Question

1) Explain with halogens are deactivators, but ortho, para directors, instead of m directors, in an EAS reaction.

2) Draw the energy diagrams for the first step of an electrophilic aromatic substitution reactions of a monosubstituted benzene, when the substituent is an electron-withdrawing group (E), an electron-donating group (D), and a halogen (X), respectively.

Answer 1

1. Halogens are deactivated, but ortho para directing in EAS:

Halogens withdraws electrons through inductive effect and releases electrons through resonance. Through inductive effect, halogen destabilize the intermediate carbocation formed during the electrophilic substitution.

Through resonance, halogen tends to stabilize the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes the net electron withdrawal and thus net deactivation. The resonance effect tends to oppose the inductive effect for the attack of ortho- and para- positions and hence make the deactivation less for ortho- and para- attack. Reactivity is thus controlled by the stronger inductive effect and the orientation is controlled by resonance effect.

2. Energy Profile diagram for the first step of an electrophilic aromatic substitution:

The first step is the attack of the electrophile which is the rate determining step. The transition state 1 is followed by the carbocation intermediate.

The electron withdrawing group destabilize the carbocation intermediate hence it decrease the rate of reaction. So more activation energy is required for the step 1.

Electron donating group stabilize the carbocation intermediate hence it increases the rate of reaction. So for the step 1, less activation energy will be required.

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