So the first step is harder attacking the electrophile this is the rate determining step Known as ring deactivators 2 Electron releasing Called ring activators They increase the rate relative to benzene Orientation o There are three possible products when you subs a benzene ring with a Electrophilic group already on it In ortho carbocation indetermediate is delocalized So you get 3 resonance structures Meta carbocation intermediate is also delocalized So you get another 3 resonance structures Para again You get NINE resonance structures o Electron withdrawing group ex aldehyde o With an electron releasing functional group it wants the positive charge So electron releasing ortho and para So electron withdrawing Our experiment Split into halfs o One half with rate another half with orientation One report though First half rate o Take several aromatic compounds in the presence of bromine o We will do a relative rates of bromination experiment o Come to lab with a predicted order or rates o Solvent is 15 M acetic acid Weird solvent cannot happen under aqueous conditions because the rings aren t soluble o We need to activate the bromine so it doesn t take forever Second half orientation o We will nitrate bromobenzene o Bromine is a weak ring deactivator But strong ortho partha directors o All meta directers are ring deactivators but not all ortho para directors are ring activators Exception halogens o What s the control Split when it comes to rate and orientation Steric hindrance Br is big Nitro group is big So it makes sense But when you use chlorine expect doesn t happen So it s not size electronegativity Cl more electronegative than bromine Most you can expect is 8 mg o Could also get nothing so not important Always add stronger acid to weaker acid 04 09 2015 04 09 2015