Theory of orientation: orientation or directive effect can be explained by studying the all the possible resonance structure of the sigma complex formed as a result of the electrophile at ortho, meta and para positions for different types of monosubstituted benzene.
a) Ortho-para directing groups which having electron releasing inductive effect (+I effect): Ex: Alkyl groups (-R) has +I effect i.e electron releasing effect. Let’s study the sigma complexes(carbocation intermediate) which are formed by attacked of electrophile (+E) at ortho, para and meta position of mono substituted benzene ( toluene).
The sigma complex or carbocation intermediate is a resonance hybrid of three Structure. Alkyl group has electron releasing effect so it is disperse the positive charges and stabilize the carboncation. Structure 1&5 effect is maximum because +ve charge is present at the position of carbon where the methyl group is attached.
b) Ortho-para directing groups which are having electron withdrawing (-I effect) and electron releasing (+R or +M effect) resonance or mesomeric effect: Ex -NH2 group. Various sigma complexes or intermediate resulting from attract of electrophile Ortho-para and meta positions.
At Ortho and para positions the sigma complex obtained resonance hybrid of four Structure, at meta position only three. Structure 4 and 7 are more stable as positive charges delocalised on the nitrogen atom as well as ring carbon atom.
c) Meta directing groups: all meta directing groups ( -NO2, CN, COOH,-CHO, -SO3H) are electron withdrawing in nature. They have both electron withdrawing inductive and resonance effect i.e -I and -R effect.
Intermediate carbocation resulting from ortho, Para and meta effect:
Structure 3 and 5 are highly unstable as electron withdrawing nitro groups attached to the carbon atom which are having +ve charge. So sigma complex intermediate from Ortho ¶ attack are resonance hybrid of 2 Structure. Ortho-para attacking sigma complex are less stable than meta attack. Electrophilic substitution reaction is occurs slowly in nitrobenzene than Benzene.
The next time I come to this blog I will be sure to make better points.
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