Depto. Química Orgánica

ORTHO-PARA DIRECTORS
SEAr

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15/07/2017
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Ortho-para directors can be either activators or weak deactivators.
Activators
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The ring of aniline is so activated that its bromination is performed without catalyst and is difficult to be stopped at the first step.

Could you anticipate the serious inconvenient of using in this very reaction a Lewis acid as a possible catalyst? See problems and exercises...
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The orientation is ortho-para because the positive charge in the corresponding benzenonium ions is directly stabilized by resonance with the nitrogen's lone electron pair.
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Weak Deactivators
The best example is the halogens. Their electronegativity makes them withdraw some electron density from the ring thus somewhat hindering the SEAr.
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Despite the electron-withdrawing effect of the halogens, the ring still retains a reasonable electron density for the SEAr to proceed at a good rate.
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