Stronger Donor Wins
OK, now that we’ve discussed electrophilic aromatic substitution a bit and the influence of directing groups, here’s yet another question.
What do we do when there’s more than one group on a benzene?
Sometimes it’s fairly easy to figure this out. Let’s say you have an aromatic ring with CH3 and NO2 on it, arranged ortho to each other.
- CH3 is an ortho-para director, so it will direct electrophiles to C4 and C6 (but not C2 – nitro is there already).
- The NO2, being a meta director (ortho-para avoider, as I like to say) will also direct electrophiles to C4 and C6.
- In this case C6 is a slightly more sterically hindered carbon than C6 since it’s adjacent to the CH3 group.
- So C-4 would be favored.
What if the two groups direct to different positions? In the second example below:
- CH3 directs to C2, C4, and C6, while NO2 directs to C5.
- Here, we can apply a rule: the stronger activator wins. Remember that the dominant product will reflect the most stable carbocation formed during the reaction. So we can ignore the influence of the nitro group and focus on the directing influence of the methyl group.
- Here, C2 is notably more sterically hindered than C4 and C6, since it’s in between two groups.
- So the dominant product will have the electrophile either at C4 or C6.
So there are really four questions to ask:
- What groups are present?
- Which is the more activating group?
- Where will they direct electrophiles to on the ring?
- Which is the least sterically hindered of those positions?
Tomorrow: what do electrophilic aromatic substitutions have in common with simple arithmetic questions?
Thanks for reading! James