'A' Level Chemistry Problem Analysis: Electron Withdrawing / Releasing Groups |
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How do you differentiate between electron withdrawing groups and electron releasing groups (or benzene ring activator/deactivator)? Thought process: Look for positive formal charges (eg. on N of the NO2 group) or positive partial charges (eg. on C of COOH group) to identify electron withdrawing by induction effect (and where possible because of pi bonds present, eg. NO2 group, electron withdrawing by resonance effect as well). Look for lone pairs which may be delocalized into the benzene ring by resonance; a resonance contributor sees a lone pair (on the atom bonded to the benzene ring) becoming a pi bond with the benzene ring, with a negative formal charge spread out over the ortho and para C atoms of the benzene ring. Viewed from another perspective, we can say for instance, that the p orbitals of the halogen (eg. chlorine of chlorobenzene) overlaps with the pi orbitals of the benzene ring, giving the C-X (eg. C-Cl) bond partial double bond character. Groups which donate electrons inductively (and/or by hyperconjugation), as well as groups which donate electrons by resonance, are both ortho-para directing, though interestingly for different reasons. Such groups activate the benzene ring towards electrophilic aromatic substitution, as they donate electrons into the benzene ring's pi system, making the pi electrons of benzene a stronger nucleophile. Groups which withdraw electrons by induction or by resonance, are meta directing, and deactivate the benzene ring against electrophilic aromatic substitution, as they withdraw electrons away from the pi system of the benzene ring, making the pi electrons of benzene a weaker nucleophile. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums. |
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