This document discusses how substituents on benzene rings affect the reactivity and orientation of mono-substituted benzene compounds in electrophilic substitution reactions. It notes that for mono-substituted benzene, three possible constitutional isomers may form depending on whether substitution occurs at the ortho, meta, or para sites. The nature of the substituent influences which isomer is favored, with methoxybenzene producing mainly the para isomer and nitrobenzene favoring the meta isomer. The meta carbocation intermediate is less stable than the ortho intermediate due to inability to delocalize the positive charge through resonance to carbon 1.

1. 1
UNIT I: Benzene and its
derivatives
1. Substituent's
2. Effect of substituent's on reactivity and
orientation of mono substituted benzene
compounds towards electrophilic
substitution reaction

2. 2

3. 3

4. 4

5. II. The second factor that becomes important in reactions of substituted benzenes concerns
the site at which electrophilic substitution occurs.
Since a mono-substituted benzene ring has two equivalent ortho-sites, two equivalent meta-
sites and a unique para-site, three possible constitutional isomers may be formed in such a
substitution.
If reaction occurs equally well at all available sites, the expected statistical mixture of
isomeric products would be 40% ortho, 40% meta and 20% para.
Again we find that the nature of the substituent influences this product ratio in a dramatic
fashion. Bromination of methoxybenzene (anisole) is very fast and gives mainly the para-
bromo isomer, accompanied by 10% of the ortho-isomer and only a trace of the meta-
isomer.
Bromination of nitrobenzene requires strong heating and produces the meta-bromo isomer as
the chief product.
5

6. 6

7. 7
HAAAT
NCCC=S
N

8. 8

9. 9

10. 10

11. 11

12. 12

13. 13

14. 14
Attack of the electrophile at C-3 results in a carbocation which can be
delocalized via resonance to C2, C4, and C6. However, there’s no way to “move”
the carbocation to C1, which means that there’s no reasonable resonance form
where all atoms have full octets.
This makes the meta- carbocation intermediate much less stable than the ortho-
carbocation intermediate.

15. 15

16. 16

17. 17
Why are halogens ortho- para-
directors?