I cant remember where I have seen this...
For electrophilic substituition, if say you have 3-methylbenzoic
acid reacting with the conc. nitric acid and conc. sulphuric acid,
where will the NO2 group be added since there's no common
position?
Will the acid group take priority or will the methyl group take
priority?
Is there a list of priority of groups in this case? Is this the
same priorities as when naming?
I googled and this is the priority list for aromatic cpds I have
found out:
1. R group
2. NH2 grp
3. OH gp
4. aldehyde grp
5. COOH gp
6. SO3H gp
The carboyxl group is deactivating and meta directing, due to a
partial positive charge on its C atom. The methyl group is
electron donating by induction and activating, and is hence ortho
and para directing.
Activating directors outweigh (ie. are more influential than)
deactivating directors.
Therefore, in this case, the nitronium cation, a sufficiently
electrophilic species,will substitute away a proton on a
position ortho or para to the methyl group.
Considering that the carboxyl group presents a greater steric
hinderance as compared to the methyl group, the major product will
be the nitro group substituting ortho to the methyl group (ie. 4th
C on the benzene), and the minor product will be the nitro group
substituting para to the methyl group (ie. 6th C on the
benzene).