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
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
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
Consider $–OH$ group attached on ${C_6}{H_6}$ nucleus having two unshared pair of electron on $O$ atom. One of the such unshared pair of electron shifts towards nucleus following $+E$ effect and $+M$ effect and give rise to an increase in electron density at $o-$ and p-position and thus provide seat for electrophile to attack these centres.
Note: 1. A free electron pair charge or increased density at $o-$ and $p-$position activates ring for further substitution. That is why nucleus having $o-$ and $p-$ directing groups are more reactive than benzene for ${S_E}$ reactions.
2. The $o-$ and p-directing groups (except halogens) are thus also known as activating groups.
3. Halogens, although increase electron density at $o-$ and $p-$ positions due to $+E$ and $+M$ effect, but they also deactivate the ring due to strong $–I$ effect. (Halogens are strong electron withdrawing groups).
