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Fundamental concepts of Organic reaction Mechanisms

  • The movement of electrons can be represented by double headed arrow in polar reactions.
  • The movement of electrons can be represented by single headed arrow in radical reactions.
  • Heterolysis is the unsymmetrical cleavage of a covalent bond to form a cation and an anion.
  • Homolysis is the symmetrical cleavage of a covalent bond to form two radicals.
  • Short-lived carbocations, carbanions, and carbon radicals, which can often be detected (but not isolated) in polar and radical reactions, are called reactive intermediates.
  • The shape of the reactive intermediate depends on the hybridization of the carbon atom carrying the charge or unpaired electron.
  • Electronic effects
    • the polarization of electrons in inductive effect and hyper conjugation refers to σ bonds, while the delocalization of lone pairs of electrons and electrons in σ bonds refers to mesomeric effects.

  • +I/+M groups are electron-donating groups that stabilize carbocations
    I/-M groups are electron-withdrawing groups that stabilize carbanions.
A combination of a number of contributing resonance forms is called resonance hybrid.
  • More often mesomeric and inductive effects oppose one another, but work sometimes in the same direction
  • Mesomeric effects are generally stronger and can be more effective over longer distances than inductive effects and hyperconjugation.
  • Conjugated molecules contain alternating single and double bonds.
Nucleophiles and electrophiles
  • Nucleophiles are negatively charged ions or neutral molecules that donate a pair of electrons.
  • The molecules that accept a pair of electrons is called electrophiles or positively charged molecules.
  • Anions are strong nucleophiles (of large atoms that are not highly electronegative) or neutral molecules containing a lone pair of electrons (on an atom that is not highly electronegative).

  • The carbon bonded to highly electronegative atoms is called strong electrophiles.

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