Why is there a +R or -R effect?

AI Thread Summary
The discussion centers on the behavior of substituent groups in resonance structures, specifically how certain groups can either attract or repel the delocalization of pi electrons. Electron-donating groups, known as +R groups, such as -OH, -OR, -SH, -NH2, and -NR2, contribute excess electrons, facilitating electron delocalization towards them. Conversely, electron-withdrawing groups, or -R groups, including -NO2, carbonyl groups (C=O), -C≡N, -COOH, -SO3H, CO₂, and SO₂, create a deficit of electrons, causing delocalization to be directed away from these groups. Understanding these interactions is crucial for predicting the stability and reactivity of various organic compounds. Examples illustrate how the presence of these groups influences molecular behavior through resonance effects.
kay
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What I mean to say is that why are some substituent groups such that(during resonance) the pi electrons' delocalisation is directed away from it while some groups are such that delocalisation of pi electrons is towards those groups?
Can you kindly give some examples too?
 
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+R groups have excess electrons like -OH, -OR, -SH, -SR, -NH2, -NR2 etc.
and -R have deficit of electrons like -NO2, Carbony group (C=O), -C≡N, -COOH, -SO3H, CO₂ , SO₂ etc.
 

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