{"id":28993,"date":"2025-07-17T12:44:59","date_gmt":"2025-07-17T11:44:59","guid":{"rendered":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28993"},"modified":"2025-07-22T09:02:02","modified_gmt":"2025-07-22T08:02:02","slug":"typical-electron-withdrawing-groups-are-o-m-directors-rather-than-m-directors-in-electrophilic-aromatic-substitution","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=28993","title":{"rendered":"“Typical Electron-Withdrawing Groups Are o, m-Directors Rather than m-Directors in Electrophilic Aromatic Substitution”"},"content":{"rendered":"
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The title of this post comes from an article published in a special virtual issue on the theme “Physical Organic Chemistry: Never Out of Style<\/em>“[1]<\/a><\/span> There, Paul Rablen presents the case that the amount of o<\/em> (ortho) product in electrophilic substitution of a phenyl ring bearing an EWG (electron withdrawing group) is often large enough to merit changing the long held rule-of-thumb for EWGs from being just meta directors into these substituents are best understood as ortho, meta-directors, with a preference for meta.<\/em> I cannot help but add here a citation[2]<\/a><\/span> to the earliest publication I can find showing tables of both o,p<\/em> and m<\/em>-directing groups and dating from 1887, so this rule is 138 years old (at least).<\/p>\n

Here\u00a0I thought I might show some computational models (\u03c9B97XD\/Def2-QZVPP\/SCRF=Dichloromethane)[3]<\/a><\/span> derived from the relative stability of the Wheland or \u03c3-complex produced by protonating the Ph-EWG molecule in the three possible positions on the ring – and now taking the opportunity to add some unusual EWGs to the table to explore how far this effect might be pushed.<\/p>\n

\"\"<\/a><\/p>\n

I start by looking at the results reported for benzonitrile\u00a0(EWG = CN), for typical product distributions: <\/p>\n

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  1. o<\/em>– (~16%), m<\/em>– (~82%) and p<\/em>– (~2%) are cited for nitronium ion as electrophile\n<\/li>\n
  2. o<\/em>– (23%), m<\/em>– ( 74%) and p<\/em>– (3% ) for chlorination\n<\/li>\n
  3. o<\/em>– (34%), m<\/em>– (55%) and p<\/em>– (1%) for uncatalysed bromination (see [4]<\/a><\/span> for an unexpectedly complex mechanism and kinetic analysis of this particular reaction)\n<\/li>\n
  4. \u03c3-complex calculations [5]<\/a><\/span> which result in values of o<\/em>– (43%), m<\/em>– (55%) and p<\/em>– (2%) for benzonitrile.\n