Electrophilic Aromatic Substitution
Our next topic is reactions of aromatic rings. Yup, more C=C double bonds! You might be thinking, been there done that, but not so fast.These C=C bonds are worthy of a class of their own. Why? Well because if you subject them to the conditions of an addition reaction, nothing will happen. Turns out their conjugated ring system (review aromatic rings and resonance structures in Structure 9 and Structure 10 videos) makes them really stable. Problematically, an addition reaction would result in loss of aromaticity since you would break api bond and replace it with two new sigma bonds. (Stubborn rings!) So, what reactions can you do with aromatic rings?
Substitution reactions! But remember you cannot do a SN1or SN2 reaction on a sp2 hybridized carbon atom which means it’s time to learn a new mechanism for substitution!
In this pack we will learn how to do Electrophilic AromaticSubstitution (EAS) reactions. Let’s dissect this name a bit further. This reaction will take an aromatic ring and substitute a hydrogen atom for an electrophile.
The pi bonds of an aromatic ring are still electron rich, so similar to what you saw in the addition reactions, the first step is to attack the electrophile using a pi bond. But since this does break aromaticity (but only temporarily) you need a really good electrophile to get this reaction going. That is why you will also learn how about new electrophiles formed from AlCl3and HNO3/H2SO4.
As you will see in the videos recommended below, the pi bond is reformed, and aromaticity is restored, by removing the H from the carbon that got the electrophile, similar to what you did in elimination reactions. Another thing to keep in mind is that if a substituent is present on the aromatic ring, this will direct the substitution to occur on specific carbons.
Watch the videos below to review the new content associated with EAS reactions including
1. Mechanism for substitution on a sp2 hybridized carbon
2. Formation of good electrophiles
3. The possibility of rearrangements of electrophiles
4. Directing effects of substituents on an aromatic ring
Then check your understanding by going through the practice puzzles.