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
9.6 Competitive orienting effect of two substituents
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
1. Two groups reinforce (provide same place for ${S_E}$ reaction) each other
Electrophilic attacks $o-$ and $p-$ positions of each $C{H_3}$ group, which are same and only one possible product is formed.
Electrophile attacks $o-$ and p-positions of $C{H_3}$ group and $m$-positions of $-N{O_2}$ group which are same and only one possible product is formed.
2. Two groups, one $o-$ and p-directing and one meta directing provides different positions for ${S_E}$ reactions: The $o-$ and p-directing group activates the ring by supplying electrons at $o-$ and $p-$positions, whereas $m-$directing group deactivates ring by withdrawing electrons from $o-$ and p-positions. In such cases the power of $o-$ and $p-$directing group overpowers the $m-$directing group e.g.
$C{H_3}$ group supplies electrons to 2,6 and 4 positions; $N{O_2}$ withdraws electrons from 2,6,4 positions. The power of $C{H_3}$ group predominates and no substitution occurs at $m-$position of –$N{O_2}$ group i.e.
3. Two $o-$ and $p-$directing groups – one is strong activating and other one is weak: The $o-$ and $p-$directing group having strong nature to supply electron pair and $o-$ and $p-$position overpowers the effect of other $o-$ and $p-$directing group having weak nature to supply electron pair at $o-$ and $p-$position e.g. $OC{H_3}$ is relatively stronger $o-$ and $p-$directing group than methyl group and thus substitution occurs with respect o- and p-positions of $OC{H_3}$ group.
Note:
1. $p-$position is not vacant for $OC{H_3}$ group and thus only at $o-$position substitution occurs.
2. No substitution at $o-$position of $C{H_3}$ group.
4. Two weak activating or deactivating groups or two strong activating or deactivating groups are present: All possible isomers are formed.
