Why is there a +R or -R effect?

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SUMMARY

The discussion centers on the +R (positive resonance) and -R (negative resonance) effects of substituent groups in organic chemistry. +R groups, such as -OH, -OR, -SH, -SR, -NH2, and -NR2, donate electron density through resonance, enhancing the stability of adjacent positive charges. Conversely, -R groups, including -NO2, carbonyl groups (C=O), -C≡N, -COOH, -SO3H, CO₂, and SO₂, withdraw electron density, destabilizing adjacent positive charges. Understanding these effects is crucial for predicting the reactivity and stability of organic compounds.

PREREQUISITES
  • Understanding of resonance structures in organic chemistry
  • Familiarity with electron donating and withdrawing groups
  • Knowledge of functional groups and their properties
  • Basic principles of molecular stability and reactivity
NEXT STEPS
  • Study the impact of resonance on acidity and basicity in organic compounds
  • Explore the role of substituent effects in electrophilic aromatic substitution reactions
  • Learn about the stability of carbocations and carbanions in relation to resonance
  • Investigate the influence of resonance on reaction mechanisms in organic synthesis
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Chemistry students, organic chemists, and anyone interested in understanding the effects of substituents on molecular stability and reactivity.

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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