Organic Chemistry: Some Basic Principles and TechniquesNEET Chemistry · Class 11 · NCERT Chapter 8

Inductive effect is the permanent polarisation of sigma (σ) bonds due to electronegativity differences. It operates through the sigma framework and decreases rapidly with distance. Resonance involves the delocalisation of pi (π) electrons (or lone pairs) across a conjugated system. Resonance is a property of the whole molecule, not just one bond, and is described by drawing contributing structures. Both effects influence reactivity and stability but operate through different mechanisms.

The more alkyl groups attached to the carbocation carbon, the more stable it is: tertiary (3°) > secondary (2°) > primary (1°) > methyl (CH₃⁺). Alkyl groups donate electron density by the +I inductive effect and hyperconjugation, which disperses the positive charge and stabilises the carbocation. Resonance stabilisation (when a lone pair or double bond is adjacent) provides even greater stability.

Hyperconjugation is the delocalisation of electrons from C-H (or C-C) sigma bonds adjacent to a carbocation, radical, or double bond. The sigma bond electrons of the C-H bond interact with the empty p orbital (in carbocations) or pi system (in alkenes). More alpha-H atoms means more hyperconjugation, which means greater stability. This explains why more substituted alkenes and carbocations are more stable.

Valid resonance structures must: (1) have the same arrangement of atoms (only electron positions change); (2) have the same number of paired and unpaired electrons; (3) obey valency rules — no atom should exceed its maximum valency; (4) be most stable when all atoms have complete octets (electronegative atoms should not carry positive charges unless unavoidable). Resonance structures are not actual structures; the real molecule is a resonance hybrid of all contributing structures.

When multiple functional groups are present, use the priority order: carboxylic acid (-COOH) > anhydride > ester (-COO-) > acyl halide > amide (-CONH₂) > aldehyde (-CHO) > ketone (-CO-) > alcohol (-OH) > amine (-NH₂) > alkene (C=C) > alkyne (C≡C). The highest priority group is the principal characteristic group (named as a suffix). Lower priority groups are named as prefixes. Number the chain to give the principal group the lowest locant.

A nucleophile ("nucleus-lover") is an electron-rich species that attacks electron-poor centres. It has a lone pair or pi electrons to donate. Examples: OH⁻, CN⁻, NH₃, H₂O, Cl⁻. An electrophile ("electron-lover") is an electron-deficient species that attacks electron-rich centres. Examples: H⁺, BF₃, carbocations (R⁺), AlCl₃. In a reaction, the nucleophile donates the electron pair to form a new bond with the electrophile.