Covers the inductive effect with its negative and positive forms and example groups, then the mesomeric effect with +M and -M substituents, and a brief note on the electromeric effect. Goes on to resonance structures and reaction classifications by bond change and by chemical conversion, with examples of radicals, nucleophiles and electrophiles. Closing sections address activation energy and the transition state, complex reaction types, and hyperconjugation, Hückel's rule and electromeric examples.
Topic 4 of 42 in the MU Sofia syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 9 sections, about 1,101 words (7 min of reading), with 21 flashcards.
In this note
Electronic effects in organic molecules.
Resonance structure
Activation energy. Activated complex and transition state.
Complex chemical reactions – competitive (parallel), successive, reversible and chain reactions. Heat of chemical reactions.
High-Yield Additions: Electronic Effects in Organic Molecules
Hyperconjugation (No-bond Resonance)
Aromaticity & Hückel’s Rule
Electromeric Effect – Examples
Cleavage Types
Electronic effects in organic molecules.
Inductive effect
Definition: A permanent shift of electron density in a molecule due tothe electronegativity differences between its atoms.
Explanation: More electronegative atoms attract electron density towards themselves, creating a negative I-effect (-I). Less electronegative atoms push electron density away, resulting in a positive I-effect (+I).
Examples:
Negative I-effect: F, Cl, Br, OH, NO2, CN (pull electron density) (see image showing when F was added to the compound it pulled e- away from the OH group making it more acidic )
Positive I-effect: CH3, alkyl groups (push electron density)
Note: The I-effect weakens with increasing distance through the chain of atoms.
Mesomeric effect
Definition:
Mesomeric effect describes the electron density shift in a molecule due to the delocalization of π-electrons or lone pairs in a conjugated system.
Caused by the movement of π-electrons towards or away from a substituent group.
Represented by "M" and analyzed using resonance structures.
Types:
+M effect (positive): Electron density increases in the molecule due to π-electron donation from the substituent.
-M effect (negative): Electron density decreases in the molecule due to π-electron withdrawal by the substituent.
Examples: NO2, -CN, -SO3H, -COOH, -COOR.
Electrometer effect
Definition: Similar to the M-effect, but involves a temporary shift of electrons in response to an external stimulus, like an approaching electrophile.
These effects illustrate the differences in electron behaviour when elements other than carbon and Hydrogen participate in molecular bonds
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