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#1 2010-09-08 09:08:48

orgopete
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Why o- or p-nitrobenzene react faster nucleophilic arom subst

Yahoo!Answers in chemistry wrote:

Why does an ortho- or para-nitro substituted benzene reaction faster in a nucleophilic aromatic substitution reaction?

The answer requires understanding the mechanism of the substitution. In an ortho or para-isomer, a nucleophile can add to the aromatic ring in which the intermediate can be stabilized by the nitro-group. This intermediate is the Meisenheimer complex. This cannot be drawn if the attack occurs at the meta-position and therefore groups in this position are not substituted as readily. The mechanism is shown below:

       http://www.curvedarrow.com/chem/ZZ785A8AF6.jpg

This example is excerpted from A Handbook of Organic Chemistry Mechanisms, available at http://www.curvedarrowpress.com/ or Amazon.com

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