Chemistry > Aromatic Compounds > 9.0 Ortho / Para Ratio
Aromatic Compounds
1.0 The Structure of Benzene
1.1 A Resonance Picture of Benzene
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
2.0 Electrophilic Aromatic Substitution Reactions
3.0 Nitration
4.0 Sulphonation
5.0 Halogenation
6.0 Friedel-Crafts Alkylation
7.0 Friedel-Crafts Acylation
8.0 Orientation and Reactivity in Electrophilic Aromatic Substitution
8.1 Donation of electrons into a benzene ring by resonance
8.2 Withdrawal of electrons from a benzene ring by resonance
9.0 Ortho / Para Ratio
9.1 Directive influence of the groups during substitutions in benzene ring
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
10.0 Reactions of Alkyl Benzenes
9.5 Mechanism of m-directing groups
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
8.2 Withdrawal of electrons from a benzene ring by resonance
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
The m-directing groups except $CC{l_3}$, either possess a –ve charge or the atom of such group attached to nucleus has a multiple bond on it. The $–E,$ $–M$ and $–I$ effect of all such groups leading the displacement of the electrons away from the nucleus and withdraw the electron from $o-$ and $p-$ position, thereby producing a decrease in electron density at $o-$ and p-position.
The electron density at meta position remains unchanged, however, the relative electron density at meta position becomes more and thus electrophile attacks $m-$ positions in all such cases.
Note:
1. The meta position electron density remains unaffected in presence of $m-$ directing groups and thus these groups provide no extra facility for electrophile. On the country they deactivate the ring for further substitution. That is why presence of $m-$directing group on benzene nucleus give rise to ${S_E}$ reactions with difficulty in comparison to benzene itself.
2. $-CC{l_3},$ $ - NH_3^ + $ and $NR_3^ + $ groups undergo –I effect followed with –M effect.
