Ortho/para versus meta directors in benzene chemistry
Students taking organic chemistry second semester, learn benzene reactions. Benzene is a cyclic ring consisting of six carbons and three double bonds.
When a benzene has one or more substituents already on, these substituents determine where the next substituent will go.
The substituents can be divided into two categories: ortho/para and meta.
Ortho is the location on the benzene ring right next to the substituent.
Para is the location opposite the substituent.
Meta is one carbon away from the substituent.
How can we recognize whether a substituent is ortho/para versus meta?
Here are some examples:
Ortho/Para directors - OH, NH2, R, OR, Cl, Br, F, I, SH
Meta directors - CF3, COH, COOH, COOR, CN, NO2
Trick to memorizing directors:
Ortho/para directors are alkyl group or generally have lone pairs on the element attached to the benzene ring.
Meta directors usually have an element with a double bond o attached to the benzene ring.
When there are multiple substituents on the benzene ring, how do we know where the next one will go?
The most activating substituent always wins. Ortho/para substituents are always more activating than meta.
For example if there is OH and NO2 on the ring, OH will be responsible for directing the next reaction since it is more activating than NO2.
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